1 @c Copyright (C) 1988,1989,1992,1993,1994,1996,1998,1999,2000,2001,2002, 2003
2 @c Free Software Foundation, Inc.
3 @c This is part of the GCC manual.
4 @c For copying conditions, see the file gcc.texi.
7 @chapter C Implementation-defined behavior
8 @cindex implementation-defined behavior, C language
10 A conforming implementation of ISO C is required to document its
11 choice of behavior in each of the areas that are designated
12 ``implementation defined.'' The following lists all such areas,
13 along with the section number from the ISO/IEC 9899:1999 standard.
16 * Translation implementation::
17 * Environment implementation::
18 * Identifiers implementation::
19 * Characters implementation::
20 * Integers implementation::
21 * Floating point implementation::
22 * Arrays and pointers implementation::
23 * Hints implementation::
24 * Structures unions enumerations and bit-fields implementation::
25 * Qualifiers implementation::
26 * Preprocessing directives implementation::
27 * Library functions implementation::
28 * Architecture implementation::
29 * Locale-specific behavior implementation::
32 @node Translation implementation
37 @cite{How a diagnostic is identified (3.10, 5.1.1.3).}
39 Diagnostics consist of all the output sent to stderr by GCC.
42 @cite{Whether each nonempty sequence of white-space characters other than
43 new-line is retained or replaced by one space character in translation
47 @node Environment implementation
50 The behavior of these points are dependent on the implementation
51 of the C library, and are not defined by GCC itself.
53 @node Identifiers implementation
58 @cite{Which additional multibyte characters may appear in identifiers
59 and their correspondence to universal character names (6.4.2).}
62 @cite{The number of significant initial characters in an identifier
65 For internal names, all characters are significant. For external names,
66 the number of significant characters are defined by the linker; for
67 almost all targets, all characters are significant.
71 @node Characters implementation
76 @cite{The number of bits in a byte (3.6).}
79 @cite{The values of the members of the execution character set (5.2.1).}
82 @cite{The unique value of the member of the execution character set produced
83 for each of the standard alphabetic escape sequences (5.2.2).}
86 @cite{The value of a @code{char} object into which has been stored any
87 character other than a member of the basic execution character set (6.2.5).}
90 @cite{Which of @code{signed char} or @code{unsigned char} has the same range,
91 representation, and behavior as ``plain'' @code{char} (6.2.5, 6.3.1.1).}
94 @cite{The mapping of members of the source character set (in character
95 constants and string literals) to members of the execution character
96 set (6.4.4.4, 5.1.1.2).}
99 @cite{The value of an integer character constant containing more than one
100 character or containing a character or escape sequence that does not map
101 to a single-byte execution character (6.4.4.4).}
104 @cite{The value of a wide character constant containing more than one
105 multibyte character, or containing a multibyte character or escape
106 sequence not represented in the extended execution character set (6.4.4.4).}
109 @cite{The current locale used to convert a wide character constant consisting
110 of a single multibyte character that maps to a member of the extended
111 execution character set into a corresponding wide character code (6.4.4.4).}
114 @cite{The current locale used to convert a wide string literal into
115 corresponding wide character codes (6.4.5).}
118 @cite{The value of a string literal containing a multibyte character or escape
119 sequence not represented in the execution character set (6.4.5).}
122 @node Integers implementation
127 @cite{Any extended integer types that exist in the implementation (6.2.5).}
130 @cite{Whether signed integer types are represented using sign and magnitude,
131 two's complement, or one's complement, and whether the extraordinary value
132 is a trap representation or an ordinary value (6.2.6.2).}
134 GCC supports only two's complement integer types, and all bit patterns
138 @cite{The rank of any extended integer type relative to another extended
139 integer type with the same precision (6.3.1.1).}
142 @cite{The result of, or the signal raised by, converting an integer to a
143 signed integer type when the value cannot be represented in an object of
144 that type (6.3.1.3).}
147 @cite{The results of some bitwise operations on signed integers (6.5).}
150 @node Floating point implementation
151 @section Floating point
155 @cite{The accuracy of the floating-point operations and of the library
156 functions in @code{<math.h>} and @code{<complex.h>} that return floating-point
157 results (5.2.4.2.2).}
160 @cite{The rounding behaviors characterized by non-standard values
161 of @code{FLT_ROUNDS} @gol
165 @cite{The evaluation methods characterized by non-standard negative
166 values of @code{FLT_EVAL_METHOD} (5.2.4.2.2).}
169 @cite{The direction of rounding when an integer is converted to a
170 floating-point number that cannot exactly represent the original
174 @cite{The direction of rounding when a floating-point number is
175 converted to a narrower floating-point number (6.3.1.5).}
178 @cite{How the nearest representable value or the larger or smaller
179 representable value immediately adjacent to the nearest representable
180 value is chosen for certain floating constants (6.4.4.2).}
183 @cite{Whether and how floating expressions are contracted when not
184 disallowed by the @code{FP_CONTRACT} pragma (6.5).}
187 @cite{The default state for the @code{FENV_ACCESS} pragma (7.6.1).}
190 @cite{Additional floating-point exceptions, rounding modes, environments,
191 and classifications, and their macro names (7.6, 7.12).}
194 @cite{The default state for the @code{FP_CONTRACT} pragma (7.12.2).}
197 @cite{Whether the ``inexact'' floating-point exception can be raised
198 when the rounded result actually does equal the mathematical result
199 in an IEC 60559 conformant implementation (F.9).}
202 @cite{Whether the ``underflow'' (and ``inexact'') floating-point
203 exception can be raised when a result is tiny but not inexact in an
204 IEC 60559 conformant implementation (F.9).}
208 @node Arrays and pointers implementation
209 @section Arrays and pointers
213 @cite{The result of converting a pointer to an integer or
214 vice versa (6.3.2.3).}
216 A cast from pointer to integer discards most-significant bits if the
217 pointer representation is larger than the integer type,
218 sign-extends@footnote{Future versions of GCC may zero-extend, or use
219 a target-defined @code{ptr_extend} pattern. Do not rely on sign extension.}
220 if the pointer representation is smaller than the integer type, otherwise
221 the bits are unchanged.
222 @c ??? We've always claimed that pointers were unsigned entities.
223 @c Shouldn't we therefore be doing zero-extension? If so, the bug
224 @c is in convert_to_integer, where we call type_for_size and request
225 @c a signed integral type. On the other hand, it might be most useful
226 @c for the target if we extend according to POINTERS_EXTEND_UNSIGNED.
228 A cast from integer to pointer discards most-significant bits if the
229 pointer representation is smaller than the integer type, extends according
230 to the signedness of the integer type if the pointer representation
231 is larger than the integer type, otherwise the bits are unchanged.
233 When casting from pointer to integer and back again, the resulting
234 pointer must reference the same object as the original pointer, otherwise
235 the behavior is undefined. That is, one may not use integer arithmetic to
236 avoid the undefined behavior of pointer arithmetic as proscribed in 6.5.6/8.
239 @cite{The size of the result of subtracting two pointers to elements
240 of the same array (6.5.6).}
244 @node Hints implementation
249 @cite{The extent to which suggestions made by using the @code{register}
250 storage-class specifier are effective (6.7.1).}
252 The @code{register} specifier affects code generation only in these ways:
256 When used as part of the register variable extension, see
257 @ref{Explicit Reg Vars}.
260 When @option{-O0} is in use, the compiler allocates distinct stack
261 memory for all variables that do not have the @code{register}
262 storage-class specifier; if @code{register} is specified, the variable
263 may have a shorter lifespan than the code would indicate and may never
267 On some rare x86 targets, @code{setjmp} doesn't save the registers in
268 all circumstances. In those cases, GCC doesn't allocate any variables
269 in registers unless they are marked @code{register}.
274 @cite{The extent to which suggestions made by using the inline function
275 specifier are effective (6.7.4).}
277 GCC will not inline any functions if the @option{-fno-inline} option is
278 used or if @option{-O0} is used. Otherwise, GCC may still be unable to
279 inline a function for many reasons; the @option{-Winline} option may be
280 used to determine if a function has not been inlined and why not.
284 @node Structures unions enumerations and bit-fields implementation
285 @section Structures, unions, enumerations, and bit-fields
289 @cite{Whether a ``plain'' int bit-field is treated as a @code{signed int}
290 bit-field or as an @code{unsigned int} bit-field (6.7.2, 6.7.2.1).}
293 @cite{Allowable bit-field types other than @code{_Bool}, @code{signed int},
294 and @code{unsigned int} (6.7.2.1).}
297 @cite{Whether a bit-field can straddle a storage-unit boundary (6.7.2.1).}
300 @cite{The order of allocation of bit-fields within a unit (6.7.2.1).}
303 @cite{The alignment of non-bit-field members of structures (6.7.2.1).}
306 @cite{The integer type compatible with each enumerated type (6.7.2.2).}
310 @node Qualifiers implementation
315 @cite{What constitutes an access to an object that has volatile-qualified
320 @node Preprocessing directives implementation
321 @section Preprocessing directives
325 @cite{How sequences in both forms of header names are mapped to headers
326 or external source file names (6.4.7).}
329 @cite{Whether the value of a character constant in a constant expression
330 that controls conditional inclusion matches the value of the same character
331 constant in the execution character set (6.10.1).}
334 @cite{Whether the value of a single-character character constant in a
335 constant expression that controls conditional inclusion may have a
336 negative value (6.10.1).}
339 @cite{The places that are searched for an included @samp{<>} delimited
340 header, and how the places are specified or the header is
341 identified (6.10.2).}
344 @cite{How the named source file is searched for in an included @samp{""}
345 delimited header (6.10.2).}
348 @cite{The method by which preprocessing tokens (possibly resulting from
349 macro expansion) in a @code{#include} directive are combined into a header
353 @cite{The nesting limit for @code{#include} processing (6.10.2).}
355 GCC imposes a limit of 200 nested @code{#include}s.
358 @cite{Whether the @samp{#} operator inserts a @samp{\} character before
359 the @samp{\} character that begins a universal character name in a
360 character constant or string literal (6.10.3.2).}
363 @cite{The behavior on each recognized non-@code{STDC #pragma}
367 @cite{The definitions for @code{__DATE__} and @code{__TIME__} when
368 respectively, the date and time of translation are not available (6.10.8).}
370 If the date and time are not available, @code{__DATE__} expands to
371 @code{@w{"??? ?? ????"}} and @code{__TIME__} expands to
376 @node Library functions implementation
377 @section Library functions
379 The behavior of these points are dependent on the implementation
380 of the C library, and are not defined by GCC itself.
382 @node Architecture implementation
383 @section Architecture
387 @cite{The values or expressions assigned to the macros specified in the
388 headers @code{<float.h>}, @code{<limits.h>}, and @code{<stdint.h>}
389 (5.2.4.2, 7.18.2, 7.18.3).}
392 @cite{The number, order, and encoding of bytes in any object
393 (when not explicitly specified in this International Standard) (6.2.6.1).}
396 @cite{The value of the result of the sizeof operator (6.5.3.4).}
400 @node Locale-specific behavior implementation
401 @section Locale-specific behavior
403 The behavior of these points are dependent on the implementation
404 of the C library, and are not defined by GCC itself.
407 @chapter Extensions to the C Language Family
408 @cindex extensions, C language
409 @cindex C language extensions
412 GNU C provides several language features not found in ISO standard C@.
413 (The @option{-pedantic} option directs GCC to print a warning message if
414 any of these features is used.) To test for the availability of these
415 features in conditional compilation, check for a predefined macro
416 @code{__GNUC__}, which is always defined under GCC@.
418 These extensions are available in C and Objective-C@. Most of them are
419 also available in C++. @xref{C++ Extensions,,Extensions to the
420 C++ Language}, for extensions that apply @emph{only} to C++.
422 Some features that are in ISO C99 but not C89 or C++ are also, as
423 extensions, accepted by GCC in C89 mode and in C++.
426 * Statement Exprs:: Putting statements and declarations inside expressions.
427 * Local Labels:: Labels local to a block.
428 * Labels as Values:: Getting pointers to labels, and computed gotos.
429 * Nested Functions:: As in Algol and Pascal, lexical scoping of functions.
430 * Constructing Calls:: Dispatching a call to another function.
431 * Typeof:: @code{typeof}: referring to the type of an expression.
432 * Lvalues:: Using @samp{?:}, @samp{,} and casts in lvalues.
433 * Conditionals:: Omitting the middle operand of a @samp{?:} expression.
434 * Long Long:: Double-word integers---@code{long long int}.
435 * Complex:: Data types for complex numbers.
436 * Hex Floats:: Hexadecimal floating-point constants.
437 * Zero Length:: Zero-length arrays.
438 * Variable Length:: Arrays whose length is computed at run time.
439 * Empty Structures:: Structures with no members.
440 * Variadic Macros:: Macros with a variable number of arguments.
441 * Escaped Newlines:: Slightly looser rules for escaped newlines.
442 * Subscripting:: Any array can be subscripted, even if not an lvalue.
443 * Pointer Arith:: Arithmetic on @code{void}-pointers and function pointers.
444 * Initializers:: Non-constant initializers.
445 * Compound Literals:: Compound literals give structures, unions
447 * Designated Inits:: Labeling elements of initializers.
448 * Cast to Union:: Casting to union type from any member of the union.
449 * Case Ranges:: `case 1 ... 9' and such.
450 * Mixed Declarations:: Mixing declarations and code.
451 * Function Attributes:: Declaring that functions have no side effects,
452 or that they can never return.
453 * Attribute Syntax:: Formal syntax for attributes.
454 * Function Prototypes:: Prototype declarations and old-style definitions.
455 * C++ Comments:: C++ comments are recognized.
456 * Dollar Signs:: Dollar sign is allowed in identifiers.
457 * Character Escapes:: @samp{\e} stands for the character @key{ESC}.
458 * Variable Attributes:: Specifying attributes of variables.
459 * Type Attributes:: Specifying attributes of types.
460 * Alignment:: Inquiring about the alignment of a type or variable.
461 * Inline:: Defining inline functions (as fast as macros).
462 * Extended Asm:: Assembler instructions with C expressions as operands.
463 (With them you can define ``built-in'' functions.)
464 * Constraints:: Constraints for asm operands
465 * Asm Labels:: Specifying the assembler name to use for a C symbol.
466 * Explicit Reg Vars:: Defining variables residing in specified registers.
467 * Alternate Keywords:: @code{__const__}, @code{__asm__}, etc., for header files.
468 * Incomplete Enums:: @code{enum foo;}, with details to follow.
469 * Function Names:: Printable strings which are the name of the current
471 * Return Address:: Getting the return or frame address of a function.
472 * Vector Extensions:: Using vector instructions through built-in functions.
473 * Other Builtins:: Other built-in functions.
474 * Target Builtins:: Built-in functions specific to particular targets.
475 * Pragmas:: Pragmas accepted by GCC.
476 * Unnamed Fields:: Unnamed struct/union fields within structs/unions.
477 * Thread-Local:: Per-thread variables.
480 @node Statement Exprs
481 @section Statements and Declarations in Expressions
482 @cindex statements inside expressions
483 @cindex declarations inside expressions
484 @cindex expressions containing statements
485 @cindex macros, statements in expressions
487 @c the above section title wrapped and causes an underfull hbox.. i
488 @c changed it from "within" to "in". --mew 4feb93
489 A compound statement enclosed in parentheses may appear as an expression
490 in GNU C@. This allows you to use loops, switches, and local variables
491 within an expression.
493 Recall that a compound statement is a sequence of statements surrounded
494 by braces; in this construct, parentheses go around the braces. For
498 (@{ int y = foo (); int z;
505 is a valid (though slightly more complex than necessary) expression
506 for the absolute value of @code{foo ()}.
508 The last thing in the compound statement should be an expression
509 followed by a semicolon; the value of this subexpression serves as the
510 value of the entire construct. (If you use some other kind of statement
511 last within the braces, the construct has type @code{void}, and thus
512 effectively no value.)
514 This feature is especially useful in making macro definitions ``safe'' (so
515 that they evaluate each operand exactly once). For example, the
516 ``maximum'' function is commonly defined as a macro in standard C as
520 #define max(a,b) ((a) > (b) ? (a) : (b))
524 @cindex side effects, macro argument
525 But this definition computes either @var{a} or @var{b} twice, with bad
526 results if the operand has side effects. In GNU C, if you know the
527 type of the operands (here let's assume @code{int}), you can define
528 the macro safely as follows:
531 #define maxint(a,b) \
532 (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @})
535 Embedded statements are not allowed in constant expressions, such as
536 the value of an enumeration constant, the width of a bit-field, or
537 the initial value of a static variable.
539 If you don't know the type of the operand, you can still do this, but you
540 must use @code{typeof} (@pxref{Typeof}).
542 In G++, the result value of a statement expression undergoes array and
543 function pointer decay, and is returned by value to the enclosing
544 expression. For instance, if @code{A} is a class, then
553 will construct a temporary @code{A} object to hold the result of the
554 statement expression, and that will be used to invoke @code{Foo}.
555 Therefore the @code{this} pointer observed by @code{Foo} will not be the
558 Any temporaries created within a statement within a statement expression
559 will be destroyed at the statement's end. This makes statement
560 expressions inside macros slightly different from function calls. In
561 the latter case temporaries introduced during argument evaluation will
562 be destroyed at the end of the statement that includes the function
563 call. In the statement expression case they will be destroyed during
564 the statement expression. For instance,
567 #define macro(a) (@{__typeof__(a) b = (a); b + 3; @})
568 template<typename T> T function(T a) @{ T b = a; return b + 3; @}
578 will have different places where temporaries are destroyed. For the
579 @code{macro} case, the temporary @code{X} will be destroyed just after
580 the initialization of @code{b}. In the @code{function} case that
581 temporary will be destroyed when the function returns.
583 These considerations mean that it is probably a bad idea to use
584 statement-expressions of this form in header files that are designed to
585 work with C++. (Note that some versions of the GNU C Library contained
586 header files using statement-expression that lead to precisely this
590 @section Locally Declared Labels
592 @cindex macros, local labels
594 GCC allows you to declare @dfn{local labels} in any nested block
595 scope. A local label is just like an ordinary label, but you can
596 only reference it (with a @code{goto} statement, or by taking its
597 address) within the block in which it was declared.
599 A local label declaration looks like this:
602 __label__ @var{label};
609 __label__ @var{label1}, @var{label2}, /* @r{@dots{}} */;
612 Local label declarations must come at the beginning of the block,
613 before any ordinary declarations or statements.
615 The label declaration defines the label @emph{name}, but does not define
616 the label itself. You must do this in the usual way, with
617 @code{@var{label}:}, within the statements of the statement expression.
619 The local label feature is useful for complex macros. If a macro
620 contains nested loops, a @code{goto} can be useful for breaking out of
621 them. However, an ordinary label whose scope is the whole function
622 cannot be used: if the macro can be expanded several times in one
623 function, the label will be multiply defined in that function. A
624 local label avoids this problem. For example:
627 #define SEARCH(value, array, target) \
630 typeof (target) _SEARCH_target = (target); \
631 typeof (*(array)) *_SEARCH_array = (array); \
634 for (i = 0; i < max; i++) \
635 for (j = 0; j < max; j++) \
636 if (_SEARCH_array[i][j] == _SEARCH_target) \
637 @{ (value) = i; goto found; @} \
643 This could also be written using a statement-expression:
646 #define SEARCH(array, target) \
649 typeof (target) _SEARCH_target = (target); \
650 typeof (*(array)) *_SEARCH_array = (array); \
653 for (i = 0; i < max; i++) \
654 for (j = 0; j < max; j++) \
655 if (_SEARCH_array[i][j] == _SEARCH_target) \
656 @{ value = i; goto found; @} \
663 Local label declarations also make the labels they declare visible to
664 nested functions, if there are any. @xref{Nested Functions}, for details.
666 @node Labels as Values
667 @section Labels as Values
668 @cindex labels as values
669 @cindex computed gotos
670 @cindex goto with computed label
671 @cindex address of a label
673 You can get the address of a label defined in the current function
674 (or a containing function) with the unary operator @samp{&&}. The
675 value has type @code{void *}. This value is a constant and can be used
676 wherever a constant of that type is valid. For example:
684 To use these values, you need to be able to jump to one. This is done
685 with the computed goto statement@footnote{The analogous feature in
686 Fortran is called an assigned goto, but that name seems inappropriate in
687 C, where one can do more than simply store label addresses in label
688 variables.}, @code{goto *@var{exp};}. For example,
695 Any expression of type @code{void *} is allowed.
697 One way of using these constants is in initializing a static array that
698 will serve as a jump table:
701 static void *array[] = @{ &&foo, &&bar, &&hack @};
704 Then you can select a label with indexing, like this:
711 Note that this does not check whether the subscript is in bounds---array
712 indexing in C never does that.
714 Such an array of label values serves a purpose much like that of the
715 @code{switch} statement. The @code{switch} statement is cleaner, so
716 use that rather than an array unless the problem does not fit a
717 @code{switch} statement very well.
719 Another use of label values is in an interpreter for threaded code.
720 The labels within the interpreter function can be stored in the
721 threaded code for super-fast dispatching.
723 You may not use this mechanism to jump to code in a different function.
724 If you do that, totally unpredictable things will happen. The best way to
725 avoid this is to store the label address only in automatic variables and
726 never pass it as an argument.
728 An alternate way to write the above example is
731 static const int array[] = @{ &&foo - &&foo, &&bar - &&foo,
733 goto *(&&foo + array[i]);
737 This is more friendly to code living in shared libraries, as it reduces
738 the number of dynamic relocations that are needed, and by consequence,
739 allows the data to be read-only.
741 @node Nested Functions
742 @section Nested Functions
743 @cindex nested functions
744 @cindex downward funargs
747 A @dfn{nested function} is a function defined inside another function.
748 (Nested functions are not supported for GNU C++.) The nested function's
749 name is local to the block where it is defined. For example, here we
750 define a nested function named @code{square}, and call it twice:
754 foo (double a, double b)
756 double square (double z) @{ return z * z; @}
758 return square (a) + square (b);
763 The nested function can access all the variables of the containing
764 function that are visible at the point of its definition. This is
765 called @dfn{lexical scoping}. For example, here we show a nested
766 function which uses an inherited variable named @code{offset}:
770 bar (int *array, int offset, int size)
772 int access (int *array, int index)
773 @{ return array[index + offset]; @}
776 for (i = 0; i < size; i++)
777 /* @r{@dots{}} */ access (array, i) /* @r{@dots{}} */
782 Nested function definitions are permitted within functions in the places
783 where variable definitions are allowed; that is, in any block, before
784 the first statement in the block.
786 It is possible to call the nested function from outside the scope of its
787 name by storing its address or passing the address to another function:
790 hack (int *array, int size)
792 void store (int index, int value)
793 @{ array[index] = value; @}
795 intermediate (store, size);
799 Here, the function @code{intermediate} receives the address of
800 @code{store} as an argument. If @code{intermediate} calls @code{store},
801 the arguments given to @code{store} are used to store into @code{array}.
802 But this technique works only so long as the containing function
803 (@code{hack}, in this example) does not exit.
805 If you try to call the nested function through its address after the
806 containing function has exited, all hell will break loose. If you try
807 to call it after a containing scope level has exited, and if it refers
808 to some of the variables that are no longer in scope, you may be lucky,
809 but it's not wise to take the risk. If, however, the nested function
810 does not refer to anything that has gone out of scope, you should be
813 GCC implements taking the address of a nested function using a technique
814 called @dfn{trampolines}. A paper describing them is available as
817 @uref{http://people.debian.org/~aaronl/Usenix88-lexic.pdf}.
819 A nested function can jump to a label inherited from a containing
820 function, provided the label was explicitly declared in the containing
821 function (@pxref{Local Labels}). Such a jump returns instantly to the
822 containing function, exiting the nested function which did the
823 @code{goto} and any intermediate functions as well. Here is an example:
827 bar (int *array, int offset, int size)
830 int access (int *array, int index)
834 return array[index + offset];
838 for (i = 0; i < size; i++)
839 /* @r{@dots{}} */ access (array, i) /* @r{@dots{}} */
843 /* @r{Control comes here from @code{access}
844 if it detects an error.} */
851 A nested function always has internal linkage. Declaring one with
852 @code{extern} is erroneous. If you need to declare the nested function
853 before its definition, use @code{auto} (which is otherwise meaningless
854 for function declarations).
857 bar (int *array, int offset, int size)
860 auto int access (int *, int);
862 int access (int *array, int index)
866 return array[index + offset];
872 @node Constructing Calls
873 @section Constructing Function Calls
874 @cindex constructing calls
875 @cindex forwarding calls
877 Using the built-in functions described below, you can record
878 the arguments a function received, and call another function
879 with the same arguments, without knowing the number or types
882 You can also record the return value of that function call,
883 and later return that value, without knowing what data type
884 the function tried to return (as long as your caller expects
887 @deftypefn {Built-in Function} {void *} __builtin_apply_args ()
888 This built-in function returns a pointer to data
889 describing how to perform a call with the same arguments as were passed
890 to the current function.
892 The function saves the arg pointer register, structure value address,
893 and all registers that might be used to pass arguments to a function
894 into a block of memory allocated on the stack. Then it returns the
895 address of that block.
898 @deftypefn {Built-in Function} {void *} __builtin_apply (void (*@var{function})(), void *@var{arguments}, size_t @var{size})
899 This built-in function invokes @var{function}
900 with a copy of the parameters described by @var{arguments}
903 The value of @var{arguments} should be the value returned by
904 @code{__builtin_apply_args}. The argument @var{size} specifies the size
905 of the stack argument data, in bytes.
907 This function returns a pointer to data describing
908 how to return whatever value was returned by @var{function}. The data
909 is saved in a block of memory allocated on the stack.
911 It is not always simple to compute the proper value for @var{size}. The
912 value is used by @code{__builtin_apply} to compute the amount of data
913 that should be pushed on the stack and copied from the incoming argument
917 @deftypefn {Built-in Function} {void} __builtin_return (void *@var{result})
918 This built-in function returns the value described by @var{result} from
919 the containing function. You should specify, for @var{result}, a value
920 returned by @code{__builtin_apply}.
924 @section Referring to a Type with @code{typeof}
927 @cindex macros, types of arguments
929 Another way to refer to the type of an expression is with @code{typeof}.
930 The syntax of using of this keyword looks like @code{sizeof}, but the
931 construct acts semantically like a type name defined with @code{typedef}.
933 There are two ways of writing the argument to @code{typeof}: with an
934 expression or with a type. Here is an example with an expression:
941 This assumes that @code{x} is an array of pointers to functions;
942 the type described is that of the values of the functions.
944 Here is an example with a typename as the argument:
951 Here the type described is that of pointers to @code{int}.
953 If you are writing a header file that must work when included in ISO C
954 programs, write @code{__typeof__} instead of @code{typeof}.
955 @xref{Alternate Keywords}.
957 A @code{typeof}-construct can be used anywhere a typedef name could be
958 used. For example, you can use it in a declaration, in a cast, or inside
959 of @code{sizeof} or @code{typeof}.
961 @code{typeof} is often useful in conjunction with the
962 statements-within-expressions feature. Here is how the two together can
963 be used to define a safe ``maximum'' macro that operates on any
964 arithmetic type and evaluates each of its arguments exactly once:
968 (@{ typeof (a) _a = (a); \
969 typeof (b) _b = (b); \
970 _a > _b ? _a : _b; @})
973 @cindex underscores in variables in macros
974 @cindex @samp{_} in variables in macros
975 @cindex local variables in macros
976 @cindex variables, local, in macros
977 @cindex macros, local variables in
979 The reason for using names that start with underscores for the local
980 variables is to avoid conflicts with variable names that occur within the
981 expressions that are substituted for @code{a} and @code{b}. Eventually we
982 hope to design a new form of declaration syntax that allows you to declare
983 variables whose scopes start only after their initializers; this will be a
984 more reliable way to prevent such conflicts.
987 Some more examples of the use of @code{typeof}:
991 This declares @code{y} with the type of what @code{x} points to.
998 This declares @code{y} as an array of such values.
1005 This declares @code{y} as an array of pointers to characters:
1008 typeof (typeof (char *)[4]) y;
1012 It is equivalent to the following traditional C declaration:
1018 To see the meaning of the declaration using @code{typeof}, and why it
1019 might be a useful way to write, let's rewrite it with these macros:
1022 #define pointer(T) typeof(T *)
1023 #define array(T, N) typeof(T [N])
1027 Now the declaration can be rewritten this way:
1030 array (pointer (char), 4) y;
1034 Thus, @code{array (pointer (char), 4)} is the type of arrays of 4
1035 pointers to @code{char}.
1038 @emph{Compatibility Note:} In addition to @code{typeof}, GCC 2 supported
1039 a more limited extension which permitted one to write
1042 typedef @var{T} = @var{expr};
1046 with the effect of declaring @var{T} to have the type of the expression
1047 @var{expr}. This extension does not work with GCC 3 (versions between
1048 3.0 and 3.2 will crash; 3.2.1 and later give an error). Code which
1049 relies on it should be rewritten to use @code{typeof}:
1052 typedef typeof(@var{expr}) @var{T};
1056 This will work with all versions of GCC@.
1059 @section Generalized Lvalues
1060 @cindex compound expressions as lvalues
1061 @cindex expressions, compound, as lvalues
1062 @cindex conditional expressions as lvalues
1063 @cindex expressions, conditional, as lvalues
1064 @cindex casts as lvalues
1065 @cindex generalized lvalues
1066 @cindex lvalues, generalized
1067 @cindex extensions, @code{?:}
1068 @cindex @code{?:} extensions
1070 Compound expressions, conditional expressions and casts are allowed as
1071 lvalues provided their operands are lvalues. This means that you can take
1072 their addresses or store values into them.
1074 Standard C++ allows compound expressions and conditional expressions
1075 as lvalues, and permits casts to reference type, so use of this
1076 extension is not supported for C++ code.
1078 For example, a compound expression can be assigned, provided the last
1079 expression in the sequence is an lvalue. These two expressions are
1087 Similarly, the address of the compound expression can be taken. These two
1088 expressions are equivalent:
1095 A conditional expression is a valid lvalue if its type is not void and the
1096 true and false branches are both valid lvalues. For example, these two
1097 expressions are equivalent:
1101 (a ? b = 5 : (c = 5))
1104 A cast is a valid lvalue if its operand is an lvalue. A simple
1105 assignment whose left-hand side is a cast works by converting the
1106 right-hand side first to the specified type, then to the type of the
1107 inner left-hand side expression. After this is stored, the value is
1108 converted back to the specified type to become the value of the
1109 assignment. Thus, if @code{a} has type @code{char *}, the following two
1110 expressions are equivalent:
1114 (int)(a = (char *)(int)5)
1117 An assignment-with-arithmetic operation such as @samp{+=} applied to a cast
1118 performs the arithmetic using the type resulting from the cast, and then
1119 continues as in the previous case. Therefore, these two expressions are
1124 (int)(a = (char *)(int) ((int)a + 5))
1127 You cannot take the address of an lvalue cast, because the use of its
1128 address would not work out coherently. Suppose that @code{&(int)f} were
1129 permitted, where @code{f} has type @code{float}. Then the following
1130 statement would try to store an integer bit-pattern where a floating
1131 point number belongs:
1137 This is quite different from what @code{(int)f = 1} would do---that
1138 would convert 1 to floating point and store it. Rather than cause this
1139 inconsistency, we think it is better to prohibit use of @samp{&} on a cast.
1141 If you really do want an @code{int *} pointer with the address of
1142 @code{f}, you can simply write @code{(int *)&f}.
1145 @section Conditionals with Omitted Operands
1146 @cindex conditional expressions, extensions
1147 @cindex omitted middle-operands
1148 @cindex middle-operands, omitted
1149 @cindex extensions, @code{?:}
1150 @cindex @code{?:} extensions
1152 The middle operand in a conditional expression may be omitted. Then
1153 if the first operand is nonzero, its value is the value of the conditional
1156 Therefore, the expression
1163 has the value of @code{x} if that is nonzero; otherwise, the value of
1166 This example is perfectly equivalent to
1172 @cindex side effect in ?:
1173 @cindex ?: side effect
1175 In this simple case, the ability to omit the middle operand is not
1176 especially useful. When it becomes useful is when the first operand does,
1177 or may (if it is a macro argument), contain a side effect. Then repeating
1178 the operand in the middle would perform the side effect twice. Omitting
1179 the middle operand uses the value already computed without the undesirable
1180 effects of recomputing it.
1183 @section Double-Word Integers
1184 @cindex @code{long long} data types
1185 @cindex double-word arithmetic
1186 @cindex multiprecision arithmetic
1187 @cindex @code{LL} integer suffix
1188 @cindex @code{ULL} integer suffix
1190 ISO C99 supports data types for integers that are at least 64 bits wide,
1191 and as an extension GCC supports them in C89 mode and in C++.
1192 Simply write @code{long long int} for a signed integer, or
1193 @code{unsigned long long int} for an unsigned integer. To make an
1194 integer constant of type @code{long long int}, add the suffix @samp{LL}
1195 to the integer. To make an integer constant of type @code{unsigned long
1196 long int}, add the suffix @samp{ULL} to the integer.
1198 You can use these types in arithmetic like any other integer types.
1199 Addition, subtraction, and bitwise boolean operations on these types
1200 are open-coded on all types of machines. Multiplication is open-coded
1201 if the machine supports fullword-to-doubleword a widening multiply
1202 instruction. Division and shifts are open-coded only on machines that
1203 provide special support. The operations that are not open-coded use
1204 special library routines that come with GCC@.
1206 There may be pitfalls when you use @code{long long} types for function
1207 arguments, unless you declare function prototypes. If a function
1208 expects type @code{int} for its argument, and you pass a value of type
1209 @code{long long int}, confusion will result because the caller and the
1210 subroutine will disagree about the number of bytes for the argument.
1211 Likewise, if the function expects @code{long long int} and you pass
1212 @code{int}. The best way to avoid such problems is to use prototypes.
1215 @section Complex Numbers
1216 @cindex complex numbers
1217 @cindex @code{_Complex} keyword
1218 @cindex @code{__complex__} keyword
1220 ISO C99 supports complex floating data types, and as an extension GCC
1221 supports them in C89 mode and in C++, and supports complex integer data
1222 types which are not part of ISO C99. You can declare complex types
1223 using the keyword @code{_Complex}. As an extension, the older GNU
1224 keyword @code{__complex__} is also supported.
1226 For example, @samp{_Complex double x;} declares @code{x} as a
1227 variable whose real part and imaginary part are both of type
1228 @code{double}. @samp{_Complex short int y;} declares @code{y} to
1229 have real and imaginary parts of type @code{short int}; this is not
1230 likely to be useful, but it shows that the set of complex types is
1233 To write a constant with a complex data type, use the suffix @samp{i} or
1234 @samp{j} (either one; they are equivalent). For example, @code{2.5fi}
1235 has type @code{_Complex float} and @code{3i} has type
1236 @code{_Complex int}. Such a constant always has a pure imaginary
1237 value, but you can form any complex value you like by adding one to a
1238 real constant. This is a GNU extension; if you have an ISO C99
1239 conforming C library (such as GNU libc), and want to construct complex
1240 constants of floating type, you should include @code{<complex.h>} and
1241 use the macros @code{I} or @code{_Complex_I} instead.
1243 @cindex @code{__real__} keyword
1244 @cindex @code{__imag__} keyword
1245 To extract the real part of a complex-valued expression @var{exp}, write
1246 @code{__real__ @var{exp}}. Likewise, use @code{__imag__} to
1247 extract the imaginary part. This is a GNU extension; for values of
1248 floating type, you should use the ISO C99 functions @code{crealf},
1249 @code{creal}, @code{creall}, @code{cimagf}, @code{cimag} and
1250 @code{cimagl}, declared in @code{<complex.h>} and also provided as
1251 built-in functions by GCC@.
1253 @cindex complex conjugation
1254 The operator @samp{~} performs complex conjugation when used on a value
1255 with a complex type. This is a GNU extension; for values of
1256 floating type, you should use the ISO C99 functions @code{conjf},
1257 @code{conj} and @code{conjl}, declared in @code{<complex.h>} and also
1258 provided as built-in functions by GCC@.
1260 GCC can allocate complex automatic variables in a noncontiguous
1261 fashion; it's even possible for the real part to be in a register while
1262 the imaginary part is on the stack (or vice-versa). Only the DWARF2
1263 debug info format can represent this, so use of DWARF2 is recommended.
1264 If you are using the stabs debug info format, GCC describes a noncontiguous
1265 complex variable as if it were two separate variables of noncomplex type.
1266 If the variable's actual name is @code{foo}, the two fictitious
1267 variables are named @code{foo$real} and @code{foo$imag}. You can
1268 examine and set these two fictitious variables with your debugger.
1274 ISO C99 supports floating-point numbers written not only in the usual
1275 decimal notation, such as @code{1.55e1}, but also numbers such as
1276 @code{0x1.fp3} written in hexadecimal format. As a GNU extension, GCC
1277 supports this in C89 mode (except in some cases when strictly
1278 conforming) and in C++. In that format the
1279 @samp{0x} hex introducer and the @samp{p} or @samp{P} exponent field are
1280 mandatory. The exponent is a decimal number that indicates the power of
1281 2 by which the significant part will be multiplied. Thus @samp{0x1.f} is
1288 @samp{p3} multiplies it by 8, and the value of @code{0x1.fp3}
1289 is the same as @code{1.55e1}.
1291 Unlike for floating-point numbers in the decimal notation the exponent
1292 is always required in the hexadecimal notation. Otherwise the compiler
1293 would not be able to resolve the ambiguity of, e.g., @code{0x1.f}. This
1294 could mean @code{1.0f} or @code{1.9375} since @samp{f} is also the
1295 extension for floating-point constants of type @code{float}.
1298 @section Arrays of Length Zero
1299 @cindex arrays of length zero
1300 @cindex zero-length arrays
1301 @cindex length-zero arrays
1302 @cindex flexible array members
1304 Zero-length arrays are allowed in GNU C@. They are very useful as the
1305 last element of a structure which is really a header for a variable-length
1314 struct line *thisline = (struct line *)
1315 malloc (sizeof (struct line) + this_length);
1316 thisline->length = this_length;
1319 In ISO C90, you would have to give @code{contents} a length of 1, which
1320 means either you waste space or complicate the argument to @code{malloc}.
1322 In ISO C99, you would use a @dfn{flexible array member}, which is
1323 slightly different in syntax and semantics:
1327 Flexible array members are written as @code{contents[]} without
1331 Flexible array members have incomplete type, and so the @code{sizeof}
1332 operator may not be applied. As a quirk of the original implementation
1333 of zero-length arrays, @code{sizeof} evaluates to zero.
1336 Flexible array members may only appear as the last member of a
1337 @code{struct} that is otherwise non-empty.
1340 A structure containing a flexible array member, or a union containing
1341 such a structure (possibly recursively), may not be a member of a
1342 structure or an element of an array. (However, these uses are
1343 permitted by GCC as extensions.)
1346 GCC versions before 3.0 allowed zero-length arrays to be statically
1347 initialized, as if they were flexible arrays. In addition to those
1348 cases that were useful, it also allowed initializations in situations
1349 that would corrupt later data. Non-empty initialization of zero-length
1350 arrays is now treated like any case where there are more initializer
1351 elements than the array holds, in that a suitable warning about "excess
1352 elements in array" is given, and the excess elements (all of them, in
1353 this case) are ignored.
1355 Instead GCC allows static initialization of flexible array members.
1356 This is equivalent to defining a new structure containing the original
1357 structure followed by an array of sufficient size to contain the data.
1358 I.e.@: in the following, @code{f1} is constructed as if it were declared
1364 @} f1 = @{ 1, @{ 2, 3, 4 @} @};
1367 struct f1 f1; int data[3];
1368 @} f2 = @{ @{ 1 @}, @{ 2, 3, 4 @} @};
1372 The convenience of this extension is that @code{f1} has the desired
1373 type, eliminating the need to consistently refer to @code{f2.f1}.
1375 This has symmetry with normal static arrays, in that an array of
1376 unknown size is also written with @code{[]}.
1378 Of course, this extension only makes sense if the extra data comes at
1379 the end of a top-level object, as otherwise we would be overwriting
1380 data at subsequent offsets. To avoid undue complication and confusion
1381 with initialization of deeply nested arrays, we simply disallow any
1382 non-empty initialization except when the structure is the top-level
1383 object. For example:
1386 struct foo @{ int x; int y[]; @};
1387 struct bar @{ struct foo z; @};
1389 struct foo a = @{ 1, @{ 2, 3, 4 @} @}; // @r{Valid.}
1390 struct bar b = @{ @{ 1, @{ 2, 3, 4 @} @} @}; // @r{Invalid.}
1391 struct bar c = @{ @{ 1, @{ @} @} @}; // @r{Valid.}
1392 struct foo d[1] = @{ @{ 1 @{ 2, 3, 4 @} @} @}; // @r{Invalid.}
1395 @node Empty Structures
1396 @section Structures With No Members
1397 @cindex empty structures
1398 @cindex zero-size structures
1400 GCC permits a C structure to have no members:
1407 The structure will have size zero. In C++, empty structures are part
1408 of the language. G++ treats empty structures as if they had a single
1409 member of type @code{char}.
1411 @node Variable Length
1412 @section Arrays of Variable Length
1413 @cindex variable-length arrays
1414 @cindex arrays of variable length
1417 Variable-length automatic arrays are allowed in ISO C99, and as an
1418 extension GCC accepts them in C89 mode and in C++. (However, GCC's
1419 implementation of variable-length arrays does not yet conform in detail
1420 to the ISO C99 standard.) These arrays are
1421 declared like any other automatic arrays, but with a length that is not
1422 a constant expression. The storage is allocated at the point of
1423 declaration and deallocated when the brace-level is exited. For
1428 concat_fopen (char *s1, char *s2, char *mode)
1430 char str[strlen (s1) + strlen (s2) + 1];
1433 return fopen (str, mode);
1437 @cindex scope of a variable length array
1438 @cindex variable-length array scope
1439 @cindex deallocating variable length arrays
1440 Jumping or breaking out of the scope of the array name deallocates the
1441 storage. Jumping into the scope is not allowed; you get an error
1444 @cindex @code{alloca} vs variable-length arrays
1445 You can use the function @code{alloca} to get an effect much like
1446 variable-length arrays. The function @code{alloca} is available in
1447 many other C implementations (but not in all). On the other hand,
1448 variable-length arrays are more elegant.
1450 There are other differences between these two methods. Space allocated
1451 with @code{alloca} exists until the containing @emph{function} returns.
1452 The space for a variable-length array is deallocated as soon as the array
1453 name's scope ends. (If you use both variable-length arrays and
1454 @code{alloca} in the same function, deallocation of a variable-length array
1455 will also deallocate anything more recently allocated with @code{alloca}.)
1457 You can also use variable-length arrays as arguments to functions:
1461 tester (int len, char data[len][len])
1467 The length of an array is computed once when the storage is allocated
1468 and is remembered for the scope of the array in case you access it with
1471 If you want to pass the array first and the length afterward, you can
1472 use a forward declaration in the parameter list---another GNU extension.
1476 tester (int len; char data[len][len], int len)
1482 @cindex parameter forward declaration
1483 The @samp{int len} before the semicolon is a @dfn{parameter forward
1484 declaration}, and it serves the purpose of making the name @code{len}
1485 known when the declaration of @code{data} is parsed.
1487 You can write any number of such parameter forward declarations in the
1488 parameter list. They can be separated by commas or semicolons, but the
1489 last one must end with a semicolon, which is followed by the ``real''
1490 parameter declarations. Each forward declaration must match a ``real''
1491 declaration in parameter name and data type. ISO C99 does not support
1492 parameter forward declarations.
1494 @node Variadic Macros
1495 @section Macros with a Variable Number of Arguments.
1496 @cindex variable number of arguments
1497 @cindex macro with variable arguments
1498 @cindex rest argument (in macro)
1499 @cindex variadic macros
1501 In the ISO C standard of 1999, a macro can be declared to accept a
1502 variable number of arguments much as a function can. The syntax for
1503 defining the macro is similar to that of a function. Here is an
1507 #define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
1510 Here @samp{@dots{}} is a @dfn{variable argument}. In the invocation of
1511 such a macro, it represents the zero or more tokens until the closing
1512 parenthesis that ends the invocation, including any commas. This set of
1513 tokens replaces the identifier @code{__VA_ARGS__} in the macro body
1514 wherever it appears. See the CPP manual for more information.
1516 GCC has long supported variadic macros, and used a different syntax that
1517 allowed you to give a name to the variable arguments just like any other
1518 argument. Here is an example:
1521 #define debug(format, args...) fprintf (stderr, format, args)
1524 This is in all ways equivalent to the ISO C example above, but arguably
1525 more readable and descriptive.
1527 GNU CPP has two further variadic macro extensions, and permits them to
1528 be used with either of the above forms of macro definition.
1530 In standard C, you are not allowed to leave the variable argument out
1531 entirely; but you are allowed to pass an empty argument. For example,
1532 this invocation is invalid in ISO C, because there is no comma after
1539 GNU CPP permits you to completely omit the variable arguments in this
1540 way. In the above examples, the compiler would complain, though since
1541 the expansion of the macro still has the extra comma after the format
1544 To help solve this problem, CPP behaves specially for variable arguments
1545 used with the token paste operator, @samp{##}. If instead you write
1548 #define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
1551 and if the variable arguments are omitted or empty, the @samp{##}
1552 operator causes the preprocessor to remove the comma before it. If you
1553 do provide some variable arguments in your macro invocation, GNU CPP
1554 does not complain about the paste operation and instead places the
1555 variable arguments after the comma. Just like any other pasted macro
1556 argument, these arguments are not macro expanded.
1558 @node Escaped Newlines
1559 @section Slightly Looser Rules for Escaped Newlines
1560 @cindex escaped newlines
1561 @cindex newlines (escaped)
1563 Recently, the preprocessor has relaxed its treatment of escaped
1564 newlines. Previously, the newline had to immediately follow a
1565 backslash. The current implementation allows whitespace in the form
1566 of spaces, horizontal and vertical tabs, and form feeds between the
1567 backslash and the subsequent newline. The preprocessor issues a
1568 warning, but treats it as a valid escaped newline and combines the two
1569 lines to form a single logical line. This works within comments and
1570 tokens, as well as between tokens. Comments are @emph{not} treated as
1571 whitespace for the purposes of this relaxation, since they have not
1572 yet been replaced with spaces.
1575 @section Non-Lvalue Arrays May Have Subscripts
1576 @cindex subscripting
1577 @cindex arrays, non-lvalue
1579 @cindex subscripting and function values
1580 In ISO C99, arrays that are not lvalues still decay to pointers, and
1581 may be subscripted, although they may not be modified or used after
1582 the next sequence point and the unary @samp{&} operator may not be
1583 applied to them. As an extension, GCC allows such arrays to be
1584 subscripted in C89 mode, though otherwise they do not decay to
1585 pointers outside C99 mode. For example,
1586 this is valid in GNU C though not valid in C89:
1590 struct foo @{int a[4];@};
1596 return f().a[index];
1602 @section Arithmetic on @code{void}- and Function-Pointers
1603 @cindex void pointers, arithmetic
1604 @cindex void, size of pointer to
1605 @cindex function pointers, arithmetic
1606 @cindex function, size of pointer to
1608 In GNU C, addition and subtraction operations are supported on pointers to
1609 @code{void} and on pointers to functions. This is done by treating the
1610 size of a @code{void} or of a function as 1.
1612 A consequence of this is that @code{sizeof} is also allowed on @code{void}
1613 and on function types, and returns 1.
1615 @opindex Wpointer-arith
1616 The option @option{-Wpointer-arith} requests a warning if these extensions
1620 @section Non-Constant Initializers
1621 @cindex initializers, non-constant
1622 @cindex non-constant initializers
1624 As in standard C++ and ISO C99, the elements of an aggregate initializer for an
1625 automatic variable are not required to be constant expressions in GNU C@.
1626 Here is an example of an initializer with run-time varying elements:
1629 foo (float f, float g)
1631 float beat_freqs[2] = @{ f-g, f+g @};
1636 @node Compound Literals
1637 @section Compound Literals
1638 @cindex constructor expressions
1639 @cindex initializations in expressions
1640 @cindex structures, constructor expression
1641 @cindex expressions, constructor
1642 @cindex compound literals
1643 @c The GNU C name for what C99 calls compound literals was "constructor expressions".
1645 ISO C99 supports compound literals. A compound literal looks like
1646 a cast containing an initializer. Its value is an object of the
1647 type specified in the cast, containing the elements specified in
1648 the initializer; it is an lvalue. As an extension, GCC supports
1649 compound literals in C89 mode and in C++.
1651 Usually, the specified type is a structure. Assume that
1652 @code{struct foo} and @code{structure} are declared as shown:
1655 struct foo @{int a; char b[2];@} structure;
1659 Here is an example of constructing a @code{struct foo} with a compound literal:
1662 structure = ((struct foo) @{x + y, 'a', 0@});
1666 This is equivalent to writing the following:
1670 struct foo temp = @{x + y, 'a', 0@};
1675 You can also construct an array. If all the elements of the compound literal
1676 are (made up of) simple constant expressions, suitable for use in
1677 initializers of objects of static storage duration, then the compound
1678 literal can be coerced to a pointer to its first element and used in
1679 such an initializer, as shown here:
1682 char **foo = (char *[]) @{ "x", "y", "z" @};
1685 Compound literals for scalar types and union types are is
1686 also allowed, but then the compound literal is equivalent
1689 As a GNU extension, GCC allows initialization of objects with static storage
1690 duration by compound literals (which is not possible in ISO C99, because
1691 the initializer is not a constant).
1692 It is handled as if the object was initialized only with the bracket
1693 enclosed list if compound literal's and object types match.
1694 The initializer list of the compound literal must be constant.
1695 If the object being initialized has array type of unknown size, the size is
1696 determined by compound literal size.
1699 static struct foo x = (struct foo) @{1, 'a', 'b'@};
1700 static int y[] = (int []) @{1, 2, 3@};
1701 static int z[] = (int [3]) @{1@};
1705 The above lines are equivalent to the following:
1707 static struct foo x = @{1, 'a', 'b'@};
1708 static int y[] = @{1, 2, 3@};
1709 static int z[] = @{1, 0, 0@};
1712 @node Designated Inits
1713 @section Designated Initializers
1714 @cindex initializers with labeled elements
1715 @cindex labeled elements in initializers
1716 @cindex case labels in initializers
1717 @cindex designated initializers
1719 Standard C89 requires the elements of an initializer to appear in a fixed
1720 order, the same as the order of the elements in the array or structure
1723 In ISO C99 you can give the elements in any order, specifying the array
1724 indices or structure field names they apply to, and GNU C allows this as
1725 an extension in C89 mode as well. This extension is not
1726 implemented in GNU C++.
1728 To specify an array index, write
1729 @samp{[@var{index}] =} before the element value. For example,
1732 int a[6] = @{ [4] = 29, [2] = 15 @};
1739 int a[6] = @{ 0, 0, 15, 0, 29, 0 @};
1743 The index values must be constant expressions, even if the array being
1744 initialized is automatic.
1746 An alternative syntax for this which has been obsolete since GCC 2.5 but
1747 GCC still accepts is to write @samp{[@var{index}]} before the element
1748 value, with no @samp{=}.
1750 To initialize a range of elements to the same value, write
1751 @samp{[@var{first} ... @var{last}] = @var{value}}. This is a GNU
1752 extension. For example,
1755 int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @};
1759 If the value in it has side-effects, the side-effects will happen only once,
1760 not for each initialized field by the range initializer.
1763 Note that the length of the array is the highest value specified
1766 In a structure initializer, specify the name of a field to initialize
1767 with @samp{.@var{fieldname} =} before the element value. For example,
1768 given the following structure,
1771 struct point @{ int x, y; @};
1775 the following initialization
1778 struct point p = @{ .y = yvalue, .x = xvalue @};
1785 struct point p = @{ xvalue, yvalue @};
1788 Another syntax which has the same meaning, obsolete since GCC 2.5, is
1789 @samp{@var{fieldname}:}, as shown here:
1792 struct point p = @{ y: yvalue, x: xvalue @};
1796 The @samp{[@var{index}]} or @samp{.@var{fieldname}} is known as a
1797 @dfn{designator}. You can also use a designator (or the obsolete colon
1798 syntax) when initializing a union, to specify which element of the union
1799 should be used. For example,
1802 union foo @{ int i; double d; @};
1804 union foo f = @{ .d = 4 @};
1808 will convert 4 to a @code{double} to store it in the union using
1809 the second element. By contrast, casting 4 to type @code{union foo}
1810 would store it into the union as the integer @code{i}, since it is
1811 an integer. (@xref{Cast to Union}.)
1813 You can combine this technique of naming elements with ordinary C
1814 initialization of successive elements. Each initializer element that
1815 does not have a designator applies to the next consecutive element of the
1816 array or structure. For example,
1819 int a[6] = @{ [1] = v1, v2, [4] = v4 @};
1826 int a[6] = @{ 0, v1, v2, 0, v4, 0 @};
1829 Labeling the elements of an array initializer is especially useful
1830 when the indices are characters or belong to an @code{enum} type.
1835 = @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1,
1836 ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @};
1839 @cindex designator lists
1840 You can also write a series of @samp{.@var{fieldname}} and
1841 @samp{[@var{index}]} designators before an @samp{=} to specify a
1842 nested subobject to initialize; the list is taken relative to the
1843 subobject corresponding to the closest surrounding brace pair. For
1844 example, with the @samp{struct point} declaration above:
1847 struct point ptarray[10] = @{ [2].y = yv2, [2].x = xv2, [0].x = xv0 @};
1851 If the same field is initialized multiple times, it will have value from
1852 the last initialization. If any such overridden initialization has
1853 side-effect, it is unspecified whether the side-effect happens or not.
1854 Currently, gcc will discard them and issue a warning.
1857 @section Case Ranges
1859 @cindex ranges in case statements
1861 You can specify a range of consecutive values in a single @code{case} label,
1865 case @var{low} ... @var{high}:
1869 This has the same effect as the proper number of individual @code{case}
1870 labels, one for each integer value from @var{low} to @var{high}, inclusive.
1872 This feature is especially useful for ranges of ASCII character codes:
1878 @strong{Be careful:} Write spaces around the @code{...}, for otherwise
1879 it may be parsed wrong when you use it with integer values. For example,
1894 @section Cast to a Union Type
1895 @cindex cast to a union
1896 @cindex union, casting to a
1898 A cast to union type is similar to other casts, except that the type
1899 specified is a union type. You can specify the type either with
1900 @code{union @var{tag}} or with a typedef name. A cast to union is actually
1901 a constructor though, not a cast, and hence does not yield an lvalue like
1902 normal casts. (@xref{Compound Literals}.)
1904 The types that may be cast to the union type are those of the members
1905 of the union. Thus, given the following union and variables:
1908 union foo @{ int i; double d; @};
1914 both @code{x} and @code{y} can be cast to type @code{union foo}.
1916 Using the cast as the right-hand side of an assignment to a variable of
1917 union type is equivalent to storing in a member of the union:
1922 u = (union foo) x @equiv{} u.i = x
1923 u = (union foo) y @equiv{} u.d = y
1926 You can also use the union cast as a function argument:
1929 void hack (union foo);
1931 hack ((union foo) x);
1934 @node Mixed Declarations
1935 @section Mixed Declarations and Code
1936 @cindex mixed declarations and code
1937 @cindex declarations, mixed with code
1938 @cindex code, mixed with declarations
1940 ISO C99 and ISO C++ allow declarations and code to be freely mixed
1941 within compound statements. As an extension, GCC also allows this in
1942 C89 mode. For example, you could do:
1951 Each identifier is visible from where it is declared until the end of
1952 the enclosing block.
1954 @node Function Attributes
1955 @section Declaring Attributes of Functions
1956 @cindex function attributes
1957 @cindex declaring attributes of functions
1958 @cindex functions that never return
1959 @cindex functions that have no side effects
1960 @cindex functions in arbitrary sections
1961 @cindex functions that behave like malloc
1962 @cindex @code{volatile} applied to function
1963 @cindex @code{const} applied to function
1964 @cindex functions with @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style arguments
1965 @cindex functions with non-null pointer arguments
1966 @cindex functions that are passed arguments in registers on the 386
1967 @cindex functions that pop the argument stack on the 386
1968 @cindex functions that do not pop the argument stack on the 386
1970 In GNU C, you declare certain things about functions called in your program
1971 which help the compiler optimize function calls and check your code more
1974 The keyword @code{__attribute__} allows you to specify special
1975 attributes when making a declaration. This keyword is followed by an
1976 attribute specification inside double parentheses. The following
1977 attributes are currently defined for functions on all targets:
1978 @code{noreturn}, @code{noinline}, @code{always_inline},
1979 @code{pure}, @code{const}, @code{nothrow},
1980 @code{format}, @code{format_arg}, @code{no_instrument_function},
1981 @code{section}, @code{constructor}, @code{destructor}, @code{used},
1982 @code{unused}, @code{deprecated}, @code{weak}, @code{malloc},
1983 @code{alias}, @code{warn_unused_result} and @code{nonnull}. Several other
1984 attributes are defined for functions on particular target systems. Other
1985 attributes, including @code{section} are supported for variables declarations
1986 (@pxref{Variable Attributes}) and for types (@pxref{Type Attributes}).
1988 You may also specify attributes with @samp{__} preceding and following
1989 each keyword. This allows you to use them in header files without
1990 being concerned about a possible macro of the same name. For example,
1991 you may use @code{__noreturn__} instead of @code{noreturn}.
1993 @xref{Attribute Syntax}, for details of the exact syntax for using
1997 @cindex @code{noreturn} function attribute
1999 A few standard library functions, such as @code{abort} and @code{exit},
2000 cannot return. GCC knows this automatically. Some programs define
2001 their own functions that never return. You can declare them
2002 @code{noreturn} to tell the compiler this fact. For example,
2006 void fatal () __attribute__ ((noreturn));
2009 fatal (/* @r{@dots{}} */)
2011 /* @r{@dots{}} */ /* @r{Print error message.} */ /* @r{@dots{}} */
2017 The @code{noreturn} keyword tells the compiler to assume that
2018 @code{fatal} cannot return. It can then optimize without regard to what
2019 would happen if @code{fatal} ever did return. This makes slightly
2020 better code. More importantly, it helps avoid spurious warnings of
2021 uninitialized variables.
2023 The @code{noreturn} keyword does not affect the exceptional path when that
2024 applies: a @code{noreturn}-marked function may still return to the caller
2025 by throwing an exception.
2027 Do not assume that registers saved by the calling function are
2028 restored before calling the @code{noreturn} function.
2030 It does not make sense for a @code{noreturn} function to have a return
2031 type other than @code{void}.
2033 The attribute @code{noreturn} is not implemented in GCC versions
2034 earlier than 2.5. An alternative way to declare that a function does
2035 not return, which works in the current version and in some older
2036 versions, is as follows:
2039 typedef void voidfn ();
2041 volatile voidfn fatal;
2044 @cindex @code{noinline} function attribute
2046 This function attribute prevents a function from being considered for
2049 @cindex @code{always_inline} function attribute
2051 Generally, functions are not inlined unless optimization is specified.
2052 For functions declared inline, this attribute inlines the function even
2053 if no optimization level was specified.
2055 @cindex @code{pure} function attribute
2057 Many functions have no effects except the return value and their
2058 return value depends only on the parameters and/or global variables.
2059 Such a function can be subject
2060 to common subexpression elimination and loop optimization just as an
2061 arithmetic operator would be. These functions should be declared
2062 with the attribute @code{pure}. For example,
2065 int square (int) __attribute__ ((pure));
2069 says that the hypothetical function @code{square} is safe to call
2070 fewer times than the program says.
2072 Some of common examples of pure functions are @code{strlen} or @code{memcmp}.
2073 Interesting non-pure functions are functions with infinite loops or those
2074 depending on volatile memory or other system resource, that may change between
2075 two consecutive calls (such as @code{feof} in a multithreading environment).
2077 The attribute @code{pure} is not implemented in GCC versions earlier
2079 @cindex @code{const} function attribute
2081 Many functions do not examine any values except their arguments, and
2082 have no effects except the return value. Basically this is just slightly
2083 more strict class than the @code{pure} attribute above, since function is not
2084 allowed to read global memory.
2086 @cindex pointer arguments
2087 Note that a function that has pointer arguments and examines the data
2088 pointed to must @emph{not} be declared @code{const}. Likewise, a
2089 function that calls a non-@code{const} function usually must not be
2090 @code{const}. It does not make sense for a @code{const} function to
2093 The attribute @code{const} is not implemented in GCC versions earlier
2094 than 2.5. An alternative way to declare that a function has no side
2095 effects, which works in the current version and in some older versions,
2099 typedef int intfn ();
2101 extern const intfn square;
2104 This approach does not work in GNU C++ from 2.6.0 on, since the language
2105 specifies that the @samp{const} must be attached to the return value.
2107 @cindex @code{nothrow} function attribute
2109 The @code{nothrow} attribute is used to inform the compiler that a
2110 function cannot throw an exception. For example, most functions in
2111 the standard C library can be guaranteed not to throw an exception
2112 with the notable exceptions of @code{qsort} and @code{bsearch} that
2113 take function pointer arguments. The @code{nothrow} attribute is not
2114 implemented in GCC versions earlier than 3.2.
2116 @item format (@var{archetype}, @var{string-index}, @var{first-to-check})
2117 @cindex @code{format} function attribute
2119 The @code{format} attribute specifies that a function takes @code{printf},
2120 @code{scanf}, @code{strftime} or @code{strfmon} style arguments which
2121 should be type-checked against a format string. For example, the
2126 my_printf (void *my_object, const char *my_format, ...)
2127 __attribute__ ((format (printf, 2, 3)));
2131 causes the compiler to check the arguments in calls to @code{my_printf}
2132 for consistency with the @code{printf} style format string argument
2135 The parameter @var{archetype} determines how the format string is
2136 interpreted, and should be @code{printf}, @code{scanf}, @code{strftime}
2137 or @code{strfmon}. (You can also use @code{__printf__},
2138 @code{__scanf__}, @code{__strftime__} or @code{__strfmon__}.) The
2139 parameter @var{string-index} specifies which argument is the format
2140 string argument (starting from 1), while @var{first-to-check} is the
2141 number of the first argument to check against the format string. For
2142 functions where the arguments are not available to be checked (such as
2143 @code{vprintf}), specify the third parameter as zero. In this case the
2144 compiler only checks the format string for consistency. For
2145 @code{strftime} formats, the third parameter is required to be zero.
2146 Since non-static C++ methods have an implicit @code{this} argument, the
2147 arguments of such methods should be counted from two, not one, when
2148 giving values for @var{string-index} and @var{first-to-check}.
2150 In the example above, the format string (@code{my_format}) is the second
2151 argument of the function @code{my_print}, and the arguments to check
2152 start with the third argument, so the correct parameters for the format
2153 attribute are 2 and 3.
2155 @opindex ffreestanding
2156 The @code{format} attribute allows you to identify your own functions
2157 which take format strings as arguments, so that GCC can check the
2158 calls to these functions for errors. The compiler always (unless
2159 @option{-ffreestanding} is used) checks formats
2160 for the standard library functions @code{printf}, @code{fprintf},
2161 @code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime},
2162 @code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such
2163 warnings are requested (using @option{-Wformat}), so there is no need to
2164 modify the header file @file{stdio.h}. In C99 mode, the functions
2165 @code{snprintf}, @code{vsnprintf}, @code{vscanf}, @code{vfscanf} and
2166 @code{vsscanf} are also checked. Except in strictly conforming C
2167 standard modes, the X/Open function @code{strfmon} is also checked as
2168 are @code{printf_unlocked} and @code{fprintf_unlocked}.
2169 @xref{C Dialect Options,,Options Controlling C Dialect}.
2171 @item format_arg (@var{string-index})
2172 @cindex @code{format_arg} function attribute
2173 @opindex Wformat-nonliteral
2174 The @code{format_arg} attribute specifies that a function takes a format
2175 string for a @code{printf}, @code{scanf}, @code{strftime} or
2176 @code{strfmon} style function and modifies it (for example, to translate
2177 it into another language), so the result can be passed to a
2178 @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style
2179 function (with the remaining arguments to the format function the same
2180 as they would have been for the unmodified string). For example, the
2185 my_dgettext (char *my_domain, const char *my_format)
2186 __attribute__ ((format_arg (2)));
2190 causes the compiler to check the arguments in calls to a @code{printf},
2191 @code{scanf}, @code{strftime} or @code{strfmon} type function, whose
2192 format string argument is a call to the @code{my_dgettext} function, for
2193 consistency with the format string argument @code{my_format}. If the
2194 @code{format_arg} attribute had not been specified, all the compiler
2195 could tell in such calls to format functions would be that the format
2196 string argument is not constant; this would generate a warning when
2197 @option{-Wformat-nonliteral} is used, but the calls could not be checked
2198 without the attribute.
2200 The parameter @var{string-index} specifies which argument is the format
2201 string argument (starting from one). Since non-static C++ methods have
2202 an implicit @code{this} argument, the arguments of such methods should
2203 be counted from two.
2205 The @code{format-arg} attribute allows you to identify your own
2206 functions which modify format strings, so that GCC can check the
2207 calls to @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon}
2208 type function whose operands are a call to one of your own function.
2209 The compiler always treats @code{gettext}, @code{dgettext}, and
2210 @code{dcgettext} in this manner except when strict ISO C support is
2211 requested by @option{-ansi} or an appropriate @option{-std} option, or
2212 @option{-ffreestanding} is used. @xref{C Dialect Options,,Options
2213 Controlling C Dialect}.
2215 @item nonnull (@var{arg-index}, @dots{})
2216 @cindex @code{nonnull} function attribute
2217 The @code{nonnull} attribute specifies that some function parameters should
2218 be non-null pointers. For instance, the declaration:
2222 my_memcpy (void *dest, const void *src, size_t len)
2223 __attribute__((nonnull (1, 2)));
2227 causes the compiler to check that, in calls to @code{my_memcpy},
2228 arguments @var{dest} and @var{src} are non-null. If the compiler
2229 determines that a null pointer is passed in an argument slot marked
2230 as non-null, and the @option{-Wnonnull} option is enabled, a warning
2231 is issued. The compiler may also choose to make optimizations based
2232 on the knowledge that certain function arguments will not be null.
2234 If no argument index list is given to the @code{nonnull} attribute,
2235 all pointer arguments are marked as non-null. To illustrate, the
2236 following declaration is equivalent to the previous example:
2240 my_memcpy (void *dest, const void *src, size_t len)
2241 __attribute__((nonnull));
2244 @item no_instrument_function
2245 @cindex @code{no_instrument_function} function attribute
2246 @opindex finstrument-functions
2247 If @option{-finstrument-functions} is given, profiling function calls will
2248 be generated at entry and exit of most user-compiled functions.
2249 Functions with this attribute will not be so instrumented.
2251 @item section ("@var{section-name}")
2252 @cindex @code{section} function attribute
2253 Normally, the compiler places the code it generates in the @code{text} section.
2254 Sometimes, however, you need additional sections, or you need certain
2255 particular functions to appear in special sections. The @code{section}
2256 attribute specifies that a function lives in a particular section.
2257 For example, the declaration:
2260 extern void foobar (void) __attribute__ ((section ("bar")));
2264 puts the function @code{foobar} in the @code{bar} section.
2266 Some file formats do not support arbitrary sections so the @code{section}
2267 attribute is not available on all platforms.
2268 If you need to map the entire contents of a module to a particular
2269 section, consider using the facilities of the linker instead.
2273 @cindex @code{constructor} function attribute
2274 @cindex @code{destructor} function attribute
2275 The @code{constructor} attribute causes the function to be called
2276 automatically before execution enters @code{main ()}. Similarly, the
2277 @code{destructor} attribute causes the function to be called
2278 automatically after @code{main ()} has completed or @code{exit ()} has
2279 been called. Functions with these attributes are useful for
2280 initializing data that will be used implicitly during the execution of
2283 These attributes are not currently implemented for Objective-C@.
2285 @cindex @code{unused} attribute.
2287 This attribute, attached to a function, means that the function is meant
2288 to be possibly unused. GCC will not produce a warning for this
2291 @cindex @code{used} attribute.
2293 This attribute, attached to a function, means that code must be emitted
2294 for the function even if it appears that the function is not referenced.
2295 This is useful, for example, when the function is referenced only in
2298 @cindex @code{deprecated} attribute.
2300 The @code{deprecated} attribute results in a warning if the function
2301 is used anywhere in the source file. This is useful when identifying
2302 functions that are expected to be removed in a future version of a
2303 program. The warning also includes the location of the declaration
2304 of the deprecated function, to enable users to easily find further
2305 information about why the function is deprecated, or what they should
2306 do instead. Note that the warnings only occurs for uses:
2309 int old_fn () __attribute__ ((deprecated));
2311 int (*fn_ptr)() = old_fn;
2314 results in a warning on line 3 but not line 2.
2316 The @code{deprecated} attribute can also be used for variables and
2317 types (@pxref{Variable Attributes}, @pxref{Type Attributes}.)
2319 @item warn_unused_result
2320 @cindex @code{warn_unused_result} attribute
2321 The @code{warn_unused_result} attribute causes a warning to be emitted
2322 if a caller of the function with this attribute does not use its
2323 return value. This is useful for functions where not checking
2324 the result is either a security problem or always a bug, such as
2328 int fn () __attribute__ ((warn_unused_result));
2331 if (fn () < 0) return -1;
2337 results in warning on line 5.
2340 @cindex @code{weak} attribute
2341 The @code{weak} attribute causes the declaration to be emitted as a weak
2342 symbol rather than a global. This is primarily useful in defining
2343 library functions which can be overridden in user code, though it can
2344 also be used with non-function declarations. Weak symbols are supported
2345 for ELF targets, and also for a.out targets when using the GNU assembler
2349 @cindex @code{malloc} attribute
2350 The @code{malloc} attribute is used to tell the compiler that a function
2351 may be treated as if it were the malloc function. The compiler assumes
2352 that calls to malloc result in pointers that cannot alias anything.
2353 This will often improve optimization.
2355 @item alias ("@var{target}")
2356 @cindex @code{alias} attribute
2357 The @code{alias} attribute causes the declaration to be emitted as an
2358 alias for another symbol, which must be specified. For instance,
2361 void __f () @{ /* @r{Do something.} */; @}
2362 void f () __attribute__ ((weak, alias ("__f")));
2365 declares @samp{f} to be a weak alias for @samp{__f}. In C++, the
2366 mangled name for the target must be used.
2368 Not all target machines support this attribute.
2370 @item visibility ("@var{visibility_type}")
2371 @cindex @code{visibility} attribute
2372 The @code{visibility} attribute on ELF targets causes the declaration
2373 to be emitted with default, hidden, protected or internal visibility.
2376 void __attribute__ ((visibility ("protected")))
2377 f () @{ /* @r{Do something.} */; @}
2378 int i __attribute__ ((visibility ("hidden")));
2381 See the ELF gABI for complete details, but the short story is:
2385 Default visibility is the normal case for ELF. This value is
2386 available for the visibility attribute to override other options
2387 that may change the assumed visibility of symbols.
2390 Hidden visibility indicates that the symbol will not be placed into
2391 the dynamic symbol table, so no other @dfn{module} (executable or
2392 shared library) can reference it directly.
2395 Protected visibility indicates that the symbol will be placed in the
2396 dynamic symbol table, but that references within the defining module
2397 will bind to the local symbol. That is, the symbol cannot be overridden
2401 Internal visibility is like hidden visibility, but with additional
2402 processor specific semantics. Unless otherwise specified by the psABI,
2403 gcc defines internal visibility to mean that the function is @emph{never}
2404 called from another module. Note that hidden symbols, while they cannot
2405 be referenced directly by other modules, can be referenced indirectly via
2406 function pointers. By indicating that a symbol cannot be called from
2407 outside the module, gcc may for instance omit the load of a PIC register
2408 since it is known that the calling function loaded the correct value.
2411 Not all ELF targets support this attribute.
2413 @item regparm (@var{number})
2414 @cindex @code{regparm} attribute
2415 @cindex functions that are passed arguments in registers on the 386
2416 On the Intel 386, the @code{regparm} attribute causes the compiler to
2417 pass up to @var{number} integer arguments in registers EAX,
2418 EDX, and ECX instead of on the stack. Functions that take a
2419 variable number of arguments will continue to be passed all of their
2420 arguments on the stack.
2422 Beware that on some ELF systems this attribute is unsuitable for
2423 global functions in shared libraries with lazy binding (which is the
2424 default). Lazy binding will send the first call via resolving code in
2425 the loader, which might assume EAX, EDX and ECX can be clobbered, as
2426 per the standard calling conventions. Solaris 8 is affected by this.
2427 GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be
2428 safe since the loaders there save all registers. (Lazy binding can be
2429 disabled with the linker or the loader if desired, to avoid the
2433 @cindex functions that pop the argument stack on the 386
2434 On the Intel 386, the @code{stdcall} attribute causes the compiler to
2435 assume that the called function will pop off the stack space used to
2436 pass arguments, unless it takes a variable number of arguments.
2439 @cindex functions that pop the argument stack on the 386
2440 On the Intel 386, the @code{fastcall} attribute causes the compiler to
2441 pass the first two arguments in the registers ECX and EDX. Subsequent
2442 arguments are passed on the stack. The called function will pop the
2443 arguments off the stack. If the number of arguments is variable all
2444 arguments are pushed on the stack.
2447 @cindex functions that do pop the argument stack on the 386
2449 On the Intel 386, the @code{cdecl} attribute causes the compiler to
2450 assume that the calling function will pop off the stack space used to
2451 pass arguments. This is
2452 useful to override the effects of the @option{-mrtd} switch.
2454 @item longcall/shortcall
2455 @cindex functions called via pointer on the RS/6000 and PowerPC
2456 On the RS/6000 and PowerPC, the @code{longcall} attribute causes the
2457 compiler to always call this function via a pointer, just as it would if
2458 the @option{-mlongcall} option had been specified. The @code{shortcall}
2459 attribute causes the compiler not to do this. These attributes override
2460 both the @option{-mlongcall} switch and the @code{#pragma longcall}
2463 @xref{RS/6000 and PowerPC Options}, for more information on whether long
2464 calls are necessary.
2466 @item long_call/short_call
2467 @cindex indirect calls on ARM
2468 This attribute specifies how a particular function is called on
2469 ARM@. Both attributes override the @option{-mlong-calls} (@pxref{ARM Options})
2470 command line switch and @code{#pragma long_calls} settings. The
2471 @code{long_call} attribute causes the compiler to always call the
2472 function by first loading its address into a register and then using the
2473 contents of that register. The @code{short_call} attribute always places
2474 the offset to the function from the call site into the @samp{BL}
2475 instruction directly.
2477 @item function_vector
2478 @cindex calling functions through the function vector on the H8/300 processors
2479 Use this attribute on the H8/300 and H8/300H to indicate that the specified
2480 function should be called through the function vector. Calling a
2481 function through the function vector will reduce code size, however;
2482 the function vector has a limited size (maximum 128 entries on the H8/300
2483 and 64 entries on the H8/300H) and shares space with the interrupt vector.
2485 You must use GAS and GLD from GNU binutils version 2.7 or later for
2486 this attribute to work correctly.
2489 @cindex interrupt handler functions
2490 Use this attribute on the ARM, AVR, C4x, M32R/D and Xstormy16 ports to indicate
2491 that the specified function is an interrupt handler. The compiler will
2492 generate function entry and exit sequences suitable for use in an
2493 interrupt handler when this attribute is present.
2495 Note, interrupt handlers for the H8/300, H8/300H and SH processors can
2496 be specified via the @code{interrupt_handler} attribute.
2498 Note, on the AVR, interrupts will be enabled inside the function.
2500 Note, for the ARM, you can specify the kind of interrupt to be handled by
2501 adding an optional parameter to the interrupt attribute like this:
2504 void f () __attribute__ ((interrupt ("IRQ")));
2507 Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF@.
2509 @item interrupt_handler
2510 @cindex interrupt handler functions on the H8/300 and SH processors
2511 Use this attribute on the H8/300, H8/300H and SH to indicate that the
2512 specified function is an interrupt handler. The compiler will generate
2513 function entry and exit sequences suitable for use in an interrupt
2514 handler when this attribute is present.
2517 Use this attribute on the SH to indicate an @code{interrupt_handler}
2518 function should switch to an alternate stack. It expects a string
2519 argument that names a global variable holding the address of the
2524 void f () __attribute__ ((interrupt_handler,
2525 sp_switch ("alt_stack")));
2529 Use this attribute on the SH for an @code{interrupt_handle} to return using
2530 @code{trapa} instead of @code{rte}. This attribute expects an integer
2531 argument specifying the trap number to be used.
2534 @cindex eight bit data on the H8/300 and H8/300H
2535 Use this attribute on the H8/300 and H8/300H to indicate that the specified
2536 variable should be placed into the eight bit data section.
2537 The compiler will generate more efficient code for certain operations
2538 on data in the eight bit data area. Note the eight bit data area is limited to
2541 You must use GAS and GLD from GNU binutils version 2.7 or later for
2542 this attribute to work correctly.
2545 @cindex tiny data section on the H8/300H
2546 Use this attribute on the H8/300H to indicate that the specified
2547 variable should be placed into the tiny data section.
2548 The compiler will generate more efficient code for loads and stores
2549 on data in the tiny data section. Note the tiny data area is limited to
2550 slightly under 32kbytes of data.
2553 @cindex signal handler functions on the AVR processors
2554 Use this attribute on the AVR to indicate that the specified
2555 function is a signal handler. The compiler will generate function
2556 entry and exit sequences suitable for use in a signal handler when this
2557 attribute is present. Interrupts will be disabled inside the function.
2560 @cindex function without a prologue/epilogue code
2561 Use this attribute on the ARM, AVR, C4x and IP2K ports to indicate that the
2562 specified function does not need prologue/epilogue sequences generated by
2563 the compiler. It is up to the programmer to provide these sequences.
2565 @item model (@var{model-name})
2566 @cindex function addressability on the M32R/D
2567 @cindex variable addressability on the IA-64
2569 On the M32R/D, use this attribute to set the addressability of an
2570 object, and of the code generated for a function. The identifier
2571 @var{model-name} is one of @code{small}, @code{medium}, or
2572 @code{large}, representing each of the code models.
2574 Small model objects live in the lower 16MB of memory (so that their
2575 addresses can be loaded with the @code{ld24} instruction), and are
2576 callable with the @code{bl} instruction.
2578 Medium model objects may live anywhere in the 32-bit address space (the
2579 compiler will generate @code{seth/add3} instructions to load their addresses),
2580 and are callable with the @code{bl} instruction.
2582 Large model objects may live anywhere in the 32-bit address space (the
2583 compiler will generate @code{seth/add3} instructions to load their addresses),
2584 and may not be reachable with the @code{bl} instruction (the compiler will
2585 generate the much slower @code{seth/add3/jl} instruction sequence).
2587 On IA-64, use this attribute to set the addressability of an object.
2588 At present, the only supported identifier for @var{model-name} is
2589 @code{small}, indicating addressability via ``small'' (22-bit)
2590 addresses (so that their addresses can be loaded with the @code{addl}
2591 instruction). Caveat: such addressing is by definition not position
2592 independent and hence this attribute must not be used for objects
2593 defined by shared libraries.
2596 @cindex functions which handle memory bank switching
2597 On 68HC11 and 68HC12 the @code{far} attribute causes the compiler to
2598 use a calling convention that takes care of switching memory banks when
2599 entering and leaving a function. This calling convention is also the
2600 default when using the @option{-mlong-calls} option.
2602 On 68HC12 the compiler will use the @code{call} and @code{rtc} instructions
2603 to call and return from a function.
2605 On 68HC11 the compiler will generate a sequence of instructions
2606 to invoke a board-specific routine to switch the memory bank and call the
2607 real function. The board-specific routine simulates a @code{call}.
2608 At the end of a function, it will jump to a board-specific routine
2609 instead of using @code{rts}. The board-specific return routine simulates
2613 @cindex functions which do not handle memory bank switching on 68HC11/68HC12
2614 On 68HC11 and 68HC12 the @code{near} attribute causes the compiler to
2615 use the normal calling convention based on @code{jsr} and @code{rts}.
2616 This attribute can be used to cancel the effect of the @option{-mlong-calls}
2620 @cindex @code{__declspec(dllimport)}
2621 On Windows targets, the @code{dllimport} attribute causes the compiler
2622 to reference a function or variable via a global pointer to a pointer
2623 that is set up by the Windows dll library. The pointer name is formed by
2624 combining @code{_imp__} and the function or variable name. The attribute
2625 implies @code{extern} storage.
2627 Currently, the attribute is ignored for inlined functions. If the
2628 attribute is applied to a symbol @emph{definition}, an error is reported.
2629 If a symbol previously declared @code{dllimport} is later defined, the
2630 attribute is ignored in subsequent references, and a warning is emitted.
2631 The attribute is also overridden by a subsequent declaration as
2634 When applied to C++ classes, the attribute marks non-inlined
2635 member functions and static data members as imports. However, the
2636 attribute is ignored for virtual methods to allow creation of vtables
2639 On cygwin, mingw and arm-pe targets, @code{__declspec(dllimport)} is
2640 recognized as a synonym for @code{__attribute__ ((dllimport))} for
2641 compatibility with other Windows compilers.
2643 The use of the @code{dllimport} attribute on functions is not necessary,
2644 but provides a small performance benefit by eliminating a thunk in the
2645 dll. The use of the @code{dllimport} attribute on imported variables was
2646 required on older versions of GNU ld, but can now be avoided by passing
2647 the @option{--enable-auto-import} switch to ld. As with functions, using
2648 the attribute for a variable eliminates a thunk in the dll.
2650 One drawback to using this attribute is that a pointer to a function or
2651 variable marked as dllimport cannot be used as a constant address. The
2652 attribute can be disabled for functions by setting the
2653 @option{-mnop-fun-dllimport} flag.
2656 @cindex @code{__declspec(dllexport)}
2657 On Windows targets the @code{dllexport} attribute causes the compiler to
2658 provide a global pointer to a pointer in a dll, so that it can be
2659 referenced with the @code{dllimport} attribute. The pointer name is
2660 formed by combining @code{_imp__} and the function or variable name.
2662 Currently, the @code{dllexport}attribute is ignored for inlined
2663 functions, but export can be forced by using the
2664 @option{-fkeep-inline-functions} flag. The attribute is also ignored for
2667 When applied to C++ classes. the attribute marks defined non-inlined
2668 member functions and static data members as exports. Static consts
2669 initialized in-class are not marked unless they are also defined
2672 On cygwin, mingw and arm-pe targets, @code{__declspec(dllexport)} is
2673 recognized as a synonym for @code{__attribute__ ((dllexport))} for
2674 compatibility with other Windows compilers.
2676 Alternative methods for including the symbol in the dll's export table
2677 are to use a .def file with an @code{EXPORTS} section or, with GNU ld,
2678 using the @option{--export-all} linker flag.
2682 You can specify multiple attributes in a declaration by separating them
2683 by commas within the double parentheses or by immediately following an
2684 attribute declaration with another attribute declaration.
2686 @cindex @code{#pragma}, reason for not using
2687 @cindex pragma, reason for not using
2688 Some people object to the @code{__attribute__} feature, suggesting that
2689 ISO C's @code{#pragma} should be used instead. At the time
2690 @code{__attribute__} was designed, there were two reasons for not doing
2695 It is impossible to generate @code{#pragma} commands from a macro.
2698 There is no telling what the same @code{#pragma} might mean in another
2702 These two reasons applied to almost any application that might have been
2703 proposed for @code{#pragma}. It was basically a mistake to use
2704 @code{#pragma} for @emph{anything}.
2706 The ISO C99 standard includes @code{_Pragma}, which now allows pragmas
2707 to be generated from macros. In addition, a @code{#pragma GCC}
2708 namespace is now in use for GCC-specific pragmas. However, it has been
2709 found convenient to use @code{__attribute__} to achieve a natural
2710 attachment of attributes to their corresponding declarations, whereas
2711 @code{#pragma GCC} is of use for constructs that do not naturally form
2712 part of the grammar. @xref{Other Directives,,Miscellaneous
2713 Preprocessing Directives, cpp, The GNU C Preprocessor}.
2715 @node Attribute Syntax
2716 @section Attribute Syntax
2717 @cindex attribute syntax
2719 This section describes the syntax with which @code{__attribute__} may be
2720 used, and the constructs to which attribute specifiers bind, for the C
2721 language. Some details may vary for C++ and Objective-C@. Because of
2722 infelicities in the grammar for attributes, some forms described here
2723 may not be successfully parsed in all cases.
2725 There are some problems with the semantics of attributes in C++. For
2726 example, there are no manglings for attributes, although they may affect
2727 code generation, so problems may arise when attributed types are used in
2728 conjunction with templates or overloading. Similarly, @code{typeid}
2729 does not distinguish between types with different attributes. Support
2730 for attributes in C++ may be restricted in future to attributes on
2731 declarations only, but not on nested declarators.
2733 @xref{Function Attributes}, for details of the semantics of attributes
2734 applying to functions. @xref{Variable Attributes}, for details of the
2735 semantics of attributes applying to variables. @xref{Type Attributes},
2736 for details of the semantics of attributes applying to structure, union
2737 and enumerated types.
2739 An @dfn{attribute specifier} is of the form
2740 @code{__attribute__ ((@var{attribute-list}))}. An @dfn{attribute list}
2741 is a possibly empty comma-separated sequence of @dfn{attributes}, where
2742 each attribute is one of the following:
2746 Empty. Empty attributes are ignored.
2749 A word (which may be an identifier such as @code{unused}, or a reserved
2750 word such as @code{const}).
2753 A word, followed by, in parentheses, parameters for the attribute.
2754 These parameters take one of the following forms:
2758 An identifier. For example, @code{mode} attributes use this form.
2761 An identifier followed by a comma and a non-empty comma-separated list
2762 of expressions. For example, @code{format} attributes use this form.
2765 A possibly empty comma-separated list of expressions. For example,
2766 @code{format_arg} attributes use this form with the list being a single
2767 integer constant expression, and @code{alias} attributes use this form
2768 with the list being a single string constant.
2772 An @dfn{attribute specifier list} is a sequence of one or more attribute
2773 specifiers, not separated by any other tokens.
2775 In GNU C, an attribute specifier list may appear after the colon following a
2776 label, other than a @code{case} or @code{default} label. The only
2777 attribute it makes sense to use after a label is @code{unused}. This
2778 feature is intended for code generated by programs which contains labels
2779 that may be unused but which is compiled with @option{-Wall}. It would
2780 not normally be appropriate to use in it human-written code, though it
2781 could be useful in cases where the code that jumps to the label is
2782 contained within an @code{#ifdef} conditional. GNU C++ does not permit
2783 such placement of attribute lists, as it is permissible for a
2784 declaration, which could begin with an attribute list, to be labelled in
2785 C++. Declarations cannot be labelled in C90 or C99, so the ambiguity
2786 does not arise there.
2788 An attribute specifier list may appear as part of a @code{struct},
2789 @code{union} or @code{enum} specifier. It may go either immediately
2790 after the @code{struct}, @code{union} or @code{enum} keyword, or after
2791 the closing brace. It is ignored if the content of the structure, union
2792 or enumerated type is not defined in the specifier in which the
2793 attribute specifier list is used---that is, in usages such as
2794 @code{struct __attribute__((foo)) bar} with no following opening brace.
2795 Where attribute specifiers follow the closing brace, they are considered
2796 to relate to the structure, union or enumerated type defined, not to any
2797 enclosing declaration the type specifier appears in, and the type
2798 defined is not complete until after the attribute specifiers.
2799 @c Otherwise, there would be the following problems: a shift/reduce
2800 @c conflict between attributes binding the struct/union/enum and
2801 @c binding to the list of specifiers/qualifiers; and "aligned"
2802 @c attributes could use sizeof for the structure, but the size could be
2803 @c changed later by "packed" attributes.
2805 Otherwise, an attribute specifier appears as part of a declaration,
2806 counting declarations of unnamed parameters and type names, and relates
2807 to that declaration (which may be nested in another declaration, for
2808 example in the case of a parameter declaration), or to a particular declarator
2809 within a declaration. Where an
2810 attribute specifier is applied to a parameter declared as a function or
2811 an array, it should apply to the function or array rather than the
2812 pointer to which the parameter is implicitly converted, but this is not
2813 yet correctly implemented.
2815 Any list of specifiers and qualifiers at the start of a declaration may
2816 contain attribute specifiers, whether or not such a list may in that
2817 context contain storage class specifiers. (Some attributes, however,
2818 are essentially in the nature of storage class specifiers, and only make
2819 sense where storage class specifiers may be used; for example,
2820 @code{section}.) There is one necessary limitation to this syntax: the
2821 first old-style parameter declaration in a function definition cannot
2822 begin with an attribute specifier, because such an attribute applies to
2823 the function instead by syntax described below (which, however, is not
2824 yet implemented in this case). In some other cases, attribute
2825 specifiers are permitted by this grammar but not yet supported by the
2826 compiler. All attribute specifiers in this place relate to the
2827 declaration as a whole. In the obsolescent usage where a type of
2828 @code{int} is implied by the absence of type specifiers, such a list of
2829 specifiers and qualifiers may be an attribute specifier list with no
2830 other specifiers or qualifiers.
2832 An attribute specifier list may appear immediately before a declarator
2833 (other than the first) in a comma-separated list of declarators in a
2834 declaration of more than one identifier using a single list of
2835 specifiers and qualifiers. Such attribute specifiers apply
2836 only to the identifier before whose declarator they appear. For
2840 __attribute__((noreturn)) void d0 (void),
2841 __attribute__((format(printf, 1, 2))) d1 (const char *, ...),
2846 the @code{noreturn} attribute applies to all the functions
2847 declared; the @code{format} attribute only applies to @code{d1}.
2849 An attribute specifier list may appear immediately before the comma,
2850 @code{=} or semicolon terminating the declaration of an identifier other
2851 than a function definition. At present, such attribute specifiers apply
2852 to the declared object or function, but in future they may attach to the
2853 outermost adjacent declarator. In simple cases there is no difference,
2854 but, for example, in
2857 void (****f)(void) __attribute__((noreturn));
2861 at present the @code{noreturn} attribute applies to @code{f}, which
2862 causes a warning since @code{f} is not a function, but in future it may
2863 apply to the function @code{****f}. The precise semantics of what
2864 attributes in such cases will apply to are not yet specified. Where an
2865 assembler name for an object or function is specified (@pxref{Asm
2866 Labels}), at present the attribute must follow the @code{asm}
2867 specification; in future, attributes before the @code{asm} specification
2868 may apply to the adjacent declarator, and those after it to the declared
2871 An attribute specifier list may, in future, be permitted to appear after
2872 the declarator in a function definition (before any old-style parameter
2873 declarations or the function body).
2875 Attribute specifiers may be mixed with type qualifiers appearing inside
2876 the @code{[]} of a parameter array declarator, in the C99 construct by
2877 which such qualifiers are applied to the pointer to which the array is
2878 implicitly converted. Such attribute specifiers apply to the pointer,
2879 not to the array, but at present this is not implemented and they are
2882 An attribute specifier list may appear at the start of a nested
2883 declarator. At present, there are some limitations in this usage: the
2884 attributes correctly apply to the declarator, but for most individual
2885 attributes the semantics this implies are not implemented.
2886 When attribute specifiers follow the @code{*} of a pointer
2887 declarator, they may be mixed with any type qualifiers present.
2888 The following describes the formal semantics of this syntax. It will make the
2889 most sense if you are familiar with the formal specification of
2890 declarators in the ISO C standard.
2892 Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration @code{T
2893 D1}, where @code{T} contains declaration specifiers that specify a type
2894 @var{Type} (such as @code{int}) and @code{D1} is a declarator that
2895 contains an identifier @var{ident}. The type specified for @var{ident}
2896 for derived declarators whose type does not include an attribute
2897 specifier is as in the ISO C standard.
2899 If @code{D1} has the form @code{( @var{attribute-specifier-list} D )},
2900 and the declaration @code{T D} specifies the type
2901 ``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
2902 @code{T D1} specifies the type ``@var{derived-declarator-type-list}
2903 @var{attribute-specifier-list} @var{Type}'' for @var{ident}.
2905 If @code{D1} has the form @code{*
2906 @var{type-qualifier-and-attribute-specifier-list} D}, and the
2907 declaration @code{T D} specifies the type
2908 ``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
2909 @code{T D1} specifies the type ``@var{derived-declarator-type-list}
2910 @var{type-qualifier-and-attribute-specifier-list} @var{Type}'' for
2916 void (__attribute__((noreturn)) ****f) (void);
2920 specifies the type ``pointer to pointer to pointer to pointer to
2921 non-returning function returning @code{void}''. As another example,
2924 char *__attribute__((aligned(8))) *f;
2928 specifies the type ``pointer to 8-byte-aligned pointer to @code{char}''.
2929 Note again that this does not work with most attributes; for example,
2930 the usage of @samp{aligned} and @samp{noreturn} attributes given above
2931 is not yet supported.
2933 For compatibility with existing code written for compiler versions that
2934 did not implement attributes on nested declarators, some laxity is
2935 allowed in the placing of attributes. If an attribute that only applies
2936 to types is applied to a declaration, it will be treated as applying to
2937 the type of that declaration. If an attribute that only applies to
2938 declarations is applied to the type of a declaration, it will be treated
2939 as applying to that declaration; and, for compatibility with code
2940 placing the attributes immediately before the identifier declared, such
2941 an attribute applied to a function return type will be treated as
2942 applying to the function type, and such an attribute applied to an array
2943 element type will be treated as applying to the array type. If an
2944 attribute that only applies to function types is applied to a
2945 pointer-to-function type, it will be treated as applying to the pointer
2946 target type; if such an attribute is applied to a function return type
2947 that is not a pointer-to-function type, it will be treated as applying
2948 to the function type.
2950 @node Function Prototypes
2951 @section Prototypes and Old-Style Function Definitions
2952 @cindex function prototype declarations
2953 @cindex old-style function definitions
2954 @cindex promotion of formal parameters
2956 GNU C extends ISO C to allow a function prototype to override a later
2957 old-style non-prototype definition. Consider the following example:
2960 /* @r{Use prototypes unless the compiler is old-fashioned.} */
2967 /* @r{Prototype function declaration.} */
2968 int isroot P((uid_t));
2970 /* @r{Old-style function definition.} */
2972 isroot (x) /* ??? lossage here ??? */
2979 Suppose the type @code{uid_t} happens to be @code{short}. ISO C does
2980 not allow this example, because subword arguments in old-style
2981 non-prototype definitions are promoted. Therefore in this example the
2982 function definition's argument is really an @code{int}, which does not
2983 match the prototype argument type of @code{short}.
2985 This restriction of ISO C makes it hard to write code that is portable
2986 to traditional C compilers, because the programmer does not know
2987 whether the @code{uid_t} type is @code{short}, @code{int}, or
2988 @code{long}. Therefore, in cases like these GNU C allows a prototype
2989 to override a later old-style definition. More precisely, in GNU C, a
2990 function prototype argument type overrides the argument type specified
2991 by a later old-style definition if the former type is the same as the
2992 latter type before promotion. Thus in GNU C the above example is
2993 equivalent to the following:
3006 GNU C++ does not support old-style function definitions, so this
3007 extension is irrelevant.
3010 @section C++ Style Comments
3012 @cindex C++ comments
3013 @cindex comments, C++ style
3015 In GNU C, you may use C++ style comments, which start with @samp{//} and
3016 continue until the end of the line. Many other C implementations allow
3017 such comments, and they are included in the 1999 C standard. However,
3018 C++ style comments are not recognized if you specify an @option{-std}
3019 option specifying a version of ISO C before C99, or @option{-ansi}
3020 (equivalent to @option{-std=c89}).
3023 @section Dollar Signs in Identifier Names
3025 @cindex dollar signs in identifier names
3026 @cindex identifier names, dollar signs in
3028 In GNU C, you may normally use dollar signs in identifier names.
3029 This is because many traditional C implementations allow such identifiers.
3030 However, dollar signs in identifiers are not supported on a few target
3031 machines, typically because the target assembler does not allow them.
3033 @node Character Escapes
3034 @section The Character @key{ESC} in Constants
3036 You can use the sequence @samp{\e} in a string or character constant to
3037 stand for the ASCII character @key{ESC}.
3040 @section Inquiring on Alignment of Types or Variables
3042 @cindex type alignment
3043 @cindex variable alignment
3045 The keyword @code{__alignof__} allows you to inquire about how an object
3046 is aligned, or the minimum alignment usually required by a type. Its
3047 syntax is just like @code{sizeof}.
3049 For example, if the target machine requires a @code{double} value to be
3050 aligned on an 8-byte boundary, then @code{__alignof__ (double)} is 8.
3051 This is true on many RISC machines. On more traditional machine
3052 designs, @code{__alignof__ (double)} is 4 or even 2.
3054 Some machines never actually require alignment; they allow reference to any
3055 data type even at an odd address. For these machines, @code{__alignof__}
3056 reports the @emph{recommended} alignment of a type.
3058 If the operand of @code{__alignof__} is an lvalue rather than a type,
3059 its value is the required alignment for its type, taking into account
3060 any minimum alignment specified with GCC's @code{__attribute__}
3061 extension (@pxref{Variable Attributes}). For example, after this
3065 struct foo @{ int x; char y; @} foo1;
3069 the value of @code{__alignof__ (foo1.y)} is 1, even though its actual
3070 alignment is probably 2 or 4, the same as @code{__alignof__ (int)}.
3072 It is an error to ask for the alignment of an incomplete type.
3074 @node Variable Attributes
3075 @section Specifying Attributes of Variables
3076 @cindex attribute of variables
3077 @cindex variable attributes
3079 The keyword @code{__attribute__} allows you to specify special
3080 attributes of variables or structure fields. This keyword is followed
3081 by an attribute specification inside double parentheses. Some
3082 attributes are currently defined generically for variables.
3083 Other attributes are defined for variables on particular target
3084 systems. Other attributes are available for functions
3085 (@pxref{Function Attributes}) and for types (@pxref{Type Attributes}).
3086 Other front ends might define more attributes
3087 (@pxref{C++ Extensions,,Extensions to the C++ Language}).
3089 You may also specify attributes with @samp{__} preceding and following
3090 each keyword. This allows you to use them in header files without
3091 being concerned about a possible macro of the same name. For example,
3092 you may use @code{__aligned__} instead of @code{aligned}.
3094 @xref{Attribute Syntax}, for details of the exact syntax for using
3098 @cindex @code{aligned} attribute
3099 @item aligned (@var{alignment})
3100 This attribute specifies a minimum alignment for the variable or
3101 structure field, measured in bytes. For example, the declaration:
3104 int x __attribute__ ((aligned (16))) = 0;
3108 causes the compiler to allocate the global variable @code{x} on a
3109 16-byte boundary. On a 68040, this could be used in conjunction with
3110 an @code{asm} expression to access the @code{move16} instruction which
3111 requires 16-byte aligned operands.
3113 You can also specify the alignment of structure fields. For example, to
3114 create a double-word aligned @code{int} pair, you could write:
3117 struct foo @{ int x[2] __attribute__ ((aligned (8))); @};
3121 This is an alternative to creating a union with a @code{double} member
3122 that forces the union to be double-word aligned.
3124 As in the preceding examples, you can explicitly specify the alignment
3125 (in bytes) that you wish the compiler to use for a given variable or
3126 structure field. Alternatively, you can leave out the alignment factor
3127 and just ask the compiler to align a variable or field to the maximum
3128 useful alignment for the target machine you are compiling for. For
3129 example, you could write:
3132 short array[3] __attribute__ ((aligned));
3135 Whenever you leave out the alignment factor in an @code{aligned} attribute
3136 specification, the compiler automatically sets the alignment for the declared
3137 variable or field to the largest alignment which is ever used for any data
3138 type on the target machine you are compiling for. Doing this can often make
3139 copy operations more efficient, because the compiler can use whatever
3140 instructions copy the biggest chunks of memory when performing copies to
3141 or from the variables or fields that you have aligned this way.
3143 The @code{aligned} attribute can only increase the alignment; but you
3144 can decrease it by specifying @code{packed} as well. See below.
3146 Note that the effectiveness of @code{aligned} attributes may be limited
3147 by inherent limitations in your linker. On many systems, the linker is
3148 only able to arrange for variables to be aligned up to a certain maximum
3149 alignment. (For some linkers, the maximum supported alignment may
3150 be very very small.) If your linker is only able to align variables
3151 up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
3152 in an @code{__attribute__} will still only provide you with 8 byte
3153 alignment. See your linker documentation for further information.
3155 @item cleanup (@var{cleanup_function})
3156 @cindex @code{cleanup} attribute
3157 The @code{cleanup} attribute runs a function when the variable goes
3158 out of scope. This attribute can only be applied to auto function
3159 scope variables; it may not be applied to parameters or variables
3160 with static storage duration. The function must take one parameter,
3161 a pointer to a type compatible with the variable. The return value
3162 of the function (if any) is ignored.
3164 If @option{-fexceptions} is enabled, then @var{cleanup_function}
3165 will be run during the stack unwinding that happens during the
3166 processing of the exception. Note that the @code{cleanup} attribute
3167 does not allow the exception to be caught, only to perform an action.
3168 It is undefined what happens if @var{cleanup_function} does not
3173 @cindex @code{common} attribute
3174 @cindex @code{nocommon} attribute
3177 The @code{common} attribute requests GCC to place a variable in
3178 ``common'' storage. The @code{nocommon} attribute requests the
3179 opposite -- to allocate space for it directly.
3181 These attributes override the default chosen by the
3182 @option{-fno-common} and @option{-fcommon} flags respectively.
3185 @cindex @code{deprecated} attribute
3186 The @code{deprecated} attribute results in a warning if the variable
3187 is used anywhere in the source file. This is useful when identifying
3188 variables that are expected to be removed in a future version of a
3189 program. The warning also includes the location of the declaration
3190 of the deprecated variable, to enable users to easily find further
3191 information about why the variable is deprecated, or what they should
3192 do instead. Note that the warning only occurs for uses:
3195 extern int old_var __attribute__ ((deprecated));
3197 int new_fn () @{ return old_var; @}
3200 results in a warning on line 3 but not line 2.
3202 The @code{deprecated} attribute can also be used for functions and
3203 types (@pxref{Function Attributes}, @pxref{Type Attributes}.)
3205 @item mode (@var{mode})
3206 @cindex @code{mode} attribute
3207 This attribute specifies the data type for the declaration---whichever
3208 type corresponds to the mode @var{mode}. This in effect lets you
3209 request an integer or floating point type according to its width.
3211 You may also specify a mode of @samp{byte} or @samp{__byte__} to
3212 indicate the mode corresponding to a one-byte integer, @samp{word} or
3213 @samp{__word__} for the mode of a one-word integer, and @samp{pointer}
3214 or @samp{__pointer__} for the mode used to represent pointers.
3217 @cindex @code{packed} attribute
3218 The @code{packed} attribute specifies that a variable or structure field
3219 should have the smallest possible alignment---one byte for a variable,
3220 and one bit for a field, unless you specify a larger value with the
3221 @code{aligned} attribute.
3223 Here is a structure in which the field @code{x} is packed, so that it
3224 immediately follows @code{a}:
3230 int x[2] __attribute__ ((packed));
3234 @item section ("@var{section-name}")
3235 @cindex @code{section} variable attribute
3236 Normally, the compiler places the objects it generates in sections like
3237 @code{data} and @code{bss}. Sometimes, however, you need additional sections,
3238 or you need certain particular variables to appear in special sections,
3239 for example to map to special hardware. The @code{section}
3240 attribute specifies that a variable (or function) lives in a particular
3241 section. For example, this small program uses several specific section names:
3244 struct duart a __attribute__ ((section ("DUART_A"))) = @{ 0 @};
3245 struct duart b __attribute__ ((section ("DUART_B"))) = @{ 0 @};
3246 char stack[10000] __attribute__ ((section ("STACK"))) = @{ 0 @};
3247 int init_data __attribute__ ((section ("INITDATA"))) = 0;
3251 /* Initialize stack pointer */
3252 init_sp (stack + sizeof (stack));
3254 /* Initialize initialized data */
3255 memcpy (&init_data, &data, &edata - &data);
3257 /* Turn on the serial ports */
3264 Use the @code{section} attribute with an @emph{initialized} definition
3265 of a @emph{global} variable, as shown in the example. GCC issues
3266 a warning and otherwise ignores the @code{section} attribute in
3267 uninitialized variable declarations.
3269 You may only use the @code{section} attribute with a fully initialized
3270 global definition because of the way linkers work. The linker requires
3271 each object be defined once, with the exception that uninitialized
3272 variables tentatively go in the @code{common} (or @code{bss}) section
3273 and can be multiply ``defined''. You can force a variable to be
3274 initialized with the @option{-fno-common} flag or the @code{nocommon}
3277 Some file formats do not support arbitrary sections so the @code{section}
3278 attribute is not available on all platforms.
3279 If you need to map the entire contents of a module to a particular
3280 section, consider using the facilities of the linker instead.
3283 @cindex @code{shared} variable attribute
3284 On Windows, in addition to putting variable definitions in a named
3285 section, the section can also be shared among all running copies of an
3286 executable or DLL@. For example, this small program defines shared data
3287 by putting it in a named section @code{shared} and marking the section
3291 int foo __attribute__((section ("shared"), shared)) = 0;
3296 /* Read and write foo. All running
3297 copies see the same value. */
3303 You may only use the @code{shared} attribute along with @code{section}
3304 attribute with a fully initialized global definition because of the way
3305 linkers work. See @code{section} attribute for more information.
3307 The @code{shared} attribute is only available on Windows@.
3309 @item tls_model ("@var{tls_model}")
3310 @cindex @code{tls_model} attribute
3311 The @code{tls_model} attribute sets thread-local storage model
3312 (@pxref{Thread-Local}) of a particular @code{__thread} variable,
3313 overriding @code{-ftls-model=} command line switch on a per-variable
3315 The @var{tls_model} argument should be one of @code{global-dynamic},
3316 @code{local-dynamic}, @code{initial-exec} or @code{local-exec}.
3318 Not all targets support this attribute.
3320 @item transparent_union
3321 This attribute, attached to a function parameter which is a union, means
3322 that the corresponding argument may have the type of any union member,
3323 but the argument is passed as if its type were that of the first union
3324 member. For more details see @xref{Type Attributes}. You can also use
3325 this attribute on a @code{typedef} for a union data type; then it
3326 applies to all function parameters with that type.
3329 This attribute, attached to a variable, means that the variable is meant
3330 to be possibly unused. GCC will not produce a warning for this
3333 @item vector_size (@var{bytes})
3334 This attribute specifies the vector size for the variable, measured in
3335 bytes. For example, the declaration:
3338 int foo __attribute__ ((vector_size (16)));
3342 causes the compiler to set the mode for @code{foo}, to be 16 bytes,
3343 divided into @code{int} sized units. Assuming a 32-bit int (a vector of
3344 4 units of 4 bytes), the corresponding mode of @code{foo} will be V4SI@.
3346 This attribute is only applicable to integral and float scalars,
3347 although arrays, pointers, and function return values are allowed in
3348 conjunction with this construct.
3350 Aggregates with this attribute are invalid, even if they are of the same
3351 size as a corresponding scalar. For example, the declaration:
3354 struct S @{ int a; @};
3355 struct S __attribute__ ((vector_size (16))) foo;
3359 is invalid even if the size of the structure is the same as the size of
3363 The @code{weak} attribute is described in @xref{Function Attributes}.
3366 The @code{dllimport} attribute is described in @xref{Function Attributes}.
3369 The @code{dllexport} attribute is described in @xref{Function Attributes}.
3373 @subsection M32R/D Variable Attributes
3375 One attribute is currently defined for the M32R/D.
3378 @item model (@var{model-name})
3379 @cindex variable addressability on the M32R/D
3380 Use this attribute on the M32R/D to set the addressability of an object.
3381 The identifier @var{model-name} is one of @code{small}, @code{medium},
3382 or @code{large}, representing each of the code models.
3384 Small model objects live in the lower 16MB of memory (so that their
3385 addresses can be loaded with the @code{ld24} instruction).
3387 Medium and large model objects may live anywhere in the 32-bit address space
3388 (the compiler will generate @code{seth/add3} instructions to load their
3392 @subsection i386 Variable Attributes
3394 Two attributes are currently defined for i386 configurations:
3395 @code{ms_struct} and @code{gcc_struct}
3400 @cindex @code{ms_struct} attribute
3401 @cindex @code{gcc_struct} attribute
3403 If @code{packed} is used on a structure, or if bit-fields are used
3404 it may be that the Microsoft ABI packs them differently
3405 than GCC would normally pack them. Particularly when moving packed
3406 data between functions compiled with GCC and the native Microsoft compiler
3407 (either via function call or as data in a file), it may be necessary to access
3410 Currently @option{-m[no-]ms-bitfields} is provided for the Windows X86
3411 compilers to match the native Microsoft compiler.
3414 @node Type Attributes
3415 @section Specifying Attributes of Types
3416 @cindex attribute of types
3417 @cindex type attributes
3419 The keyword @code{__attribute__} allows you to specify special
3420 attributes of @code{struct} and @code{union} types when you define such
3421 types. This keyword is followed by an attribute specification inside
3422 double parentheses. Six attributes are currently defined for types:
3423 @code{aligned}, @code{packed}, @code{transparent_union}, @code{unused},
3424 @code{deprecated} and @code{may_alias}. Other attributes are defined for
3425 functions (@pxref{Function Attributes}) and for variables
3426 (@pxref{Variable Attributes}).
3428 You may also specify any one of these attributes with @samp{__}
3429 preceding and following its keyword. This allows you to use these
3430 attributes in header files without being concerned about a possible
3431 macro of the same name. For example, you may use @code{__aligned__}
3432 instead of @code{aligned}.
3434 You may specify the @code{aligned} and @code{transparent_union}
3435 attributes either in a @code{typedef} declaration or just past the
3436 closing curly brace of a complete enum, struct or union type
3437 @emph{definition} and the @code{packed} attribute only past the closing
3438 brace of a definition.
3440 You may also specify attributes between the enum, struct or union
3441 tag and the name of the type rather than after the closing brace.
3443 @xref{Attribute Syntax}, for details of the exact syntax for using
3447 @cindex @code{aligned} attribute
3448 @item aligned (@var{alignment})
3449 This attribute specifies a minimum alignment (in bytes) for variables
3450 of the specified type. For example, the declarations:
3453 struct S @{ short f[3]; @} __attribute__ ((aligned (8)));
3454 typedef int more_aligned_int __attribute__ ((aligned (8)));
3458 force the compiler to insure (as far as it can) that each variable whose
3459 type is @code{struct S} or @code{more_aligned_int} will be allocated and
3460 aligned @emph{at least} on a 8-byte boundary. On a SPARC, having all
3461 variables of type @code{struct S} aligned to 8-byte boundaries allows
3462 the compiler to use the @code{ldd} and @code{std} (doubleword load and
3463 store) instructions when copying one variable of type @code{struct S} to
3464 another, thus improving run-time efficiency.
3466 Note that the alignment of any given @code{struct} or @code{union} type
3467 is required by the ISO C standard to be at least a perfect multiple of
3468 the lowest common multiple of the alignments of all of the members of
3469 the @code{struct} or @code{union} in question. This means that you @emph{can}
3470 effectively adjust the alignment of a @code{struct} or @code{union}
3471 type by attaching an @code{aligned} attribute to any one of the members
3472 of such a type, but the notation illustrated in the example above is a
3473 more obvious, intuitive, and readable way to request the compiler to
3474 adjust the alignment of an entire @code{struct} or @code{union} type.
3476 As in the preceding example, you can explicitly specify the alignment
3477 (in bytes) that you wish the compiler to use for a given @code{struct}
3478 or @code{union} type. Alternatively, you can leave out the alignment factor
3479 and just ask the compiler to align a type to the maximum
3480 useful alignment for the target machine you are compiling for. For
3481 example, you could write:
3484 struct S @{ short f[3]; @} __attribute__ ((aligned));
3487 Whenever you leave out the alignment factor in an @code{aligned}
3488 attribute specification, the compiler automatically sets the alignment
3489 for the type to the largest alignment which is ever used for any data
3490 type on the target machine you are compiling for. Doing this can often
3491 make copy operations more efficient, because the compiler can use
3492 whatever instructions copy the biggest chunks of memory when performing
3493 copies to or from the variables which have types that you have aligned
3496 In the example above, if the size of each @code{short} is 2 bytes, then
3497 the size of the entire @code{struct S} type is 6 bytes. The smallest
3498 power of two which is greater than or equal to that is 8, so the
3499 compiler sets the alignment for the entire @code{struct S} type to 8
3502 Note that although you can ask the compiler to select a time-efficient
3503 alignment for a given type and then declare only individual stand-alone
3504 objects of that type, the compiler's ability to select a time-efficient
3505 alignment is primarily useful only when you plan to create arrays of
3506 variables having the relevant (efficiently aligned) type. If you
3507 declare or use arrays of variables of an efficiently-aligned type, then
3508 it is likely that your program will also be doing pointer arithmetic (or
3509 subscripting, which amounts to the same thing) on pointers to the
3510 relevant type, and the code that the compiler generates for these
3511 pointer arithmetic operations will often be more efficient for
3512 efficiently-aligned types than for other types.
3514 The @code{aligned} attribute can only increase the alignment; but you
3515 can decrease it by specifying @code{packed} as well. See below.
3517 Note that the effectiveness of @code{aligned} attributes may be limited
3518 by inherent limitations in your linker. On many systems, the linker is
3519 only able to arrange for variables to be aligned up to a certain maximum
3520 alignment. (For some linkers, the maximum supported alignment may
3521 be very very small.) If your linker is only able to align variables
3522 up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
3523 in an @code{__attribute__} will still only provide you with 8 byte
3524 alignment. See your linker documentation for further information.
3527 This attribute, attached to @code{struct} or @code{union} type
3528 definition, specifies that each member of the structure or union is
3529 placed to minimize the memory required. When attached to an @code{enum}
3530 definition, it indicates that the smallest integral type should be used.
3532 @opindex fshort-enums
3533 Specifying this attribute for @code{struct} and @code{union} types is
3534 equivalent to specifying the @code{packed} attribute on each of the
3535 structure or union members. Specifying the @option{-fshort-enums}
3536 flag on the line is equivalent to specifying the @code{packed}
3537 attribute on all @code{enum} definitions.
3539 In the following example @code{struct my_packed_struct}'s members are
3540 packed closely together, but the internal layout of its @code{s} member
3541 is not packed -- to do that, @code{struct my_unpacked_struct} would need to
3545 struct my_unpacked_struct
3551 struct my_packed_struct __attribute__ ((__packed__))
3555 struct my_unpacked_struct s;
3559 You may only specify this attribute on the definition of a @code{enum},
3560 @code{struct} or @code{union}, not on a @code{typedef} which does not
3561 also define the enumerated type, structure or union.
3563 @item transparent_union
3564 This attribute, attached to a @code{union} type definition, indicates
3565 that any function parameter having that union type causes calls to that
3566 function to be treated in a special way.
3568 First, the argument corresponding to a transparent union type can be of
3569 any type in the union; no cast is required. Also, if the union contains
3570 a pointer type, the corresponding argument can be a null pointer
3571 constant or a void pointer expression; and if the union contains a void
3572 pointer type, the corresponding argument can be any pointer expression.
3573 If the union member type is a pointer, qualifiers like @code{const} on
3574 the referenced type must be respected, just as with normal pointer
3577 Second, the argument is passed to the function using the calling
3578 conventions of the first member of the transparent union, not the calling
3579 conventions of the union itself. All members of the union must have the
3580 same machine representation; this is necessary for this argument passing
3583 Transparent unions are designed for library functions that have multiple
3584 interfaces for compatibility reasons. For example, suppose the
3585 @code{wait} function must accept either a value of type @code{int *} to
3586 comply with Posix, or a value of type @code{union wait *} to comply with
3587 the 4.1BSD interface. If @code{wait}'s parameter were @code{void *},
3588 @code{wait} would accept both kinds of arguments, but it would also
3589 accept any other pointer type and this would make argument type checking
3590 less useful. Instead, @code{<sys/wait.h>} might define the interface
3598 @} wait_status_ptr_t __attribute__ ((__transparent_union__));
3600 pid_t wait (wait_status_ptr_t);
3603 This interface allows either @code{int *} or @code{union wait *}
3604 arguments to be passed, using the @code{int *} calling convention.
3605 The program can call @code{wait} with arguments of either type:
3608 int w1 () @{ int w; return wait (&w); @}
3609 int w2 () @{ union wait w; return wait (&w); @}
3612 With this interface, @code{wait}'s implementation might look like this:
3615 pid_t wait (wait_status_ptr_t p)
3617 return waitpid (-1, p.__ip, 0);
3622 When attached to a type (including a @code{union} or a @code{struct}),
3623 this attribute means that variables of that type are meant to appear
3624 possibly unused. GCC will not produce a warning for any variables of
3625 that type, even if the variable appears to do nothing. This is often
3626 the case with lock or thread classes, which are usually defined and then
3627 not referenced, but contain constructors and destructors that have
3628 nontrivial bookkeeping functions.
3631 The @code{deprecated} attribute results in a warning if the type
3632 is used anywhere in the source file. This is useful when identifying
3633 types that are expected to be removed in a future version of a program.
3634 If possible, the warning also includes the location of the declaration
3635 of the deprecated type, to enable users to easily find further
3636 information about why the type is deprecated, or what they should do
3637 instead. Note that the warnings only occur for uses and then only
3638 if the type is being applied to an identifier that itself is not being
3639 declared as deprecated.
3642 typedef int T1 __attribute__ ((deprecated));
3646 typedef T1 T3 __attribute__ ((deprecated));
3647 T3 z __attribute__ ((deprecated));
3650 results in a warning on line 2 and 3 but not lines 4, 5, or 6. No
3651 warning is issued for line 4 because T2 is not explicitly
3652 deprecated. Line 5 has no warning because T3 is explicitly
3653 deprecated. Similarly for line 6.
3655 The @code{deprecated} attribute can also be used for functions and
3656 variables (@pxref{Function Attributes}, @pxref{Variable Attributes}.)
3659 Accesses to objects with types with this attribute are not subjected to
3660 type-based alias analysis, but are instead assumed to be able to alias
3661 any other type of objects, just like the @code{char} type. See
3662 @option{-fstrict-aliasing} for more information on aliasing issues.
3667 typedef short __attribute__((__may_alias__)) short_a;
3673 short_a *b = (short_a *) &a;
3677 if (a == 0x12345678)
3684 If you replaced @code{short_a} with @code{short} in the variable
3685 declaration, the above program would abort when compiled with
3686 @option{-fstrict-aliasing}, which is on by default at @option{-O2} or
3687 above in recent GCC versions.
3689 @subsection i386 Type Attributes
3691 Two attributes are currently defined for i386 configurations:
3692 @code{ms_struct} and @code{gcc_struct}
3696 @cindex @code{ms_struct}
3697 @cindex @code{gcc_struct}
3699 If @code{packed} is used on a structure, or if bit-fields are used
3700 it may be that the Microsoft ABI packs them differently
3701 than GCC would normally pack them. Particularly when moving packed
3702 data between functions compiled with GCC and the native Microsoft compiler
3703 (either via function call or as data in a file), it may be necessary to access
3706 Currently @option{-m[no-]ms-bitfields} is provided for the Windows X86
3707 compilers to match the native Microsoft compiler.
3710 To specify multiple attributes, separate them by commas within the
3711 double parentheses: for example, @samp{__attribute__ ((aligned (16),
3715 @section An Inline Function is As Fast As a Macro
3716 @cindex inline functions
3717 @cindex integrating function code
3719 @cindex macros, inline alternative
3721 By declaring a function @code{inline}, you can direct GCC to
3722 integrate that function's code into the code for its callers. This
3723 makes execution faster by eliminating the function-call overhead; in
3724 addition, if any of the actual argument values are constant, their known
3725 values may permit simplifications at compile time so that not all of the
3726 inline function's code needs to be included. The effect on code size is
3727 less predictable; object code may be larger or smaller with function
3728 inlining, depending on the particular case. Inlining of functions is an
3729 optimization and it really ``works'' only in optimizing compilation. If
3730 you don't use @option{-O}, no function is really inline.
3732 Inline functions are included in the ISO C99 standard, but there are
3733 currently substantial differences between what GCC implements and what
3734 the ISO C99 standard requires.
3736 To declare a function inline, use the @code{inline} keyword in its
3737 declaration, like this:
3747 (If you are writing a header file to be included in ISO C programs, write
3748 @code{__inline__} instead of @code{inline}. @xref{Alternate Keywords}.)
3749 You can also make all ``simple enough'' functions inline with the option
3750 @option{-finline-functions}.
3753 Note that certain usages in a function definition can make it unsuitable
3754 for inline substitution. Among these usages are: use of varargs, use of
3755 alloca, use of variable sized data types (@pxref{Variable Length}),
3756 use of computed goto (@pxref{Labels as Values}), use of nonlocal goto,
3757 and nested functions (@pxref{Nested Functions}). Using @option{-Winline}
3758 will warn when a function marked @code{inline} could not be substituted,
3759 and will give the reason for the failure.
3761 Note that in C and Objective-C, unlike C++, the @code{inline} keyword
3762 does not affect the linkage of the function.
3764 @cindex automatic @code{inline} for C++ member fns
3765 @cindex @code{inline} automatic for C++ member fns
3766 @cindex member fns, automatically @code{inline}
3767 @cindex C++ member fns, automatically @code{inline}
3768 @opindex fno-default-inline
3769 GCC automatically inlines member functions defined within the class
3770 body of C++ programs even if they are not explicitly declared
3771 @code{inline}. (You can override this with @option{-fno-default-inline};
3772 @pxref{C++ Dialect Options,,Options Controlling C++ Dialect}.)
3774 @cindex inline functions, omission of
3775 @opindex fkeep-inline-functions
3776 When a function is both inline and @code{static}, if all calls to the
3777 function are integrated into the caller, and the function's address is
3778 never used, then the function's own assembler code is never referenced.
3779 In this case, GCC does not actually output assembler code for the
3780 function, unless you specify the option @option{-fkeep-inline-functions}.
3781 Some calls cannot be integrated for various reasons (in particular,
3782 calls that precede the function's definition cannot be integrated, and
3783 neither can recursive calls within the definition). If there is a
3784 nonintegrated call, then the function is compiled to assembler code as
3785 usual. The function must also be compiled as usual if the program
3786 refers to its address, because that can't be inlined.
3788 @cindex non-static inline function
3789 When an inline function is not @code{static}, then the compiler must assume
3790 that there may be calls from other source files; since a global symbol can
3791 be defined only once in any program, the function must not be defined in
3792 the other source files, so the calls therein cannot be integrated.
3793 Therefore, a non-@code{static} inline function is always compiled on its
3794 own in the usual fashion.
3796 If you specify both @code{inline} and @code{extern} in the function
3797 definition, then the definition is used only for inlining. In no case
3798 is the function compiled on its own, not even if you refer to its
3799 address explicitly. Such an address becomes an external reference, as
3800 if you had only declared the function, and had not defined it.
3802 This combination of @code{inline} and @code{extern} has almost the
3803 effect of a macro. The way to use it is to put a function definition in
3804 a header file with these keywords, and put another copy of the
3805 definition (lacking @code{inline} and @code{extern}) in a library file.
3806 The definition in the header file will cause most calls to the function
3807 to be inlined. If any uses of the function remain, they will refer to
3808 the single copy in the library.
3810 Since GCC eventually will implement ISO C99 semantics for
3811 inline functions, it is best to use @code{static inline} only
3812 to guarantee compatibility. (The
3813 existing semantics will remain available when @option{-std=gnu89} is
3814 specified, but eventually the default will be @option{-std=gnu99} and
3815 that will implement the C99 semantics, though it does not do so yet.)
3817 GCC does not inline any functions when not optimizing unless you specify
3818 the @samp{always_inline} attribute for the function, like this:
3822 inline void foo (const char) __attribute__((always_inline));
3826 @section Assembler Instructions with C Expression Operands
3827 @cindex extended @code{asm}
3828 @cindex @code{asm} expressions
3829 @cindex assembler instructions
3832 In an assembler instruction using @code{asm}, you can specify the
3833 operands of the instruction using C expressions. This means you need not
3834 guess which registers or memory locations will contain the data you want
3837 You must specify an assembler instruction template much like what
3838 appears in a machine description, plus an operand constraint string for
3841 For example, here is how to use the 68881's @code{fsinx} instruction:
3844 asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
3848 Here @code{angle} is the C expression for the input operand while
3849 @code{result} is that of the output operand. Each has @samp{"f"} as its
3850 operand constraint, saying that a floating point register is required.
3851 The @samp{=} in @samp{=f} indicates that the operand is an output; all
3852 output operands' constraints must use @samp{=}. The constraints use the
3853 same language used in the machine description (@pxref{Constraints}).
3855 Each operand is described by an operand-constraint string followed by
3856 the C expression in parentheses. A colon separates the assembler
3857 template from the first output operand and another separates the last
3858 output operand from the first input, if any. Commas separate the
3859 operands within each group. The total number of operands is currently
3860 limited to 30; this limitation may be lifted in some future version of
3863 If there are no output operands but there are input operands, you must
3864 place two consecutive colons surrounding the place where the output
3867 As of GCC version 3.1, it is also possible to specify input and output
3868 operands using symbolic names which can be referenced within the
3869 assembler code. These names are specified inside square brackets
3870 preceding the constraint string, and can be referenced inside the
3871 assembler code using @code{%[@var{name}]} instead of a percentage sign
3872 followed by the operand number. Using named operands the above example
3876 asm ("fsinx %[angle],%[output]"
3877 : [output] "=f" (result)
3878 : [angle] "f" (angle));
3882 Note that the symbolic operand names have no relation whatsoever to
3883 other C identifiers. You may use any name you like, even those of
3884 existing C symbols, but you must ensure that no two operands within the same
3885 assembler construct use the same symbolic name.
3887 Output operand expressions must be lvalues; the compiler can check this.
3888 The input operands need not be lvalues. The compiler cannot check
3889 whether the operands have data types that are reasonable for the
3890 instruction being executed. It does not parse the assembler instruction
3891 template and does not know what it means or even whether it is valid
3892 assembler input. The extended @code{asm} feature is most often used for
3893 machine instructions the compiler itself does not know exist. If
3894 the output expression cannot be directly addressed (for example, it is a
3895 bit-field), your constraint must allow a register. In that case, GCC
3896 will use the register as the output of the @code{asm}, and then store
3897 that register into the output.
3899 The ordinary output operands must be write-only; GCC will assume that
3900 the values in these operands before the instruction are dead and need
3901 not be generated. Extended asm supports input-output or read-write
3902 operands. Use the constraint character @samp{+} to indicate such an
3903 operand and list it with the output operands.
3905 When the constraints for the read-write operand (or the operand in which
3906 only some of the bits are to be changed) allows a register, you may, as
3907 an alternative, logically split its function into two separate operands,
3908 one input operand and one write-only output operand. The connection
3909 between them is expressed by constraints which say they need to be in
3910 the same location when the instruction executes. You can use the same C
3911 expression for both operands, or different expressions. For example,
3912 here we write the (fictitious) @samp{combine} instruction with
3913 @code{bar} as its read-only source operand and @code{foo} as its
3914 read-write destination:
3917 asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
3921 The constraint @samp{"0"} for operand 1 says that it must occupy the
3922 same location as operand 0. A number in constraint is allowed only in
3923 an input operand and it must refer to an output operand.
3925 Only a number in the constraint can guarantee that one operand will be in
3926 the same place as another. The mere fact that @code{foo} is the value
3927 of both operands is not enough to guarantee that they will be in the
3928 same place in the generated assembler code. The following would not
3932 asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
3935 Various optimizations or reloading could cause operands 0 and 1 to be in
3936 different registers; GCC knows no reason not to do so. For example, the
3937 compiler might find a copy of the value of @code{foo} in one register and
3938 use it for operand 1, but generate the output operand 0 in a different
3939 register (copying it afterward to @code{foo}'s own address). Of course,
3940 since the register for operand 1 is not even mentioned in the assembler
3941 code, the result will not work, but GCC can't tell that.
3943 As of GCC version 3.1, one may write @code{[@var{name}]} instead of
3944 the operand number for a matching constraint. For example:
3947 asm ("cmoveq %1,%2,%[result]"
3948 : [result] "=r"(result)
3949 : "r" (test), "r"(new), "[result]"(old));
3952 Some instructions clobber specific hard registers. To describe this,
3953 write a third colon after the input operands, followed by the names of
3954 the clobbered hard registers (given as strings). Here is a realistic
3955 example for the VAX:
3958 asm volatile ("movc3 %0,%1,%2"
3960 : "g" (from), "g" (to), "g" (count)
3961 : "r0", "r1", "r2", "r3", "r4", "r5");
3964 You may not write a clobber description in a way that overlaps with an
3965 input or output operand. For example, you may not have an operand
3966 describing a register class with one member if you mention that register
3967 in the clobber list. Variables declared to live in specific registers
3968 (@pxref{Explicit Reg Vars}), and used as asm input or output operands must
3969 have no part mentioned in the clobber description.
3970 There is no way for you to specify that an input
3971 operand is modified without also specifying it as an output
3972 operand. Note that if all the output operands you specify are for this
3973 purpose (and hence unused), you will then also need to specify
3974 @code{volatile} for the @code{asm} construct, as described below, to
3975 prevent GCC from deleting the @code{asm} statement as unused.
3977 If you refer to a particular hardware register from the assembler code,
3978 you will probably have to list the register after the third colon to
3979 tell the compiler the register's value is modified. In some assemblers,
3980 the register names begin with @samp{%}; to produce one @samp{%} in the
3981 assembler code, you must write @samp{%%} in the input.
3983 If your assembler instruction can alter the condition code register, add
3984 @samp{cc} to the list of clobbered registers. GCC on some machines
3985 represents the condition codes as a specific hardware register;
3986 @samp{cc} serves to name this register. On other machines, the
3987 condition code is handled differently, and specifying @samp{cc} has no
3988 effect. But it is valid no matter what the machine.
3990 If your assembler instruction modifies memory in an unpredictable
3991 fashion, add @samp{memory} to the list of clobbered registers. This
3992 will cause GCC to not keep memory values cached in registers across
3993 the assembler instruction. You will also want to add the
3994 @code{volatile} keyword if the memory affected is not listed in the
3995 inputs or outputs of the @code{asm}, as the @samp{memory} clobber does
3996 not count as a side-effect of the @code{asm}.
3998 You can put multiple assembler instructions together in a single
3999 @code{asm} template, separated by the characters normally used in assembly
4000 code for the system. A combination that works in most places is a newline
4001 to break the line, plus a tab character to move to the instruction field
4002 (written as @samp{\n\t}). Sometimes semicolons can be used, if the
4003 assembler allows semicolons as a line-breaking character. Note that some
4004 assembler dialects use semicolons to start a comment.
4005 The input operands are guaranteed not to use any of the clobbered
4006 registers, and neither will the output operands' addresses, so you can
4007 read and write the clobbered registers as many times as you like. Here
4008 is an example of multiple instructions in a template; it assumes the
4009 subroutine @code{_foo} accepts arguments in registers 9 and 10:
4012 asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
4014 : "g" (from), "g" (to)
4018 Unless an output operand has the @samp{&} constraint modifier, GCC
4019 may allocate it in the same register as an unrelated input operand, on
4020 the assumption the inputs are consumed before the outputs are produced.
4021 This assumption may be false if the assembler code actually consists of
4022 more than one instruction. In such a case, use @samp{&} for each output
4023 operand that may not overlap an input. @xref{Modifiers}.
4025 If you want to test the condition code produced by an assembler
4026 instruction, you must include a branch and a label in the @code{asm}
4027 construct, as follows:
4030 asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
4036 This assumes your assembler supports local labels, as the GNU assembler
4037 and most Unix assemblers do.
4039 Speaking of labels, jumps from one @code{asm} to another are not
4040 supported. The compiler's optimizers do not know about these jumps, and
4041 therefore they cannot take account of them when deciding how to
4044 @cindex macros containing @code{asm}
4045 Usually the most convenient way to use these @code{asm} instructions is to
4046 encapsulate them in macros that look like functions. For example,
4050 (@{ double __value, __arg = (x); \
4051 asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
4056 Here the variable @code{__arg} is used to make sure that the instruction
4057 operates on a proper @code{double} value, and to accept only those
4058 arguments @code{x} which can convert automatically to a @code{double}.
4060 Another way to make sure the instruction operates on the correct data
4061 type is to use a cast in the @code{asm}. This is different from using a
4062 variable @code{__arg} in that it converts more different types. For
4063 example, if the desired type were @code{int}, casting the argument to
4064 @code{int} would accept a pointer with no complaint, while assigning the
4065 argument to an @code{int} variable named @code{__arg} would warn about
4066 using a pointer unless the caller explicitly casts it.
4068 If an @code{asm} has output operands, GCC assumes for optimization
4069 purposes the instruction has no side effects except to change the output
4070 operands. This does not mean instructions with a side effect cannot be
4071 used, but you must be careful, because the compiler may eliminate them
4072 if the output operands aren't used, or move them out of loops, or
4073 replace two with one if they constitute a common subexpression. Also,
4074 if your instruction does have a side effect on a variable that otherwise
4075 appears not to change, the old value of the variable may be reused later
4076 if it happens to be found in a register.
4078 You can prevent an @code{asm} instruction from being deleted, moved
4079 significantly, or combined, by writing the keyword @code{volatile} after
4080 the @code{asm}. For example:
4083 #define get_and_set_priority(new) \
4085 asm volatile ("get_and_set_priority %0, %1" \
4086 : "=g" (__old) : "g" (new)); \
4091 If you write an @code{asm} instruction with no outputs, GCC will know
4092 the instruction has side-effects and will not delete the instruction or
4093 move it outside of loops.
4095 The @code{volatile} keyword indicates that the instruction has
4096 important side-effects. GCC will not delete a volatile @code{asm} if
4097 it is reachable. (The instruction can still be deleted if GCC can
4098 prove that control-flow will never reach the location of the
4099 instruction.) In addition, GCC will not reschedule instructions
4100 across a volatile @code{asm} instruction. For example:
4103 *(volatile int *)addr = foo;
4104 asm volatile ("eieio" : : );
4108 Assume @code{addr} contains the address of a memory mapped device
4109 register. The PowerPC @code{eieio} instruction (Enforce In-order
4110 Execution of I/O) tells the CPU to make sure that the store to that
4111 device register happens before it issues any other I/O@.
4113 Note that even a volatile @code{asm} instruction can be moved in ways
4114 that appear insignificant to the compiler, such as across jump
4115 instructions. You can't expect a sequence of volatile @code{asm}
4116 instructions to remain perfectly consecutive. If you want consecutive
4117 output, use a single @code{asm}. Also, GCC will perform some
4118 optimizations across a volatile @code{asm} instruction; GCC does not
4119 ``forget everything'' when it encounters a volatile @code{asm}
4120 instruction the way some other compilers do.
4122 An @code{asm} instruction without any operands or clobbers (an ``old
4123 style'' @code{asm}) will be treated identically to a volatile
4124 @code{asm} instruction.
4126 It is a natural idea to look for a way to give access to the condition
4127 code left by the assembler instruction. However, when we attempted to
4128 implement this, we found no way to make it work reliably. The problem
4129 is that output operands might need reloading, which would result in
4130 additional following ``store'' instructions. On most machines, these
4131 instructions would alter the condition code before there was time to
4132 test it. This problem doesn't arise for ordinary ``test'' and
4133 ``compare'' instructions because they don't have any output operands.
4135 For reasons similar to those described above, it is not possible to give
4136 an assembler instruction access to the condition code left by previous
4139 If you are writing a header file that should be includable in ISO C
4140 programs, write @code{__asm__} instead of @code{asm}. @xref{Alternate
4143 @subsection Size of an @code{asm}
4145 Some targets require that GCC track the size of each instruction used in
4146 order to generate correct code. Because the final length of an
4147 @code{asm} is only known by the assembler, GCC must make an estimate as
4148 to how big it will be. The estimate is formed by counting the number of
4149 statements in the pattern of the @code{asm} and multiplying that by the
4150 length of the longest instruction on that processor. Statements in the
4151 @code{asm} are identified by newline characters and whatever statement
4152 separator characters are supported by the assembler; on most processors
4153 this is the `@code{;}' character.
4155 Normally, GCC's estimate is perfectly adequate to ensure that correct
4156 code is generated, but it is possible to confuse the compiler if you use
4157 pseudo instructions or assembler macros that expand into multiple real
4158 instructions or if you use assembler directives that expand to more
4159 space in the object file than would be needed for a single instruction.
4160 If this happens then the assembler will produce a diagnostic saying that
4161 a label is unreachable.
4163 @subsection i386 floating point asm operands
4165 There are several rules on the usage of stack-like regs in
4166 asm_operands insns. These rules apply only to the operands that are
4171 Given a set of input regs that die in an asm_operands, it is
4172 necessary to know which are implicitly popped by the asm, and
4173 which must be explicitly popped by gcc.
4175 An input reg that is implicitly popped by the asm must be
4176 explicitly clobbered, unless it is constrained to match an
4180 For any input reg that is implicitly popped by an asm, it is
4181 necessary to know how to adjust the stack to compensate for the pop.
4182 If any non-popped input is closer to the top of the reg-stack than
4183 the implicitly popped reg, it would not be possible to know what the
4184 stack looked like---it's not clear how the rest of the stack ``slides
4187 All implicitly popped input regs must be closer to the top of
4188 the reg-stack than any input that is not implicitly popped.
4190 It is possible that if an input dies in an insn, reload might
4191 use the input reg for an output reload. Consider this example:
4194 asm ("foo" : "=t" (a) : "f" (b));
4197 This asm says that input B is not popped by the asm, and that
4198 the asm pushes a result onto the reg-stack, i.e., the stack is one
4199 deeper after the asm than it was before. But, it is possible that
4200 reload will think that it can use the same reg for both the input and
4201 the output, if input B dies in this insn.
4203 If any input operand uses the @code{f} constraint, all output reg
4204 constraints must use the @code{&} earlyclobber.
4206 The asm above would be written as
4209 asm ("foo" : "=&t" (a) : "f" (b));
4213 Some operands need to be in particular places on the stack. All
4214 output operands fall in this category---there is no other way to
4215 know which regs the outputs appear in unless the user indicates
4216 this in the constraints.
4218 Output operands must specifically indicate which reg an output
4219 appears in after an asm. @code{=f} is not allowed: the operand
4220 constraints must select a class with a single reg.
4223 Output operands may not be ``inserted'' between existing stack regs.
4224 Since no 387 opcode uses a read/write operand, all output operands
4225 are dead before the asm_operands, and are pushed by the asm_operands.
4226 It makes no sense to push anywhere but the top of the reg-stack.
4228 Output operands must start at the top of the reg-stack: output
4229 operands may not ``skip'' a reg.
4232 Some asm statements may need extra stack space for internal
4233 calculations. This can be guaranteed by clobbering stack registers
4234 unrelated to the inputs and outputs.
4238 Here are a couple of reasonable asms to want to write. This asm
4239 takes one input, which is internally popped, and produces two outputs.
4242 asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));
4245 This asm takes two inputs, which are popped by the @code{fyl2xp1} opcode,
4246 and replaces them with one output. The user must code the @code{st(1)}
4247 clobber for reg-stack.c to know that @code{fyl2xp1} pops both inputs.
4250 asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
4256 @section Controlling Names Used in Assembler Code
4257 @cindex assembler names for identifiers
4258 @cindex names used in assembler code
4259 @cindex identifiers, names in assembler code
4261 You can specify the name to be used in the assembler code for a C
4262 function or variable by writing the @code{asm} (or @code{__asm__})
4263 keyword after the declarator as follows:
4266 int foo asm ("myfoo") = 2;
4270 This specifies that the name to be used for the variable @code{foo} in
4271 the assembler code should be @samp{myfoo} rather than the usual
4274 On systems where an underscore is normally prepended to the name of a C
4275 function or variable, this feature allows you to define names for the
4276 linker that do not start with an underscore.
4278 It does not make sense to use this feature with a non-static local
4279 variable since such variables do not have assembler names. If you are
4280 trying to put the variable in a particular register, see @ref{Explicit
4281 Reg Vars}. GCC presently accepts such code with a warning, but will
4282 probably be changed to issue an error, rather than a warning, in the
4285 You cannot use @code{asm} in this way in a function @emph{definition}; but
4286 you can get the same effect by writing a declaration for the function
4287 before its definition and putting @code{asm} there, like this:
4290 extern func () asm ("FUNC");
4297 It is up to you to make sure that the assembler names you choose do not
4298 conflict with any other assembler symbols. Also, you must not use a
4299 register name; that would produce completely invalid assembler code. GCC
4300 does not as yet have the ability to store static variables in registers.
4301 Perhaps that will be added.
4303 @node Explicit Reg Vars
4304 @section Variables in Specified Registers
4305 @cindex explicit register variables
4306 @cindex variables in specified registers
4307 @cindex specified registers
4308 @cindex registers, global allocation
4310 GNU C allows you to put a few global variables into specified hardware
4311 registers. You can also specify the register in which an ordinary
4312 register variable should be allocated.
4316 Global register variables reserve registers throughout the program.
4317 This may be useful in programs such as programming language
4318 interpreters which have a couple of global variables that are accessed
4322 Local register variables in specific registers do not reserve the
4323 registers. The compiler's data flow analysis is capable of determining
4324 where the specified registers contain live values, and where they are
4325 available for other uses. Stores into local register variables may be deleted
4326 when they appear to be dead according to dataflow analysis. References
4327 to local register variables may be deleted or moved or simplified.
4329 These local variables are sometimes convenient for use with the extended
4330 @code{asm} feature (@pxref{Extended Asm}), if you want to write one
4331 output of the assembler instruction directly into a particular register.
4332 (This will work provided the register you specify fits the constraints
4333 specified for that operand in the @code{asm}.)
4341 @node Global Reg Vars
4342 @subsection Defining Global Register Variables
4343 @cindex global register variables
4344 @cindex registers, global variables in
4346 You can define a global register variable in GNU C like this:
4349 register int *foo asm ("a5");
4353 Here @code{a5} is the name of the register which should be used. Choose a
4354 register which is normally saved and restored by function calls on your
4355 machine, so that library routines will not clobber it.
4357 Naturally the register name is cpu-dependent, so you would need to
4358 conditionalize your program according to cpu type. The register
4359 @code{a5} would be a good choice on a 68000 for a variable of pointer
4360 type. On machines with register windows, be sure to choose a ``global''
4361 register that is not affected magically by the function call mechanism.
4363 In addition, operating systems on one type of cpu may differ in how they
4364 name the registers; then you would need additional conditionals. For
4365 example, some 68000 operating systems call this register @code{%a5}.
4367 Eventually there may be a way of asking the compiler to choose a register
4368 automatically, but first we need to figure out how it should choose and
4369 how to enable you to guide the choice. No solution is evident.
4371 Defining a global register variable in a certain register reserves that
4372 register entirely for this use, at least within the current compilation.
4373 The register will not be allocated for any other purpose in the functions
4374 in the current compilation. The register will not be saved and restored by
4375 these functions. Stores into this register are never deleted even if they
4376 would appear to be dead, but references may be deleted or moved or
4379 It is not safe to access the global register variables from signal
4380 handlers, or from more than one thread of control, because the system
4381 library routines may temporarily use the register for other things (unless
4382 you recompile them specially for the task at hand).
4384 @cindex @code{qsort}, and global register variables
4385 It is not safe for one function that uses a global register variable to
4386 call another such function @code{foo} by way of a third function
4387 @code{lose} that was compiled without knowledge of this variable (i.e.@: in a
4388 different source file in which the variable wasn't declared). This is
4389 because @code{lose} might save the register and put some other value there.
4390 For example, you can't expect a global register variable to be available in
4391 the comparison-function that you pass to @code{qsort}, since @code{qsort}
4392 might have put something else in that register. (If you are prepared to
4393 recompile @code{qsort} with the same global register variable, you can
4394 solve this problem.)
4396 If you want to recompile @code{qsort} or other source files which do not
4397 actually use your global register variable, so that they will not use that
4398 register for any other purpose, then it suffices to specify the compiler
4399 option @option{-ffixed-@var{reg}}. You need not actually add a global
4400 register declaration to their source code.
4402 A function which can alter the value of a global register variable cannot
4403 safely be called from a function compiled without this variable, because it
4404 could clobber the value the caller expects to find there on return.
4405 Therefore, the function which is the entry point into the part of the
4406 program that uses the global register variable must explicitly save and
4407 restore the value which belongs to its caller.
4409 @cindex register variable after @code{longjmp}
4410 @cindex global register after @code{longjmp}
4411 @cindex value after @code{longjmp}
4414 On most machines, @code{longjmp} will restore to each global register
4415 variable the value it had at the time of the @code{setjmp}. On some
4416 machines, however, @code{longjmp} will not change the value of global
4417 register variables. To be portable, the function that called @code{setjmp}
4418 should make other arrangements to save the values of the global register
4419 variables, and to restore them in a @code{longjmp}. This way, the same
4420 thing will happen regardless of what @code{longjmp} does.
4422 All global register variable declarations must precede all function
4423 definitions. If such a declaration could appear after function
4424 definitions, the declaration would be too late to prevent the register from
4425 being used for other purposes in the preceding functions.
4427 Global register variables may not have initial values, because an
4428 executable file has no means to supply initial contents for a register.
4430 On the SPARC, there are reports that g3 @dots{} g7 are suitable
4431 registers, but certain library functions, such as @code{getwd}, as well
4432 as the subroutines for division and remainder, modify g3 and g4. g1 and
4433 g2 are local temporaries.
4435 On the 68000, a2 @dots{} a5 should be suitable, as should d2 @dots{} d7.
4436 Of course, it will not do to use more than a few of those.
4438 @node Local Reg Vars
4439 @subsection Specifying Registers for Local Variables
4440 @cindex local variables, specifying registers
4441 @cindex specifying registers for local variables
4442 @cindex registers for local variables
4444 You can define a local register variable with a specified register
4448 register int *foo asm ("a5");
4452 Here @code{a5} is the name of the register which should be used. Note
4453 that this is the same syntax used for defining global register
4454 variables, but for a local variable it would appear within a function.
4456 Naturally the register name is cpu-dependent, but this is not a
4457 problem, since specific registers are most often useful with explicit
4458 assembler instructions (@pxref{Extended Asm}). Both of these things
4459 generally require that you conditionalize your program according to
4462 In addition, operating systems on one type of cpu may differ in how they
4463 name the registers; then you would need additional conditionals. For
4464 example, some 68000 operating systems call this register @code{%a5}.
4466 Defining such a register variable does not reserve the register; it
4467 remains available for other uses in places where flow control determines
4468 the variable's value is not live. However, these registers are made
4469 unavailable for use in the reload pass; excessive use of this feature
4470 leaves the compiler too few available registers to compile certain
4473 This option does not guarantee that GCC will generate code that has
4474 this variable in the register you specify at all times. You may not
4475 code an explicit reference to this register in an @code{asm} statement
4476 and assume it will always refer to this variable.
4478 Stores into local register variables may be deleted when they appear to be dead
4479 according to dataflow analysis. References to local register variables may
4480 be deleted or moved or simplified.
4482 @node Alternate Keywords
4483 @section Alternate Keywords
4484 @cindex alternate keywords
4485 @cindex keywords, alternate
4487 @option{-ansi} and the various @option{-std} options disable certain
4488 keywords. This causes trouble when you want to use GNU C extensions, or
4489 a general-purpose header file that should be usable by all programs,
4490 including ISO C programs. The keywords @code{asm}, @code{typeof} and
4491 @code{inline} are not available in programs compiled with
4492 @option{-ansi} or @option{-std} (although @code{inline} can be used in a
4493 program compiled with @option{-std=c99}). The ISO C99 keyword
4494 @code{restrict} is only available when @option{-std=gnu99} (which will
4495 eventually be the default) or @option{-std=c99} (or the equivalent
4496 @option{-std=iso9899:1999}) is used.
4498 The way to solve these problems is to put @samp{__} at the beginning and
4499 end of each problematical keyword. For example, use @code{__asm__}
4500 instead of @code{asm}, and @code{__inline__} instead of @code{inline}.
4502 Other C compilers won't accept these alternative keywords; if you want to
4503 compile with another compiler, you can define the alternate keywords as
4504 macros to replace them with the customary keywords. It looks like this:
4512 @findex __extension__
4514 @option{-pedantic} and other options cause warnings for many GNU C extensions.
4516 prevent such warnings within one expression by writing
4517 @code{__extension__} before the expression. @code{__extension__} has no
4518 effect aside from this.
4520 @node Incomplete Enums
4521 @section Incomplete @code{enum} Types
4523 You can define an @code{enum} tag without specifying its possible values.
4524 This results in an incomplete type, much like what you get if you write
4525 @code{struct foo} without describing the elements. A later declaration
4526 which does specify the possible values completes the type.
4528 You can't allocate variables or storage using the type while it is
4529 incomplete. However, you can work with pointers to that type.
4531 This extension may not be very useful, but it makes the handling of
4532 @code{enum} more consistent with the way @code{struct} and @code{union}
4535 This extension is not supported by GNU C++.
4537 @node Function Names
4538 @section Function Names as Strings
4539 @cindex @code{__func__} identifier
4540 @cindex @code{__FUNCTION__} identifier
4541 @cindex @code{__PRETTY_FUNCTION__} identifier
4543 GCC provides three magic variables which hold the name of the current
4544 function, as a string. The first of these is @code{__func__}, which
4545 is part of the C99 standard:
4548 The identifier @code{__func__} is implicitly declared by the translator
4549 as if, immediately following the opening brace of each function
4550 definition, the declaration
4553 static const char __func__[] = "function-name";
4556 appeared, where function-name is the name of the lexically-enclosing
4557 function. This name is the unadorned name of the function.
4560 @code{__FUNCTION__} is another name for @code{__func__}. Older
4561 versions of GCC recognize only this name. However, it is not
4562 standardized. For maximum portability, we recommend you use
4563 @code{__func__}, but provide a fallback definition with the
4567 #if __STDC_VERSION__ < 199901L
4569 # define __func__ __FUNCTION__
4571 # define __func__ "<unknown>"
4576 In C, @code{__PRETTY_FUNCTION__} is yet another name for
4577 @code{__func__}. However, in C++, @code{__PRETTY_FUNCTION__} contains
4578 the type signature of the function as well as its bare name. For
4579 example, this program:
4583 extern int printf (char *, ...);
4590 printf ("__FUNCTION__ = %s\n", __FUNCTION__);
4591 printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
4609 __PRETTY_FUNCTION__ = void a::sub(int)
4612 These identifiers are not preprocessor macros. In GCC 3.3 and
4613 earlier, in C only, @code{__FUNCTION__} and @code{__PRETTY_FUNCTION__}
4614 were treated as string literals; they could be used to initialize
4615 @code{char} arrays, and they could be concatenated with other string
4616 literals. GCC 3.4 and later treat them as variables, like
4617 @code{__func__}. In C++, @code{__FUNCTION__} and
4618 @code{__PRETTY_FUNCTION__} have always been variables.
4620 @node Return Address
4621 @section Getting the Return or Frame Address of a Function
4623 These functions may be used to get information about the callers of a
4626 @deftypefn {Built-in Function} {void *} __builtin_return_address (unsigned int @var{level})
4627 This function returns the return address of the current function, or of
4628 one of its callers. The @var{level} argument is number of frames to
4629 scan up the call stack. A value of @code{0} yields the return address
4630 of the current function, a value of @code{1} yields the return address
4631 of the caller of the current function, and so forth. When inlining
4632 the expected behavior is that the function will return the address of
4633 the function that will be returned to. To work around this behavior use
4634 the @code{noinline} function attribute.
4636 The @var{level} argument must be a constant integer.
4638 On some machines it may be impossible to determine the return address of
4639 any function other than the current one; in such cases, or when the top
4640 of the stack has been reached, this function will return @code{0} or a
4641 random value. In addition, @code{__builtin_frame_address} may be used
4642 to determine if the top of the stack has been reached.
4644 This function should only be used with a nonzero argument for debugging
4648 @deftypefn {Built-in Function} {void *} __builtin_frame_address (unsigned int @var{level})
4649 This function is similar to @code{__builtin_return_address}, but it
4650 returns the address of the function frame rather than the return address
4651 of the function. Calling @code{__builtin_frame_address} with a value of
4652 @code{0} yields the frame address of the current function, a value of
4653 @code{1} yields the frame address of the caller of the current function,
4656 The frame is the area on the stack which holds local variables and saved
4657 registers. The frame address is normally the address of the first word
4658 pushed on to the stack by the function. However, the exact definition
4659 depends upon the processor and the calling convention. If the processor
4660 has a dedicated frame pointer register, and the function has a frame,
4661 then @code{__builtin_frame_address} will return the value of the frame
4664 On some machines it may be impossible to determine the frame address of
4665 any function other than the current one; in such cases, or when the top
4666 of the stack has been reached, this function will return @code{0} if
4667 the first frame pointer is properly initialized by the startup code.
4669 This function should only be used with a nonzero argument for debugging
4673 @node Vector Extensions
4674 @section Using vector instructions through built-in functions
4676 On some targets, the instruction set contains SIMD vector instructions that
4677 operate on multiple values contained in one large register at the same time.
4678 For example, on the i386 the MMX, 3Dnow! and SSE extensions can be used
4681 The first step in using these extensions is to provide the necessary data
4682 types. This should be done using an appropriate @code{typedef}:
4685 typedef int v4si __attribute__ ((mode(V4SI)));
4688 The base type @code{int} is effectively ignored by the compiler, the
4689 actual properties of the new type @code{v4si} are defined by the
4690 @code{__attribute__}. It defines the machine mode to be used; for vector
4691 types these have the form @code{V@var{n}@var{B}}; @var{n} should be the
4692 number of elements in the vector, and @var{B} should be the base mode of the
4693 individual elements. The following can be used as base modes:
4697 An integer that is as wide as the smallest addressable unit, usually 8 bits.
4699 An integer, twice as wide as a QI mode integer, usually 16 bits.
4701 An integer, four times as wide as a QI mode integer, usually 32 bits.
4703 An integer, eight times as wide as a QI mode integer, usually 64 bits.
4705 A floating point value, as wide as a SI mode integer, usually 32 bits.
4707 A floating point value, as wide as a DI mode integer, usually 64 bits.
4710 Specifying a combination that is not valid for the current architecture
4711 will cause gcc to synthesize the instructions using a narrower mode.
4712 For example, if you specify a variable of type @code{V4SI} and your
4713 architecture does not allow for this specific SIMD type, gcc will
4714 produce code that uses 4 @code{SIs}.
4716 The types defined in this manner can be used with a subset of normal C
4717 operations. Currently, gcc will allow using the following operators on
4718 these types: @code{+, -, *, /, unary minus}@.
4720 The operations behave like C++ @code{valarrays}. Addition is defined as
4721 the addition of the corresponding elements of the operands. For
4722 example, in the code below, each of the 4 elements in @var{a} will be
4723 added to the corresponding 4 elements in @var{b} and the resulting
4724 vector will be stored in @var{c}.
4727 typedef int v4si __attribute__ ((mode(V4SI)));
4734 Subtraction, multiplication, and division operate in a similar manner.
4735 Likewise, the result of using the unary minus operator on a vector type
4736 is a vector whose elements are the negative value of the corresponding
4737 elements in the operand.
4739 You can declare variables and use them in function calls and returns, as
4740 well as in assignments and some casts. You can specify a vector type as
4741 a return type for a function. Vector types can also be used as function
4742 arguments. It is possible to cast from one vector type to another,
4743 provided they are of the same size (in fact, you can also cast vectors
4744 to and from other datatypes of the same size).
4746 You cannot operate between vectors of different lengths or different
4747 signedness without a cast.
4749 A port that supports hardware vector operations, usually provides a set
4750 of built-in functions that can be used to operate on vectors. For
4751 example, a function to add two vectors and multiply the result by a
4752 third could look like this:
4755 v4si f (v4si a, v4si b, v4si c)
4757 v4si tmp = __builtin_addv4si (a, b);
4758 return __builtin_mulv4si (tmp, c);
4763 @node Other Builtins
4764 @section Other built-in functions provided by GCC
4765 @cindex built-in functions
4766 @findex __builtin_isgreater
4767 @findex __builtin_isgreaterequal
4768 @findex __builtin_isless
4769 @findex __builtin_islessequal
4770 @findex __builtin_islessgreater
4771 @findex __builtin_isunordered
4926 @findex fprintf_unlocked
4928 @findex fputs_unlocked
5013 @findex printf_unlocked
5039 @findex significandf
5040 @findex significandl
5102 GCC provides a large number of built-in functions other than the ones
5103 mentioned above. Some of these are for internal use in the processing
5104 of exceptions or variable-length argument lists and will not be
5105 documented here because they may change from time to time; we do not
5106 recommend general use of these functions.
5108 The remaining functions are provided for optimization purposes.
5110 @opindex fno-builtin
5111 GCC includes built-in versions of many of the functions in the standard
5112 C library. The versions prefixed with @code{__builtin_} will always be
5113 treated as having the same meaning as the C library function even if you
5114 specify the @option{-fno-builtin} option. (@pxref{C Dialect Options})
5115 Many of these functions are only optimized in certain cases; if they are
5116 not optimized in a particular case, a call to the library function will
5121 Outside strict ISO C mode (@option{-ansi}, @option{-std=c89} or
5122 @option{-std=c99}), the functions
5123 @code{_exit}, @code{alloca}, @code{bcmp}, @code{bzero},
5124 @code{dcgettext}, @code{dgettext}, @code{dremf}, @code{dreml},
5125 @code{drem}, @code{exp10f}, @code{exp10l}, @code{exp10}, @code{ffsll},
5126 @code{ffsl}, @code{ffs}, @code{fprintf_unlocked}, @code{fputs_unlocked},
5127 @code{gammaf}, @code{gammal}, @code{gamma}, @code{gettext},
5128 @code{index}, @code{j0f}, @code{j0l}, @code{j0}, @code{j1f}, @code{j1l},
5129 @code{j1}, @code{jnf}, @code{jnl}, @code{jn}, @code{mempcpy},
5130 @code{pow10f}, @code{pow10l}, @code{pow10}, @code{printf_unlocked},
5131 @code{rindex}, @code{scalbf}, @code{scalbl}, @code{scalb},
5132 @code{significandf}, @code{significandl}, @code{significand},
5133 @code{sincosf}, @code{sincosl}, @code{sincos}, @code{stpcpy},
5134 @code{strdup}, @code{strfmon}, @code{y0f}, @code{y0l}, @code{y0},
5135 @code{y1f}, @code{y1l}, @code{y1}, @code{ynf}, @code{ynl} and @code{yn}
5136 may be handled as built-in functions.
5137 All these functions have corresponding versions
5138 prefixed with @code{__builtin_}, which may be used even in strict C89
5141 The ISO C99 functions
5142 @code{_Exit}, @code{acoshf}, @code{acoshl}, @code{acosh}, @code{asinhf},
5143 @code{asinhl}, @code{asinh}, @code{atanhf}, @code{atanhl}, @code{atanh},
5144 @code{cabsf}, @code{cabsl}, @code{cabs}, @code{cacosf}, @code{cacoshf},
5145 @code{cacoshl}, @code{cacosh}, @code{cacosl}, @code{cacos},
5146 @code{cargf}, @code{cargl}, @code{carg}, @code{casinf}, @code{casinhf},
5147 @code{casinhl}, @code{casinh}, @code{casinl}, @code{casin},
5148 @code{catanf}, @code{catanhf}, @code{catanhl}, @code{catanh},
5149 @code{catanl}, @code{catan}, @code{cbrtf}, @code{cbrtl}, @code{cbrt},
5150 @code{ccosf}, @code{ccoshf}, @code{ccoshl}, @code{ccosh}, @code{ccosl},
5151 @code{ccos}, @code{cexpf}, @code{cexpl}, @code{cexp}, @code{cimagf},
5152 @code{cimagl}, @code{cimag},
5153 @code{conjf}, @code{conjl}, @code{conj}, @code{copysignf},
5154 @code{copysignl}, @code{copysign}, @code{cpowf}, @code{cpowl},
5155 @code{cpow}, @code{cprojf}, @code{cprojl}, @code{cproj}, @code{crealf},
5156 @code{creall}, @code{creal}, @code{csinf}, @code{csinhf}, @code{csinhl},
5157 @code{csinh}, @code{csinl}, @code{csin}, @code{csqrtf}, @code{csqrtl},
5158 @code{csqrt}, @code{ctanf}, @code{ctanhf}, @code{ctanhl}, @code{ctanh},
5159 @code{ctanl}, @code{ctan}, @code{erfcf}, @code{erfcl}, @code{erfc},
5160 @code{erff}, @code{erfl}, @code{erf}, @code{exp2f}, @code{exp2l},
5161 @code{exp2}, @code{expm1f}, @code{expm1l}, @code{expm1}, @code{fdimf},
5162 @code{fdiml}, @code{fdim}, @code{fmaf}, @code{fmal}, @code{fmaxf},
5163 @code{fmaxl}, @code{fmax}, @code{fma}, @code{fminf}, @code{fminl},
5164 @code{fmin}, @code{hypotf}, @code{hypotl}, @code{hypot}, @code{ilogbf},
5165 @code{ilogbl}, @code{ilogb}, @code{imaxabs}, @code{lgammaf},
5166 @code{lgammal}, @code{lgamma}, @code{llabs}, @code{llrintf},
5167 @code{llrintl}, @code{llrint}, @code{llroundf}, @code{llroundl},
5168 @code{llround}, @code{log1pf}, @code{log1pl}, @code{log1p},
5169 @code{log2f}, @code{log2l}, @code{log2}, @code{logbf}, @code{logbl},
5170 @code{logb}, @code{lrintf}, @code{lrintl}, @code{lrint}, @code{lroundf},
5171 @code{lroundl}, @code{lround}, @code{nearbyintf}, @code{nearbyintl},
5172 @code{nearbyint}, @code{nextafterf}, @code{nextafterl},
5173 @code{nextafter}, @code{nexttowardf}, @code{nexttowardl},
5174 @code{nexttoward}, @code{remainderf}, @code{remainderl},
5175 @code{remainder}, @code{remquof}, @code{remquol}, @code{remquo},
5176 @code{rintf}, @code{rintl}, @code{rint}, @code{roundf}, @code{roundl},
5177 @code{round}, @code{scalblnf}, @code{scalblnl}, @code{scalbln},
5178 @code{scalbnf}, @code{scalbnl}, @code{scalbn}, @code{snprintf},
5179 @code{tgammaf}, @code{tgammal}, @code{tgamma}, @code{truncf},
5180 @code{truncl}, @code{trunc}, @code{vfscanf}, @code{vscanf},
5181 @code{vsnprintf} and @code{vsscanf}
5182 are handled as built-in functions
5183 except in strict ISO C90 mode (@option{-ansi} or @option{-std=c89}).
5185 There are also built-in versions of the ISO C99 functions
5186 @code{acosf}, @code{acosl}, @code{asinf}, @code{asinl}, @code{atan2f},
5187 @code{atan2l}, @code{atanf}, @code{atanl}, @code{ceilf}, @code{ceill},
5188 @code{cosf}, @code{coshf}, @code{coshl}, @code{cosl}, @code{expf},
5189 @code{expl}, @code{fabsf}, @code{fabsl}, @code{floorf}, @code{floorl},
5190 @code{fmodf}, @code{fmodl}, @code{frexpf}, @code{frexpl}, @code{ldexpf},
5191 @code{ldexpl}, @code{log10f}, @code{log10l}, @code{logf}, @code{logl},
5192 @code{modfl}, @code{modf}, @code{powf}, @code{powl}, @code{sinf},
5193 @code{sinhf}, @code{sinhl}, @code{sinl}, @code{sqrtf}, @code{sqrtl},
5194 @code{tanf}, @code{tanhf}, @code{tanhl} and @code{tanl}
5195 that are recognized in any mode since ISO C90 reserves these names for
5196 the purpose to which ISO C99 puts them. All these functions have
5197 corresponding versions prefixed with @code{__builtin_}.
5199 The ISO C90 functions
5200 @code{abort}, @code{abs}, @code{acos}, @code{asin}, @code{atan2},
5201 @code{atan}, @code{calloc}, @code{ceil}, @code{cosh}, @code{cos},
5202 @code{exit}, @code{exp}, @code{fabs}, @code{floor}, @code{fmod},
5203 @code{fprintf}, @code{fputs}, @code{frexp}, @code{fscanf}, @code{labs},
5204 @code{ldexp}, @code{log10}, @code{log}, @code{malloc}, @code{memcmp},
5205 @code{memcpy}, @code{memset}, @code{modf}, @code{pow}, @code{printf},
5206 @code{putchar}, @code{puts}, @code{scanf}, @code{sinh}, @code{sin},
5207 @code{snprintf}, @code{sprintf}, @code{sqrt}, @code{sscanf},
5208 @code{strcat}, @code{strchr}, @code{strcmp}, @code{strcpy},
5209 @code{strcspn}, @code{strlen}, @code{strncat}, @code{strncmp},
5210 @code{strncpy}, @code{strpbrk}, @code{strrchr}, @code{strspn},
5211 @code{strstr}, @code{tanh}, @code{tan}, @code{vfprintf}, @code{vprintf}
5213 are all recognized as built-in functions unless
5214 @option{-fno-builtin} is specified (or @option{-fno-builtin-@var{function}}
5215 is specified for an individual function). All of these functions have
5216 corresponding versions prefixed with @code{__builtin_}.
5218 GCC provides built-in versions of the ISO C99 floating point comparison
5219 macros that avoid raising exceptions for unordered operands. They have
5220 the same names as the standard macros ( @code{isgreater},
5221 @code{isgreaterequal}, @code{isless}, @code{islessequal},
5222 @code{islessgreater}, and @code{isunordered}) , with @code{__builtin_}
5223 prefixed. We intend for a library implementor to be able to simply
5224 @code{#define} each standard macro to its built-in equivalent.
5226 @deftypefn {Built-in Function} int __builtin_types_compatible_p (@var{type1}, @var{type2})
5228 You can use the built-in function @code{__builtin_types_compatible_p} to
5229 determine whether two types are the same.
5231 This built-in function returns 1 if the unqualified versions of the
5232 types @var{type1} and @var{type2} (which are types, not expressions) are
5233 compatible, 0 otherwise. The result of this built-in function can be
5234 used in integer constant expressions.
5236 This built-in function ignores top level qualifiers (e.g., @code{const},
5237 @code{volatile}). For example, @code{int} is equivalent to @code{const
5240 The type @code{int[]} and @code{int[5]} are compatible. On the other
5241 hand, @code{int} and @code{char *} are not compatible, even if the size
5242 of their types, on the particular architecture are the same. Also, the
5243 amount of pointer indirection is taken into account when determining
5244 similarity. Consequently, @code{short *} is not similar to
5245 @code{short **}. Furthermore, two types that are typedefed are
5246 considered compatible if their underlying types are compatible.
5248 An @code{enum} type is considered to be compatible with another
5249 @code{enum} type. For example, @code{enum @{foo, bar@}} is similar to
5250 @code{enum @{hot, dog@}}.
5252 You would typically use this function in code whose execution varies
5253 depending on the arguments' types. For example:
5259 if (__builtin_types_compatible_p (typeof (x), long double)) \
5260 tmp = foo_long_double (tmp); \
5261 else if (__builtin_types_compatible_p (typeof (x), double)) \
5262 tmp = foo_double (tmp); \
5263 else if (__builtin_types_compatible_p (typeof (x), float)) \
5264 tmp = foo_float (tmp); \
5271 @emph{Note:} This construct is only available for C.
5275 @deftypefn {Built-in Function} @var{type} __builtin_choose_expr (@var{const_exp}, @var{exp1}, @var{exp2})
5277 You can use the built-in function @code{__builtin_choose_expr} to
5278 evaluate code depending on the value of a constant expression. This
5279 built-in function returns @var{exp1} if @var{const_exp}, which is a
5280 constant expression that must be able to be determined at compile time,
5281 is nonzero. Otherwise it returns 0.
5283 This built-in function is analogous to the @samp{? :} operator in C,
5284 except that the expression returned has its type unaltered by promotion
5285 rules. Also, the built-in function does not evaluate the expression
5286 that was not chosen. For example, if @var{const_exp} evaluates to true,
5287 @var{exp2} is not evaluated even if it has side-effects.
5289 This built-in function can return an lvalue if the chosen argument is an
5292 If @var{exp1} is returned, the return type is the same as @var{exp1}'s
5293 type. Similarly, if @var{exp2} is returned, its return type is the same
5300 __builtin_choose_expr ( \
5301 __builtin_types_compatible_p (typeof (x), double), \
5303 __builtin_choose_expr ( \
5304 __builtin_types_compatible_p (typeof (x), float), \
5306 /* @r{The void expression results in a compile-time error} \
5307 @r{when assigning the result to something.} */ \
5311 @emph{Note:} This construct is only available for C. Furthermore, the
5312 unused expression (@var{exp1} or @var{exp2} depending on the value of
5313 @var{const_exp}) may still generate syntax errors. This may change in
5318 @deftypefn {Built-in Function} int __builtin_constant_p (@var{exp})
5319 You can use the built-in function @code{__builtin_constant_p} to
5320 determine if a value is known to be constant at compile-time and hence
5321 that GCC can perform constant-folding on expressions involving that
5322 value. The argument of the function is the value to test. The function
5323 returns the integer 1 if the argument is known to be a compile-time
5324 constant and 0 if it is not known to be a compile-time constant. A
5325 return of 0 does not indicate that the value is @emph{not} a constant,
5326 but merely that GCC cannot prove it is a constant with the specified
5327 value of the @option{-O} option.
5329 You would typically use this function in an embedded application where
5330 memory was a critical resource. If you have some complex calculation,
5331 you may want it to be folded if it involves constants, but need to call
5332 a function if it does not. For example:
5335 #define Scale_Value(X) \
5336 (__builtin_constant_p (X) \
5337 ? ((X) * SCALE + OFFSET) : Scale (X))
5340 You may use this built-in function in either a macro or an inline
5341 function. However, if you use it in an inlined function and pass an
5342 argument of the function as the argument to the built-in, GCC will
5343 never return 1 when you call the inline function with a string constant
5344 or compound literal (@pxref{Compound Literals}) and will not return 1
5345 when you pass a constant numeric value to the inline function unless you
5346 specify the @option{-O} option.
5348 You may also use @code{__builtin_constant_p} in initializers for static
5349 data. For instance, you can write
5352 static const int table[] = @{
5353 __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1,
5359 This is an acceptable initializer even if @var{EXPRESSION} is not a
5360 constant expression. GCC must be more conservative about evaluating the
5361 built-in in this case, because it has no opportunity to perform
5364 Previous versions of GCC did not accept this built-in in data
5365 initializers. The earliest version where it is completely safe is
5369 @deftypefn {Built-in Function} long __builtin_expect (long @var{exp}, long @var{c})
5370 @opindex fprofile-arcs
5371 You may use @code{__builtin_expect} to provide the compiler with
5372 branch prediction information. In general, you should prefer to
5373 use actual profile feedback for this (@option{-fprofile-arcs}), as
5374 programmers are notoriously bad at predicting how their programs
5375 actually perform. However, there are applications in which this
5376 data is hard to collect.
5378 The return value is the value of @var{exp}, which should be an
5379 integral expression. The value of @var{c} must be a compile-time
5380 constant. The semantics of the built-in are that it is expected
5381 that @var{exp} == @var{c}. For example:
5384 if (__builtin_expect (x, 0))
5389 would indicate that we do not expect to call @code{foo}, since
5390 we expect @code{x} to be zero. Since you are limited to integral
5391 expressions for @var{exp}, you should use constructions such as
5394 if (__builtin_expect (ptr != NULL, 1))
5399 when testing pointer or floating-point values.
5402 @deftypefn {Built-in Function} void __builtin_prefetch (const void *@var{addr}, ...)
5403 This function is used to minimize cache-miss latency by moving data into
5404 a cache before it is accessed.
5405 You can insert calls to @code{__builtin_prefetch} into code for which
5406 you know addresses of data in memory that is likely to be accessed soon.
5407 If the target supports them, data prefetch instructions will be generated.
5408 If the prefetch is done early enough before the access then the data will
5409 be in the cache by the time it is accessed.
5411 The value of @var{addr} is the address of the memory to prefetch.
5412 There are two optional arguments, @var{rw} and @var{locality}.
5413 The value of @var{rw} is a compile-time constant one or zero; one
5414 means that the prefetch is preparing for a write to the memory address
5415 and zero, the default, means that the prefetch is preparing for a read.
5416 The value @var{locality} must be a compile-time constant integer between
5417 zero and three. A value of zero means that the data has no temporal
5418 locality, so it need not be left in the cache after the access. A value
5419 of three means that the data has a high degree of temporal locality and
5420 should be left in all levels of cache possible. Values of one and two
5421 mean, respectively, a low or moderate degree of temporal locality. The
5425 for (i = 0; i < n; i++)
5428 __builtin_prefetch (&a[i+j], 1, 1);
5429 __builtin_prefetch (&b[i+j], 0, 1);
5434 Data prefetch does not generate faults if @var{addr} is invalid, but
5435 the address expression itself must be valid. For example, a prefetch
5436 of @code{p->next} will not fault if @code{p->next} is not a valid
5437 address, but evaluation will fault if @code{p} is not a valid address.
5439 If the target does not support data prefetch, the address expression
5440 is evaluated if it includes side effects but no other code is generated
5441 and GCC does not issue a warning.
5444 @deftypefn {Built-in Function} double __builtin_huge_val (void)
5445 Returns a positive infinity, if supported by the floating-point format,
5446 else @code{DBL_MAX}. This function is suitable for implementing the
5447 ISO C macro @code{HUGE_VAL}.
5450 @deftypefn {Built-in Function} float __builtin_huge_valf (void)
5451 Similar to @code{__builtin_huge_val}, except the return type is @code{float}.
5454 @deftypefn {Built-in Function} {long double} __builtin_huge_vall (void)
5455 Similar to @code{__builtin_huge_val}, except the return
5456 type is @code{long double}.
5459 @deftypefn {Built-in Function} double __builtin_inf (void)
5460 Similar to @code{__builtin_huge_val}, except a warning is generated
5461 if the target floating-point format does not support infinities.
5462 This function is suitable for implementing the ISO C99 macro @code{INFINITY}.
5465 @deftypefn {Built-in Function} float __builtin_inff (void)
5466 Similar to @code{__builtin_inf}, except the return type is @code{float}.
5469 @deftypefn {Built-in Function} {long double} __builtin_infl (void)
5470 Similar to @code{__builtin_inf}, except the return
5471 type is @code{long double}.
5474 @deftypefn {Built-in Function} double __builtin_nan (const char *str)
5475 This is an implementation of the ISO C99 function @code{nan}.
5477 Since ISO C99 defines this function in terms of @code{strtod}, which we
5478 do not implement, a description of the parsing is in order. The string
5479 is parsed as by @code{strtol}; that is, the base is recognized by
5480 leading @samp{0} or @samp{0x} prefixes. The number parsed is placed
5481 in the significand such that the least significant bit of the number
5482 is at the least significant bit of the significand. The number is
5483 truncated to fit the significand field provided. The significand is
5484 forced to be a quiet NaN.
5486 This function, if given a string literal, is evaluated early enough
5487 that it is considered a compile-time constant.
5490 @deftypefn {Built-in Function} float __builtin_nanf (const char *str)
5491 Similar to @code{__builtin_nan}, except the return type is @code{float}.
5494 @deftypefn {Built-in Function} {long double} __builtin_nanl (const char *str)
5495 Similar to @code{__builtin_nan}, except the return type is @code{long double}.
5498 @deftypefn {Built-in Function} double __builtin_nans (const char *str)
5499 Similar to @code{__builtin_nan}, except the significand is forced
5500 to be a signaling NaN. The @code{nans} function is proposed by
5501 @uref{http://std.dkuug.dk/JTC1/SC22/WG14/www/docs/n965.htm,,WG14 N965}.
5504 @deftypefn {Built-in Function} float __builtin_nansf (const char *str)
5505 Similar to @code{__builtin_nans}, except the return type is @code{float}.
5508 @deftypefn {Built-in Function} {long double} __builtin_nansl (const char *str)
5509 Similar to @code{__builtin_nans}, except the return type is @code{long double}.
5512 @deftypefn {Built-in Function} int __builtin_ffs (unsigned int x)
5513 Returns one plus the index of the least significant 1-bit of @var{x}, or
5514 if @var{x} is zero, returns zero.
5517 @deftypefn {Built-in Function} int __builtin_clz (unsigned int x)
5518 Returns the number of leading 0-bits in @var{x}, starting at the most
5519 significant bit position. If @var{x} is 0, the result is undefined.
5522 @deftypefn {Built-in Function} int __builtin_ctz (unsigned int x)
5523 Returns the number of trailing 0-bits in @var{x}, starting at the least
5524 significant bit position. If @var{x} is 0, the result is undefined.
5527 @deftypefn {Built-in Function} int __builtin_popcount (unsigned int x)
5528 Returns the number of 1-bits in @var{x}.
5531 @deftypefn {Built-in Function} int __builtin_parity (unsigned int x)
5532 Returns the parity of @var{x}, i.@:e. the number of 1-bits in @var{x}
5536 @deftypefn {Built-in Function} int __builtin_ffsl (unsigned long)
5537 Similar to @code{__builtin_ffs}, except the argument type is
5538 @code{unsigned long}.
5541 @deftypefn {Built-in Function} int __builtin_clzl (unsigned long)
5542 Similar to @code{__builtin_clz}, except the argument type is
5543 @code{unsigned long}.
5546 @deftypefn {Built-in Function} int __builtin_ctzl (unsigned long)
5547 Similar to @code{__builtin_ctz}, except the argument type is
5548 @code{unsigned long}.
5551 @deftypefn {Built-in Function} int __builtin_popcountl (unsigned long)
5552 Similar to @code{__builtin_popcount}, except the argument type is
5553 @code{unsigned long}.
5556 @deftypefn {Built-in Function} int __builtin_parityl (unsigned long)
5557 Similar to @code{__builtin_parity}, except the argument type is
5558 @code{unsigned long}.
5561 @deftypefn {Built-in Function} int __builtin_ffsll (unsigned long long)
5562 Similar to @code{__builtin_ffs}, except the argument type is
5563 @code{unsigned long long}.
5566 @deftypefn {Built-in Function} int __builtin_clzll (unsigned long long)
5567 Similar to @code{__builtin_clz}, except the argument type is
5568 @code{unsigned long long}.
5571 @deftypefn {Built-in Function} int __builtin_ctzll (unsigned long long)
5572 Similar to @code{__builtin_ctz}, except the argument type is
5573 @code{unsigned long long}.
5576 @deftypefn {Built-in Function} int __builtin_popcountll (unsigned long long)
5577 Similar to @code{__builtin_popcount}, except the argument type is
5578 @code{unsigned long long}.
5581 @deftypefn {Built-in Function} int __builtin_parityll (unsigned long long)
5582 Similar to @code{__builtin_parity}, except the argument type is
5583 @code{unsigned long long}.
5587 @node Target Builtins
5588 @section Built-in Functions Specific to Particular Target Machines
5590 On some target machines, GCC supports many built-in functions specific
5591 to those machines. Generally these generate calls to specific machine
5592 instructions, but allow the compiler to schedule those calls.
5595 * Alpha Built-in Functions::
5596 * ARM Built-in Functions::
5597 * X86 Built-in Functions::
5598 * PowerPC AltiVec Built-in Functions::
5601 @node Alpha Built-in Functions
5602 @subsection Alpha Built-in Functions
5604 These built-in functions are available for the Alpha family of
5605 processors, depending on the command-line switches used.
5607 The following built-in functions are always available. They
5608 all generate the machine instruction that is part of the name.
5611 long __builtin_alpha_implver (void)
5612 long __builtin_alpha_rpcc (void)
5613 long __builtin_alpha_amask (long)
5614 long __builtin_alpha_cmpbge (long, long)
5615 long __builtin_alpha_extbl (long, long)
5616 long __builtin_alpha_extwl (long, long)
5617 long __builtin_alpha_extll (long, long)
5618 long __builtin_alpha_extql (long, long)
5619 long __builtin_alpha_extwh (long, long)
5620 long __builtin_alpha_extlh (long, long)
5621 long __builtin_alpha_extqh (long, long)
5622 long __builtin_alpha_insbl (long, long)
5623 long __builtin_alpha_inswl (long, long)
5624 long __builtin_alpha_insll (long, long)
5625 long __builtin_alpha_insql (long, long)
5626 long __builtin_alpha_inswh (long, long)
5627 long __builtin_alpha_inslh (long, long)
5628 long __builtin_alpha_insqh (long, long)
5629 long __builtin_alpha_mskbl (long, long)
5630 long __builtin_alpha_mskwl (long, long)
5631 long __builtin_alpha_mskll (long, long)
5632 long __builtin_alpha_mskql (long, long)
5633 long __builtin_alpha_mskwh (long, long)
5634 long __builtin_alpha_msklh (long, long)
5635 long __builtin_alpha_mskqh (long, long)
5636 long __builtin_alpha_umulh (long, long)
5637 long __builtin_alpha_zap (long, long)
5638 long __builtin_alpha_zapnot (long, long)
5641 The following built-in functions are always with @option{-mmax}
5642 or @option{-mcpu=@var{cpu}} where @var{cpu} is @code{pca56} or
5643 later. They all generate the machine instruction that is part
5647 long __builtin_alpha_pklb (long)
5648 long __builtin_alpha_pkwb (long)
5649 long __builtin_alpha_unpkbl (long)
5650 long __builtin_alpha_unpkbw (long)
5651 long __builtin_alpha_minub8 (long, long)
5652 long __builtin_alpha_minsb8 (long, long)
5653 long __builtin_alpha_minuw4 (long, long)
5654 long __builtin_alpha_minsw4 (long, long)
5655 long __builtin_alpha_maxub8 (long, long)
5656 long __builtin_alpha_maxsb8 (long, long)
5657 long __builtin_alpha_maxuw4 (long, long)
5658 long __builtin_alpha_maxsw4 (long, long)
5659 long __builtin_alpha_perr (long, long)
5662 The following built-in functions are always with @option{-mcix}
5663 or @option{-mcpu=@var{cpu}} where @var{cpu} is @code{ev67} or
5664 later. They all generate the machine instruction that is part
5668 long __builtin_alpha_cttz (long)
5669 long __builtin_alpha_ctlz (long)
5670 long __builtin_alpha_ctpop (long)
5673 The following builtins are available on systems that use the OSF/1
5674 PALcode. Normally they invoke the @code{rduniq} and @code{wruniq}
5675 PAL calls, but when invoked with @option{-mtls-kernel}, they invoke
5676 @code{rdval} and @code{wrval}.
5679 void *__builtin_thread_pointer (void)
5680 void __builtin_set_thread_pointer (void *)
5683 @node ARM Built-in Functions
5684 @subsection ARM Built-in Functions
5686 These built-in functions are available for the ARM family of
5687 processors, when the @option{-mcpu=iwmmxt} switch is used:
5690 typedef int __v2si __attribute__ ((__mode__ (__V2SI__)))
5692 v2si __builtin_arm_waddw (v2si, v2si)
5693 v2si __builtin_arm_waddw (v2si, v2si)
5694 v2si __builtin_arm_wsubw (v2si, v2si)
5695 v2si __builtin_arm_wsubw (v2si, v2si)
5696 v2si __builtin_arm_waddwss (v2si, v2si)
5697 v2si __builtin_arm_wsubwss (v2si, v2si)
5698 v2si __builtin_arm_wsubwss (v2si, v2si)
5699 v2si __builtin_arm_wsubwss (v2si, v2si)
5700 v2si __builtin_arm_wsubwss (v2si, v2si)
5701 v2si __builtin_arm_waddwus (v2si, v2si)
5702 v2si __builtin_arm_wsubwus (v2si, v2si)
5703 v2si __builtin_arm_wsubwus (v2si, v2si)
5704 v2si __builtin_arm_wmaxuw (v2si, v2si)
5705 v2si __builtin_arm_wmaxsw (v2si, v2si)
5706 v2si __builtin_arm_wavg2br (v2si, v2si)
5707 v2si __builtin_arm_wavg2hr (v2si, v2si)
5708 v2si __builtin_arm_wavg2b (v2si, v2si)
5709 v2si __builtin_arm_wavg2h (v2si, v2si)
5710 v2si __builtin_arm_waccb (v2si)
5711 v2si __builtin_arm_wacch (v2si)
5712 v2si __builtin_arm_waccw (v2si)
5713 v2si __builtin_arm_wmacs (v2si, v2si, v2si)
5714 v2si __builtin_arm_wmacsz (v2si, v2si, v2si)
5715 v2si __builtin_arm_wmacu (v2si, v2si, v2si)
5716 v2si __builtin_arm_wmacuz (v2si, v2si)
5717 v2si __builtin_arm_wsadb (v2si, v2si)
5718 v2si __builtin_arm_wsadbz (v2si, v2si)
5719 v2si __builtin_arm_wsadh (v2si, v2si)
5720 v2si __builtin_arm_wsadhz (v2si, v2si)
5721 v2si __builtin_arm_walign (v2si, v2si)
5722 v2si __builtin_arm_tmia (v2si, int, int)
5723 v2si __builtin_arm_tmiaph (v2si, int, int)
5724 v2si __builtin_arm_tmiabb (v2si, int, int)
5725 v2si __builtin_arm_tmiabt (v2si, int, int)
5726 v2si __builtin_arm_tmiatb (v2si, int, int)
5727 v2si __builtin_arm_tmiatt (v2si, int, int)
5728 int __builtin_arm_tmovmskb (v2si)
5729 int __builtin_arm_tmovmskh (v2si)
5730 int __builtin_arm_tmovmskw (v2si)
5731 v2si __builtin_arm_wmadds (v2si, v2si)
5732 v2si __builtin_arm_wmaddu (v2si, v2si)
5733 v2si __builtin_arm_wpackhss (v2si, v2si)
5734 v2si __builtin_arm_wpackwss (v2si, v2si)
5735 v2si __builtin_arm_wpackdss (v2si, v2si)
5736 v2si __builtin_arm_wpackhus (v2si, v2si)
5737 v2si __builtin_arm_wpackwus (v2si, v2si)
5738 v2si __builtin_arm_wpackdus (v2si, v2si)
5739 v2si __builtin_arm_waddb (v2si, v2si)
5740 v2si __builtin_arm_waddh (v2si, v2si)
5741 v2si __builtin_arm_waddw (v2si, v2si)
5742 v2si __builtin_arm_waddbss (v2si, v2si)
5743 v2si __builtin_arm_waddhss (v2si, v2si)
5744 v2si __builtin_arm_waddwss (v2si, v2si)
5745 v2si __builtin_arm_waddbus (v2si, v2si)
5746 v2si __builtin_arm_waddhus (v2si, v2si)
5747 v2si __builtin_arm_waddwus (v2si, v2si)
5748 v2si __builtin_arm_wsubb (v2si, v2si)
5749 v2si __builtin_arm_wsubh (v2si, v2si)
5750 v2si __builtin_arm_wsubw (v2si, v2si)
5751 v2si __builtin_arm_wsubbss (v2si, v2si)
5752 v2si __builtin_arm_wsubhss (v2si, v2si)
5753 v2si __builtin_arm_wsubwss (v2si, v2si)
5754 v2si __builtin_arm_wsubbus (v2si, v2si)
5755 v2si __builtin_arm_wsubhus (v2si, v2si)
5756 v2si __builtin_arm_wsubwus (v2si, v2si)
5757 v2si __builtin_arm_wand (v2si, v2si)
5758 v2si __builtin_arm_wandn (v2si, v2si)
5759 v2si __builtin_arm_wor (v2si, v2si)
5760 v2si __builtin_arm_wxor (v2si, v2si)
5761 v2si __builtin_arm_wcmpeqb (v2si, v2si)
5762 v2si __builtin_arm_wcmpeqh (v2si, v2si)
5763 v2si __builtin_arm_wcmpeqw (v2si, v2si)
5764 v2si __builtin_arm_wcmpgtub (v2si, v2si)
5765 v2si __builtin_arm_wcmpgtuh (v2si, v2si)
5766 v2si __builtin_arm_wcmpgtuw (v2si, v2si)
5767 v2si __builtin_arm_wcmpgtsb (v2si, v2si)
5768 v2si __builtin_arm_wcmpgtsh (v2si, v2si)
5769 v2si __builtin_arm_wcmpgtsw (v2si, v2si)
5770 int __builtin_arm_textrmsb (v2si, int)
5771 int __builtin_arm_textrmsh (v2si, int)
5772 int __builtin_arm_textrmsw (v2si, int)
5773 int __builtin_arm_textrmub (v2si, int)
5774 int __builtin_arm_textrmuh (v2si, int)
5775 int __builtin_arm_textrmuw (v2si, int)
5776 v2si __builtin_arm_tinsrb (v2si, int, int)
5777 v2si __builtin_arm_tinsrh (v2si, int, int)
5778 v2si __builtin_arm_tinsrw (v2si, int, int)
5779 v2si __builtin_arm_wmaxsw (v2si, v2si)
5780 v2si __builtin_arm_wmaxsh (v2si, v2si)
5781 v2si __builtin_arm_wmaxsb (v2si, v2si)
5782 v2si __builtin_arm_wmaxuw (v2si, v2si)
5783 v2si __builtin_arm_wmaxuh (v2si, v2si)
5784 v2si __builtin_arm_wmaxub (v2si, v2si)
5785 v2si __builtin_arm_wminsw (v2si, v2si)
5786 v2si __builtin_arm_wminsh (v2si, v2si)
5787 v2si __builtin_arm_wminsb (v2si, v2si)
5788 v2si __builtin_arm_wminuw (v2si, v2si)
5789 v2si __builtin_arm_wminuh (v2si, v2si)
5790 v2si __builtin_arm_wminub (v2si, v2si)
5791 v2si __builtin_arm_wmuluh (v2si, v2si)
5792 v2si __builtin_arm_wmulsh (v2si, v2si)
5793 v2si __builtin_arm_wmulul (v2si, v2si)
5794 v2si __builtin_arm_wshufh (v2si, int)
5795 v2si __builtin_arm_wsllh (v2si, v2si)
5796 v2si __builtin_arm_wsllw (v2si, v2si)
5797 v2si __builtin_arm_wslld (v2si, v2si)
5798 v2si __builtin_arm_wsrah (v2si, v2si)
5799 v2si __builtin_arm_wsraw (v2si, v2si)
5800 v2si __builtin_arm_wsrad (v2si, v2si)
5801 v2si __builtin_arm_wsrlh (v2si, v2si)
5802 v2si __builtin_arm_wsrlw (v2si, v2si)
5803 v2si __builtin_arm_wsrld (v2si, v2si)
5804 v2si __builtin_arm_wrorh (v2si, v2si)
5805 v2si __builtin_arm_wrorw (v2si, v2si)
5806 v2si __builtin_arm_wrord (v2si, v2si)
5807 v2si __builtin_arm_wsllhi (v2si, int)
5808 v2si __builtin_arm_wsllwi (v2si, int)
5809 v2si __builtin_arm_wslldi (v2si, v2si)
5810 v2si __builtin_arm_wsrahi (v2si, int)
5811 v2si __builtin_arm_wsrawi (v2si, int)
5812 v2si __builtin_arm_wsradi (v2si, v2si)
5813 v2si __builtin_arm_wsrlwi (v2si, int)
5814 v2si __builtin_arm_wsrldi (v2si, int)
5815 v2si __builtin_arm_wrorhi (v2si, int)
5816 v2si __builtin_arm_wrorwi (v2si, int)
5817 v2si __builtin_arm_wrordi (v2si, int)
5818 v2si __builtin_arm_wunpckihb (v2si, v2si)
5819 v2si __builtin_arm_wunpckihh (v2si, v2si)
5820 v2si __builtin_arm_wunpckihw (v2si, v2si)
5821 v2si __builtin_arm_wunpckilb (v2si, v2si)
5822 v2si __builtin_arm_wunpckilh (v2si, v2si)
5823 v2si __builtin_arm_wunpckilw (v2si, v2si)
5824 v2si __builtin_arm_wunpckehsb (v2si)
5825 v2si __builtin_arm_wunpckehsh (v2si)
5826 v2si __builtin_arm_wunpckehsw (v2si)
5827 v2si __builtin_arm_wunpckehub (v2si)
5828 v2si __builtin_arm_wunpckehuh (v2si)
5829 v2si __builtin_arm_wunpckehuw (v2si)
5830 v2si __builtin_arm_wunpckelsb (v2si)
5831 v2si __builtin_arm_wunpckelsh (v2si)
5832 v2si __builtin_arm_wunpckelsw (v2si)
5833 v2si __builtin_arm_wunpckelub (v2si)
5834 v2si __builtin_arm_wunpckeluh (v2si)
5835 v2si __builtin_arm_wunpckeluw (v2si)
5836 v2si __builtin_arm_wsubwss (v2si, v2si)
5837 v2si __builtin_arm_wsraw (v2si, v2si)
5838 v2si __builtin_arm_wsrad (v2si, v2si)
5841 @node X86 Built-in Functions
5842 @subsection X86 Built-in Functions
5844 These built-in functions are available for the i386 and x86-64 family
5845 of computers, depending on the command-line switches used.
5847 The following machine modes are available for use with MMX built-in functions
5848 (@pxref{Vector Extensions}): @code{V2SI} for a vector of two 32-bit integers,
5849 @code{V4HI} for a vector of four 16-bit integers, and @code{V8QI} for a
5850 vector of eight 8-bit integers. Some of the built-in functions operate on
5851 MMX registers as a whole 64-bit entity, these use @code{DI} as their mode.
5853 If 3Dnow extensions are enabled, @code{V2SF} is used as a mode for a vector
5854 of two 32-bit floating point values.
5856 If SSE extensions are enabled, @code{V4SF} is used for a vector of four 32-bit
5857 floating point values. Some instructions use a vector of four 32-bit
5858 integers, these use @code{V4SI}. Finally, some instructions operate on an
5859 entire vector register, interpreting it as a 128-bit integer, these use mode
5862 The following built-in functions are made available by @option{-mmmx}.
5863 All of them generate the machine instruction that is part of the name.
5866 v8qi __builtin_ia32_paddb (v8qi, v8qi)
5867 v4hi __builtin_ia32_paddw (v4hi, v4hi)
5868 v2si __builtin_ia32_paddd (v2si, v2si)
5869 v8qi __builtin_ia32_psubb (v8qi, v8qi)
5870 v4hi __builtin_ia32_psubw (v4hi, v4hi)
5871 v2si __builtin_ia32_psubd (v2si, v2si)
5872 v8qi __builtin_ia32_paddsb (v8qi, v8qi)
5873 v4hi __builtin_ia32_paddsw (v4hi, v4hi)
5874 v8qi __builtin_ia32_psubsb (v8qi, v8qi)
5875 v4hi __builtin_ia32_psubsw (v4hi, v4hi)
5876 v8qi __builtin_ia32_paddusb (v8qi, v8qi)
5877 v4hi __builtin_ia32_paddusw (v4hi, v4hi)
5878 v8qi __builtin_ia32_psubusb (v8qi, v8qi)
5879 v4hi __builtin_ia32_psubusw (v4hi, v4hi)
5880 v4hi __builtin_ia32_pmullw (v4hi, v4hi)
5881 v4hi __builtin_ia32_pmulhw (v4hi, v4hi)
5882 di __builtin_ia32_pand (di, di)
5883 di __builtin_ia32_pandn (di,di)
5884 di __builtin_ia32_por (di, di)
5885 di __builtin_ia32_pxor (di, di)
5886 v8qi __builtin_ia32_pcmpeqb (v8qi, v8qi)
5887 v4hi __builtin_ia32_pcmpeqw (v4hi, v4hi)
5888 v2si __builtin_ia32_pcmpeqd (v2si, v2si)
5889 v8qi __builtin_ia32_pcmpgtb (v8qi, v8qi)
5890 v4hi __builtin_ia32_pcmpgtw (v4hi, v4hi)
5891 v2si __builtin_ia32_pcmpgtd (v2si, v2si)
5892 v8qi __builtin_ia32_punpckhbw (v8qi, v8qi)
5893 v4hi __builtin_ia32_punpckhwd (v4hi, v4hi)
5894 v2si __builtin_ia32_punpckhdq (v2si, v2si)
5895 v8qi __builtin_ia32_punpcklbw (v8qi, v8qi)
5896 v4hi __builtin_ia32_punpcklwd (v4hi, v4hi)
5897 v2si __builtin_ia32_punpckldq (v2si, v2si)
5898 v8qi __builtin_ia32_packsswb (v4hi, v4hi)
5899 v4hi __builtin_ia32_packssdw (v2si, v2si)
5900 v8qi __builtin_ia32_packuswb (v4hi, v4hi)
5903 The following built-in functions are made available either with
5904 @option{-msse}, or with a combination of @option{-m3dnow} and
5905 @option{-march=athlon}. All of them generate the machine
5906 instruction that is part of the name.
5909 v4hi __builtin_ia32_pmulhuw (v4hi, v4hi)
5910 v8qi __builtin_ia32_pavgb (v8qi, v8qi)
5911 v4hi __builtin_ia32_pavgw (v4hi, v4hi)
5912 v4hi __builtin_ia32_psadbw (v8qi, v8qi)
5913 v8qi __builtin_ia32_pmaxub (v8qi, v8qi)
5914 v4hi __builtin_ia32_pmaxsw (v4hi, v4hi)
5915 v8qi __builtin_ia32_pminub (v8qi, v8qi)
5916 v4hi __builtin_ia32_pminsw (v4hi, v4hi)
5917 int __builtin_ia32_pextrw (v4hi, int)
5918 v4hi __builtin_ia32_pinsrw (v4hi, int, int)
5919 int __builtin_ia32_pmovmskb (v8qi)
5920 void __builtin_ia32_maskmovq (v8qi, v8qi, char *)
5921 void __builtin_ia32_movntq (di *, di)
5922 void __builtin_ia32_sfence (void)
5925 The following built-in functions are available when @option{-msse} is used.
5926 All of them generate the machine instruction that is part of the name.
5929 int __builtin_ia32_comieq (v4sf, v4sf)
5930 int __builtin_ia32_comineq (v4sf, v4sf)
5931 int __builtin_ia32_comilt (v4sf, v4sf)
5932 int __builtin_ia32_comile (v4sf, v4sf)
5933 int __builtin_ia32_comigt (v4sf, v4sf)
5934 int __builtin_ia32_comige (v4sf, v4sf)
5935 int __builtin_ia32_ucomieq (v4sf, v4sf)
5936 int __builtin_ia32_ucomineq (v4sf, v4sf)
5937 int __builtin_ia32_ucomilt (v4sf, v4sf)
5938 int __builtin_ia32_ucomile (v4sf, v4sf)
5939 int __builtin_ia32_ucomigt (v4sf, v4sf)
5940 int __builtin_ia32_ucomige (v4sf, v4sf)
5941 v4sf __builtin_ia32_addps (v4sf, v4sf)
5942 v4sf __builtin_ia32_subps (v4sf, v4sf)
5943 v4sf __builtin_ia32_mulps (v4sf, v4sf)
5944 v4sf __builtin_ia32_divps (v4sf, v4sf)
5945 v4sf __builtin_ia32_addss (v4sf, v4sf)
5946 v4sf __builtin_ia32_subss (v4sf, v4sf)
5947 v4sf __builtin_ia32_mulss (v4sf, v4sf)
5948 v4sf __builtin_ia32_divss (v4sf, v4sf)
5949 v4si __builtin_ia32_cmpeqps (v4sf, v4sf)
5950 v4si __builtin_ia32_cmpltps (v4sf, v4sf)
5951 v4si __builtin_ia32_cmpleps (v4sf, v4sf)
5952 v4si __builtin_ia32_cmpgtps (v4sf, v4sf)
5953 v4si __builtin_ia32_cmpgeps (v4sf, v4sf)
5954 v4si __builtin_ia32_cmpunordps (v4sf, v4sf)
5955 v4si __builtin_ia32_cmpneqps (v4sf, v4sf)
5956 v4si __builtin_ia32_cmpnltps (v4sf, v4sf)
5957 v4si __builtin_ia32_cmpnleps (v4sf, v4sf)
5958 v4si __builtin_ia32_cmpngtps (v4sf, v4sf)
5959 v4si __builtin_ia32_cmpngeps (v4sf, v4sf)
5960 v4si __builtin_ia32_cmpordps (v4sf, v4sf)
5961 v4si __builtin_ia32_cmpeqss (v4sf, v4sf)
5962 v4si __builtin_ia32_cmpltss (v4sf, v4sf)
5963 v4si __builtin_ia32_cmpless (v4sf, v4sf)
5964 v4si __builtin_ia32_cmpunordss (v4sf, v4sf)
5965 v4si __builtin_ia32_cmpneqss (v4sf, v4sf)
5966 v4si __builtin_ia32_cmpnlts (v4sf, v4sf)
5967 v4si __builtin_ia32_cmpnless (v4sf, v4sf)
5968 v4si __builtin_ia32_cmpordss (v4sf, v4sf)
5969 v4sf __builtin_ia32_maxps (v4sf, v4sf)
5970 v4sf __builtin_ia32_maxss (v4sf, v4sf)
5971 v4sf __builtin_ia32_minps (v4sf, v4sf)
5972 v4sf __builtin_ia32_minss (v4sf, v4sf)
5973 v4sf __builtin_ia32_andps (v4sf, v4sf)
5974 v4sf __builtin_ia32_andnps (v4sf, v4sf)
5975 v4sf __builtin_ia32_orps (v4sf, v4sf)
5976 v4sf __builtin_ia32_xorps (v4sf, v4sf)
5977 v4sf __builtin_ia32_movss (v4sf, v4sf)
5978 v4sf __builtin_ia32_movhlps (v4sf, v4sf)
5979 v4sf __builtin_ia32_movlhps (v4sf, v4sf)
5980 v4sf __builtin_ia32_unpckhps (v4sf, v4sf)
5981 v4sf __builtin_ia32_unpcklps (v4sf, v4sf)
5982 v4sf __builtin_ia32_cvtpi2ps (v4sf, v2si)
5983 v4sf __builtin_ia32_cvtsi2ss (v4sf, int)
5984 v2si __builtin_ia32_cvtps2pi (v4sf)
5985 int __builtin_ia32_cvtss2si (v4sf)
5986 v2si __builtin_ia32_cvttps2pi (v4sf)
5987 int __builtin_ia32_cvttss2si (v4sf)
5988 v4sf __builtin_ia32_rcpps (v4sf)
5989 v4sf __builtin_ia32_rsqrtps (v4sf)
5990 v4sf __builtin_ia32_sqrtps (v4sf)
5991 v4sf __builtin_ia32_rcpss (v4sf)
5992 v4sf __builtin_ia32_rsqrtss (v4sf)
5993 v4sf __builtin_ia32_sqrtss (v4sf)
5994 v4sf __builtin_ia32_shufps (v4sf, v4sf, int)
5995 void __builtin_ia32_movntps (float *, v4sf)
5996 int __builtin_ia32_movmskps (v4sf)
5999 The following built-in functions are available when @option{-msse} is used.
6002 @item v4sf __builtin_ia32_loadaps (float *)
6003 Generates the @code{movaps} machine instruction as a load from memory.
6004 @item void __builtin_ia32_storeaps (float *, v4sf)
6005 Generates the @code{movaps} machine instruction as a store to memory.
6006 @item v4sf __builtin_ia32_loadups (float *)
6007 Generates the @code{movups} machine instruction as a load from memory.
6008 @item void __builtin_ia32_storeups (float *, v4sf)
6009 Generates the @code{movups} machine instruction as a store to memory.
6010 @item v4sf __builtin_ia32_loadsss (float *)
6011 Generates the @code{movss} machine instruction as a load from memory.
6012 @item void __builtin_ia32_storess (float *, v4sf)
6013 Generates the @code{movss} machine instruction as a store to memory.
6014 @item v4sf __builtin_ia32_loadhps (v4sf, v2si *)
6015 Generates the @code{movhps} machine instruction as a load from memory.
6016 @item v4sf __builtin_ia32_loadlps (v4sf, v2si *)
6017 Generates the @code{movlps} machine instruction as a load from memory
6018 @item void __builtin_ia32_storehps (v4sf, v2si *)
6019 Generates the @code{movhps} machine instruction as a store to memory.
6020 @item void __builtin_ia32_storelps (v4sf, v2si *)
6021 Generates the @code{movlps} machine instruction as a store to memory.
6024 The following built-in functions are available when @option{-mpni} is used.
6025 All of them generate the machine instruction that is part of the name.
6028 v2df __builtin_ia32_addsubpd (v2df, v2df)
6029 v2df __builtin_ia32_addsubps (v2df, v2df)
6030 v2df __builtin_ia32_haddpd (v2df, v2df)
6031 v2df __builtin_ia32_haddps (v2df, v2df)
6032 v2df __builtin_ia32_hsubpd (v2df, v2df)
6033 v2df __builtin_ia32_hsubps (v2df, v2df)
6034 v16qi __builtin_ia32_lddqu (char const *)
6035 void __builtin_ia32_monitor (void *, unsigned int, unsigned int)
6036 v2df __builtin_ia32_movddup (v2df)
6037 v4sf __builtin_ia32_movshdup (v4sf)
6038 v4sf __builtin_ia32_movsldup (v4sf)
6039 void __builtin_ia32_mwait (unsigned int, unsigned int)
6042 The following built-in functions are available when @option{-mpni} is used.
6045 @item v2df __builtin_ia32_loadddup (double const *)
6046 Generates the @code{movddup} machine instruction as a load from memory.
6049 The following built-in functions are available when @option{-m3dnow} is used.
6050 All of them generate the machine instruction that is part of the name.
6053 void __builtin_ia32_femms (void)
6054 v8qi __builtin_ia32_pavgusb (v8qi, v8qi)
6055 v2si __builtin_ia32_pf2id (v2sf)
6056 v2sf __builtin_ia32_pfacc (v2sf, v2sf)
6057 v2sf __builtin_ia32_pfadd (v2sf, v2sf)
6058 v2si __builtin_ia32_pfcmpeq (v2sf, v2sf)
6059 v2si __builtin_ia32_pfcmpge (v2sf, v2sf)
6060 v2si __builtin_ia32_pfcmpgt (v2sf, v2sf)
6061 v2sf __builtin_ia32_pfmax (v2sf, v2sf)
6062 v2sf __builtin_ia32_pfmin (v2sf, v2sf)
6063 v2sf __builtin_ia32_pfmul (v2sf, v2sf)
6064 v2sf __builtin_ia32_pfrcp (v2sf)
6065 v2sf __builtin_ia32_pfrcpit1 (v2sf, v2sf)
6066 v2sf __builtin_ia32_pfrcpit2 (v2sf, v2sf)
6067 v2sf __builtin_ia32_pfrsqrt (v2sf)
6068 v2sf __builtin_ia32_pfrsqrtit1 (v2sf, v2sf)
6069 v2sf __builtin_ia32_pfsub (v2sf, v2sf)
6070 v2sf __builtin_ia32_pfsubr (v2sf, v2sf)
6071 v2sf __builtin_ia32_pi2fd (v2si)
6072 v4hi __builtin_ia32_pmulhrw (v4hi, v4hi)
6075 The following built-in functions are available when both @option{-m3dnow}
6076 and @option{-march=athlon} are used. All of them generate the machine
6077 instruction that is part of the name.
6080 v2si __builtin_ia32_pf2iw (v2sf)
6081 v2sf __builtin_ia32_pfnacc (v2sf, v2sf)
6082 v2sf __builtin_ia32_pfpnacc (v2sf, v2sf)
6083 v2sf __builtin_ia32_pi2fw (v2si)
6084 v2sf __builtin_ia32_pswapdsf (v2sf)
6085 v2si __builtin_ia32_pswapdsi (v2si)
6088 @node PowerPC AltiVec Built-in Functions
6089 @subsection PowerPC AltiVec Built-in Functions
6091 These built-in functions are available for the PowerPC family
6092 of computers, depending on the command-line switches used.
6094 The following machine modes are available for use with AltiVec built-in
6095 functions (@pxref{Vector Extensions}): @code{V4SI} for a vector of four
6096 32-bit integers, @code{V4SF} for a vector of four 32-bit floating point
6097 numbers, @code{V8HI} for a vector of eight 16-bit integers, and
6098 @code{V16QI} for a vector of sixteen 8-bit integers.
6100 The following functions are made available by including
6101 @code{<altivec.h>} and using @option{-maltivec} and
6102 @option{-mabi=altivec}. The functions implement the functionality
6103 described in Motorola's AltiVec Programming Interface Manual.
6105 There are a few differences from Motorola's documentation and GCC's
6106 implementation. Vector constants are done with curly braces (not
6107 parentheses). Vector initializers require no casts if the vector
6108 constant is of the same type as the variable it is initializing. The
6109 @code{vector bool} type is deprecated and will be discontinued in
6110 further revisions. Use @code{vector signed} instead. If @code{signed}
6111 or @code{unsigned} is omitted, the vector type will default to
6112 @code{signed}. Lastly, all overloaded functions are implemented with macros
6113 for the C implementation. So code the following example will not work:
6116 vec_add ((vector signed int)@{1, 2, 3, 4@}, foo);
6119 Since vec_add is a macro, the vector constant in the above example will
6120 be treated as four different arguments. Wrap the entire argument in
6121 parentheses for this to work. The C++ implementation does not use
6124 @emph{Note:} Only the @code{<altivec.h>} interface is supported.
6125 Internally, GCC uses built-in functions to achieve the functionality in
6126 the aforementioned header file, but they are not supported and are
6127 subject to change without notice.
6130 vector signed char vec_abs (vector signed char, vector signed char);
6131 vector signed short vec_abs (vector signed short, vector signed short);
6132 vector signed int vec_abs (vector signed int, vector signed int);
6133 vector signed float vec_abs (vector signed float, vector signed float);
6135 vector signed char vec_abss (vector signed char, vector signed char);
6136 vector signed short vec_abss (vector signed short, vector signed short);
6138 vector signed char vec_add (vector signed char, vector signed char);
6139 vector unsigned char vec_add (vector signed char, vector unsigned char);
6141 vector unsigned char vec_add (vector unsigned char, vector signed char);
6143 vector unsigned char vec_add (vector unsigned char,
6144 vector unsigned char);
6145 vector signed short vec_add (vector signed short, vector signed short);
6146 vector unsigned short vec_add (vector signed short,
6147 vector unsigned short);
6148 vector unsigned short vec_add (vector unsigned short,
6149 vector signed short);
6150 vector unsigned short vec_add (vector unsigned short,
6151 vector unsigned short);
6152 vector signed int vec_add (vector signed int, vector signed int);
6153 vector unsigned int vec_add (vector signed int, vector unsigned int);
6154 vector unsigned int vec_add (vector unsigned int, vector signed int);
6155 vector unsigned int vec_add (vector unsigned int, vector unsigned int);
6156 vector float vec_add (vector float, vector float);
6158 vector unsigned int vec_addc (vector unsigned int, vector unsigned int);
6160 vector unsigned char vec_adds (vector signed char,
6161 vector unsigned char);
6162 vector unsigned char vec_adds (vector unsigned char,
6163 vector signed char);
6164 vector unsigned char vec_adds (vector unsigned char,
6165 vector unsigned char);
6166 vector signed char vec_adds (vector signed char, vector signed char);
6167 vector unsigned short vec_adds (vector signed short,
6168 vector unsigned short);
6169 vector unsigned short vec_adds (vector unsigned short,
6170 vector signed short);
6171 vector unsigned short vec_adds (vector unsigned short,
6172 vector unsigned short);
6173 vector signed short vec_adds (vector signed short, vector signed short);
6175 vector unsigned int vec_adds (vector signed int, vector unsigned int);
6176 vector unsigned int vec_adds (vector unsigned int, vector signed int);
6177 vector unsigned int vec_adds (vector unsigned int, vector unsigned int);
6179 vector signed int vec_adds (vector signed int, vector signed int);
6181 vector float vec_and (vector float, vector float);
6182 vector float vec_and (vector float, vector signed int);
6183 vector float vec_and (vector signed int, vector float);
6184 vector signed int vec_and (vector signed int, vector signed int);
6185 vector unsigned int vec_and (vector signed int, vector unsigned int);
6186 vector unsigned int vec_and (vector unsigned int, vector signed int);
6187 vector unsigned int vec_and (vector unsigned int, vector unsigned int);
6188 vector signed short vec_and (vector signed short, vector signed short);
6189 vector unsigned short vec_and (vector signed short,
6190 vector unsigned short);
6191 vector unsigned short vec_and (vector unsigned short,
6192 vector signed short);
6193 vector unsigned short vec_and (vector unsigned short,
6194 vector unsigned short);
6195 vector signed char vec_and (vector signed char, vector signed char);
6196 vector unsigned char vec_and (vector signed char, vector unsigned char);
6198 vector unsigned char vec_and (vector unsigned char, vector signed char);
6200 vector unsigned char vec_and (vector unsigned char,
6201 vector unsigned char);
6203 vector float vec_andc (vector float, vector float);
6204 vector float vec_andc (vector float, vector signed int);
6205 vector float vec_andc (vector signed int, vector float);
6206 vector signed int vec_andc (vector signed int, vector signed int);
6207 vector unsigned int vec_andc (vector signed int, vector unsigned int);
6208 vector unsigned int vec_andc (vector unsigned int, vector signed int);
6209 vector unsigned int vec_andc (vector unsigned int, vector unsigned int);
6211 vector signed short vec_andc (vector signed short, vector signed short);
6213 vector unsigned short vec_andc (vector signed short,
6214 vector unsigned short);
6215 vector unsigned short vec_andc (vector unsigned short,
6216 vector signed short);
6217 vector unsigned short vec_andc (vector unsigned short,
6218 vector unsigned short);
6219 vector signed char vec_andc (vector signed char, vector signed char);
6220 vector unsigned char vec_andc (vector signed char,
6221 vector unsigned char);
6222 vector unsigned char vec_andc (vector unsigned char,
6223 vector signed char);
6224 vector unsigned char vec_andc (vector unsigned char,
6225 vector unsigned char);
6227 vector unsigned char vec_avg (vector unsigned char,
6228 vector unsigned char);
6229 vector signed char vec_avg (vector signed char, vector signed char);
6230 vector unsigned short vec_avg (vector unsigned short,
6231 vector unsigned short);
6232 vector signed short vec_avg (vector signed short, vector signed short);
6233 vector unsigned int vec_avg (vector unsigned int, vector unsigned int);
6234 vector signed int vec_avg (vector signed int, vector signed int);
6236 vector float vec_ceil (vector float);
6238 vector signed int vec_cmpb (vector float, vector float);
6240 vector signed char vec_cmpeq (vector signed char, vector signed char);
6241 vector signed char vec_cmpeq (vector unsigned char,
6242 vector unsigned char);
6243 vector signed short vec_cmpeq (vector signed short,
6244 vector signed short);
6245 vector signed short vec_cmpeq (vector unsigned short,
6246 vector unsigned short);
6247 vector signed int vec_cmpeq (vector signed int, vector signed int);
6248 vector signed int vec_cmpeq (vector unsigned int, vector unsigned int);
6249 vector signed int vec_cmpeq (vector float, vector float);
6251 vector signed int vec_cmpge (vector float, vector float);
6253 vector signed char vec_cmpgt (vector unsigned char,
6254 vector unsigned char);
6255 vector signed char vec_cmpgt (vector signed char, vector signed char);
6256 vector signed short vec_cmpgt (vector unsigned short,
6257 vector unsigned short);
6258 vector signed short vec_cmpgt (vector signed short,
6259 vector signed short);
6260 vector signed int vec_cmpgt (vector unsigned int, vector unsigned int);
6261 vector signed int vec_cmpgt (vector signed int, vector signed int);
6262 vector signed int vec_cmpgt (vector float, vector float);
6264 vector signed int vec_cmple (vector float, vector float);
6266 vector signed char vec_cmplt (vector unsigned char,
6267 vector unsigned char);
6268 vector signed char vec_cmplt (vector signed char, vector signed char);
6269 vector signed short vec_cmplt (vector unsigned short,
6270 vector unsigned short);
6271 vector signed short vec_cmplt (vector signed short,
6272 vector signed short);
6273 vector signed int vec_cmplt (vector unsigned int, vector unsigned int);
6274 vector signed int vec_cmplt (vector signed int, vector signed int);
6275 vector signed int vec_cmplt (vector float, vector float);
6277 vector float vec_ctf (vector unsigned int, const char);
6278 vector float vec_ctf (vector signed int, const char);
6280 vector signed int vec_cts (vector float, const char);
6282 vector unsigned int vec_ctu (vector float, const char);
6284 void vec_dss (const char);
6286 void vec_dssall (void);
6288 void vec_dst (void *, int, const char);
6290 void vec_dstst (void *, int, const char);
6292 void vec_dststt (void *, int, const char);
6294 void vec_dstt (void *, int, const char);
6296 vector float vec_expte (vector float, vector float);
6298 vector float vec_floor (vector float, vector float);
6300 vector float vec_ld (int, vector float *);
6301 vector float vec_ld (int, float *):
6302 vector signed int vec_ld (int, int *);
6303 vector signed int vec_ld (int, vector signed int *);
6304 vector unsigned int vec_ld (int, vector unsigned int *);
6305 vector unsigned int vec_ld (int, unsigned int *);
6306 vector signed short vec_ld (int, short *, vector signed short *);
6307 vector unsigned short vec_ld (int, unsigned short *,
6308 vector unsigned short *);
6309 vector signed char vec_ld (int, signed char *);
6310 vector signed char vec_ld (int, vector signed char *);
6311 vector unsigned char vec_ld (int, unsigned char *);
6312 vector unsigned char vec_ld (int, vector unsigned char *);
6314 vector signed char vec_lde (int, signed char *);
6315 vector unsigned char vec_lde (int, unsigned char *);
6316 vector signed short vec_lde (int, short *);
6317 vector unsigned short vec_lde (int, unsigned short *);
6318 vector float vec_lde (int, float *);
6319 vector signed int vec_lde (int, int *);
6320 vector unsigned int vec_lde (int, unsigned int *);
6322 void float vec_ldl (int, float *);
6323 void float vec_ldl (int, vector float *);
6324 void signed int vec_ldl (int, vector signed int *);
6325 void signed int vec_ldl (int, int *);
6326 void unsigned int vec_ldl (int, unsigned int *);
6327 void unsigned int vec_ldl (int, vector unsigned int *);
6328 void signed short vec_ldl (int, vector signed short *);
6329 void signed short vec_ldl (int, short *);
6330 void unsigned short vec_ldl (int, vector unsigned short *);
6331 void unsigned short vec_ldl (int, unsigned short *);
6332 void signed char vec_ldl (int, vector signed char *);
6333 void signed char vec_ldl (int, signed char *);
6334 void unsigned char vec_ldl (int, vector unsigned char *);
6335 void unsigned char vec_ldl (int, unsigned char *);
6337 vector float vec_loge (vector float);
6339 vector unsigned char vec_lvsl (int, void *, int *);
6341 vector unsigned char vec_lvsr (int, void *, int *);
6343 vector float vec_madd (vector float, vector float, vector float);
6345 vector signed short vec_madds (vector signed short, vector signed short,
6346 vector signed short);
6348 vector unsigned char vec_max (vector signed char, vector unsigned char);
6350 vector unsigned char vec_max (vector unsigned char, vector signed char);
6352 vector unsigned char vec_max (vector unsigned char,
6353 vector unsigned char);
6354 vector signed char vec_max (vector signed char, vector signed char);
6355 vector unsigned short vec_max (vector signed short,
6356 vector unsigned short);
6357 vector unsigned short vec_max (vector unsigned short,
6358 vector signed short);
6359 vector unsigned short vec_max (vector unsigned short,
6360 vector unsigned short);
6361 vector signed short vec_max (vector signed short, vector signed short);
6362 vector unsigned int vec_max (vector signed int, vector unsigned int);
6363 vector unsigned int vec_max (vector unsigned int, vector signed int);
6364 vector unsigned int vec_max (vector unsigned int, vector unsigned int);
6365 vector signed int vec_max (vector signed int, vector signed int);
6366 vector float vec_max (vector float, vector float);
6368 vector signed char vec_mergeh (vector signed char, vector signed char);
6369 vector unsigned char vec_mergeh (vector unsigned char,
6370 vector unsigned char);
6371 vector signed short vec_mergeh (vector signed short,
6372 vector signed short);
6373 vector unsigned short vec_mergeh (vector unsigned short,
6374 vector unsigned short);
6375 vector float vec_mergeh (vector float, vector float);
6376 vector signed int vec_mergeh (vector signed int, vector signed int);
6377 vector unsigned int vec_mergeh (vector unsigned int,
6378 vector unsigned int);
6380 vector signed char vec_mergel (vector signed char, vector signed char);
6381 vector unsigned char vec_mergel (vector unsigned char,
6382 vector unsigned char);
6383 vector signed short vec_mergel (vector signed short,
6384 vector signed short);
6385 vector unsigned short vec_mergel (vector unsigned short,
6386 vector unsigned short);
6387 vector float vec_mergel (vector float, vector float);
6388 vector signed int vec_mergel (vector signed int, vector signed int);
6389 vector unsigned int vec_mergel (vector unsigned int,
6390 vector unsigned int);
6392 vector unsigned short vec_mfvscr (void);
6394 vector unsigned char vec_min (vector signed char, vector unsigned char);
6396 vector unsigned char vec_min (vector unsigned char, vector signed char);
6398 vector unsigned char vec_min (vector unsigned char,
6399 vector unsigned char);
6400 vector signed char vec_min (vector signed char, vector signed char);
6401 vector unsigned short vec_min (vector signed short,
6402 vector unsigned short);
6403 vector unsigned short vec_min (vector unsigned short,
6404 vector signed short);
6405 vector unsigned short vec_min (vector unsigned short,
6406 vector unsigned short);
6407 vector signed short vec_min (vector signed short, vector signed short);
6408 vector unsigned int vec_min (vector signed int, vector unsigned int);
6409 vector unsigned int vec_min (vector unsigned int, vector signed int);
6410 vector unsigned int vec_min (vector unsigned int, vector unsigned int);
6411 vector signed int vec_min (vector signed int, vector signed int);
6412 vector float vec_min (vector float, vector float);
6414 vector signed short vec_mladd (vector signed short, vector signed short,
6415 vector signed short);
6416 vector signed short vec_mladd (vector signed short,
6417 vector unsigned short,
6418 vector unsigned short);
6419 vector signed short vec_mladd (vector unsigned short,
6420 vector signed short,
6421 vector signed short);
6422 vector unsigned short vec_mladd (vector unsigned short,
6423 vector unsigned short,
6424 vector unsigned short);
6426 vector signed short vec_mradds (vector signed short,
6427 vector signed short,
6428 vector signed short);
6430 vector unsigned int vec_msum (vector unsigned char,
6431 vector unsigned char,
6432 vector unsigned int);
6433 vector signed int vec_msum (vector signed char, vector unsigned char,
6435 vector unsigned int vec_msum (vector unsigned short,
6436 vector unsigned short,
6437 vector unsigned int);
6438 vector signed int vec_msum (vector signed short, vector signed short,
6441 vector unsigned int vec_msums (vector unsigned short,
6442 vector unsigned short,
6443 vector unsigned int);
6444 vector signed int vec_msums (vector signed short, vector signed short,
6447 void vec_mtvscr (vector signed int);
6448 void vec_mtvscr (vector unsigned int);
6449 void vec_mtvscr (vector signed short);
6450 void vec_mtvscr (vector unsigned short);
6451 void vec_mtvscr (vector signed char);
6452 void vec_mtvscr (vector unsigned char);
6454 vector unsigned short vec_mule (vector unsigned char,
6455 vector unsigned char);
6456 vector signed short vec_mule (vector signed char, vector signed char);
6457 vector unsigned int vec_mule (vector unsigned short,
6458 vector unsigned short);
6459 vector signed int vec_mule (vector signed short, vector signed short);
6461 vector unsigned short vec_mulo (vector unsigned char,
6462 vector unsigned char);
6463 vector signed short vec_mulo (vector signed char, vector signed char);
6464 vector unsigned int vec_mulo (vector unsigned short,
6465 vector unsigned short);
6466 vector signed int vec_mulo (vector signed short, vector signed short);
6468 vector float vec_nmsub (vector float, vector float, vector float);
6470 vector float vec_nor (vector float, vector float);
6471 vector signed int vec_nor (vector signed int, vector signed int);
6472 vector unsigned int vec_nor (vector unsigned int, vector unsigned int);
6473 vector signed short vec_nor (vector signed short, vector signed short);
6474 vector unsigned short vec_nor (vector unsigned short,
6475 vector unsigned short);
6476 vector signed char vec_nor (vector signed char, vector signed char);
6477 vector unsigned char vec_nor (vector unsigned char,
6478 vector unsigned char);
6480 vector float vec_or (vector float, vector float);
6481 vector float vec_or (vector float, vector signed int);
6482 vector float vec_or (vector signed int, vector float);
6483 vector signed int vec_or (vector signed int, vector signed int);
6484 vector unsigned int vec_or (vector signed int, vector unsigned int);
6485 vector unsigned int vec_or (vector unsigned int, vector signed int);
6486 vector unsigned int vec_or (vector unsigned int, vector unsigned int);
6487 vector signed short vec_or (vector signed short, vector signed short);
6488 vector unsigned short vec_or (vector signed short,
6489 vector unsigned short);
6490 vector unsigned short vec_or (vector unsigned short,
6491 vector signed short);
6492 vector unsigned short vec_or (vector unsigned short,
6493 vector unsigned short);
6494 vector signed char vec_or (vector signed char, vector signed char);
6495 vector unsigned char vec_or (vector signed char, vector unsigned char);
6496 vector unsigned char vec_or (vector unsigned char, vector signed char);
6497 vector unsigned char vec_or (vector unsigned char,
6498 vector unsigned char);
6500 vector signed char vec_pack (vector signed short, vector signed short);
6501 vector unsigned char vec_pack (vector unsigned short,
6502 vector unsigned short);
6503 vector signed short vec_pack (vector signed int, vector signed int);
6504 vector unsigned short vec_pack (vector unsigned int,
6505 vector unsigned int);
6507 vector signed short vec_packpx (vector unsigned int,
6508 vector unsigned int);
6510 vector unsigned char vec_packs (vector unsigned short,
6511 vector unsigned short);
6512 vector signed char vec_packs (vector signed short, vector signed short);
6514 vector unsigned short vec_packs (vector unsigned int,
6515 vector unsigned int);
6516 vector signed short vec_packs (vector signed int, vector signed int);
6518 vector unsigned char vec_packsu (vector unsigned short,
6519 vector unsigned short);
6520 vector unsigned char vec_packsu (vector signed short,
6521 vector signed short);
6522 vector unsigned short vec_packsu (vector unsigned int,
6523 vector unsigned int);
6524 vector unsigned short vec_packsu (vector signed int, vector signed int);
6526 vector float vec_perm (vector float, vector float,
6527 vector unsigned char);
6528 vector signed int vec_perm (vector signed int, vector signed int,
6529 vector unsigned char);
6530 vector unsigned int vec_perm (vector unsigned int, vector unsigned int,
6531 vector unsigned char);
6532 vector signed short vec_perm (vector signed short, vector signed short,
6533 vector unsigned char);
6534 vector unsigned short vec_perm (vector unsigned short,
6535 vector unsigned short,
6536 vector unsigned char);
6537 vector signed char vec_perm (vector signed char, vector signed char,
6538 vector unsigned char);
6539 vector unsigned char vec_perm (vector unsigned char,
6540 vector unsigned char,
6541 vector unsigned char);
6543 vector float vec_re (vector float);
6545 vector signed char vec_rl (vector signed char, vector unsigned char);
6546 vector unsigned char vec_rl (vector unsigned char,
6547 vector unsigned char);
6548 vector signed short vec_rl (vector signed short, vector unsigned short);
6550 vector unsigned short vec_rl (vector unsigned short,
6551 vector unsigned short);
6552 vector signed int vec_rl (vector signed int, vector unsigned int);
6553 vector unsigned int vec_rl (vector unsigned int, vector unsigned int);
6555 vector float vec_round (vector float);
6557 vector float vec_rsqrte (vector float);
6559 vector float vec_sel (vector float, vector float, vector signed int);
6560 vector float vec_sel (vector float, vector float, vector unsigned int);
6561 vector signed int vec_sel (vector signed int, vector signed int,
6563 vector signed int vec_sel (vector signed int, vector signed int,
6564 vector unsigned int);
6565 vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
6567 vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
6568 vector unsigned int);
6569 vector signed short vec_sel (vector signed short, vector signed short,
6570 vector signed short);
6571 vector signed short vec_sel (vector signed short, vector signed short,
6572 vector unsigned short);
6573 vector unsigned short vec_sel (vector unsigned short,
6574 vector unsigned short,
6575 vector signed short);
6576 vector unsigned short vec_sel (vector unsigned short,
6577 vector unsigned short,
6578 vector unsigned short);
6579 vector signed char vec_sel (vector signed char, vector signed char,
6580 vector signed char);
6581 vector signed char vec_sel (vector signed char, vector signed char,
6582 vector unsigned char);
6583 vector unsigned char vec_sel (vector unsigned char,
6584 vector unsigned char,
6585 vector signed char);
6586 vector unsigned char vec_sel (vector unsigned char,
6587 vector unsigned char,
6588 vector unsigned char);
6590 vector signed char vec_sl (vector signed char, vector unsigned char);
6591 vector unsigned char vec_sl (vector unsigned char,
6592 vector unsigned char);
6593 vector signed short vec_sl (vector signed short, vector unsigned short);
6595 vector unsigned short vec_sl (vector unsigned short,
6596 vector unsigned short);
6597 vector signed int vec_sl (vector signed int, vector unsigned int);
6598 vector unsigned int vec_sl (vector unsigned int, vector unsigned int);
6600 vector float vec_sld (vector float, vector float, const char);
6601 vector signed int vec_sld (vector signed int, vector signed int,
6603 vector unsigned int vec_sld (vector unsigned int, vector unsigned int,
6605 vector signed short vec_sld (vector signed short, vector signed short,
6607 vector unsigned short vec_sld (vector unsigned short,
6608 vector unsigned short, const char);
6609 vector signed char vec_sld (vector signed char, vector signed char,
6611 vector unsigned char vec_sld (vector unsigned char,
6612 vector unsigned char,
6615 vector signed int vec_sll (vector signed int, vector unsigned int);
6616 vector signed int vec_sll (vector signed int, vector unsigned short);
6617 vector signed int vec_sll (vector signed int, vector unsigned char);
6618 vector unsigned int vec_sll (vector unsigned int, vector unsigned int);
6619 vector unsigned int vec_sll (vector unsigned int,
6620 vector unsigned short);
6621 vector unsigned int vec_sll (vector unsigned int, vector unsigned char);
6623 vector signed short vec_sll (vector signed short, vector unsigned int);
6624 vector signed short vec_sll (vector signed short,
6625 vector unsigned short);
6626 vector signed short vec_sll (vector signed short, vector unsigned char);
6628 vector unsigned short vec_sll (vector unsigned short,
6629 vector unsigned int);
6630 vector unsigned short vec_sll (vector unsigned short,
6631 vector unsigned short);
6632 vector unsigned short vec_sll (vector unsigned short,
6633 vector unsigned char);
6634 vector signed char vec_sll (vector signed char, vector unsigned int);
6635 vector signed char vec_sll (vector signed char, vector unsigned short);
6636 vector signed char vec_sll (vector signed char, vector unsigned char);
6637 vector unsigned char vec_sll (vector unsigned char,
6638 vector unsigned int);
6639 vector unsigned char vec_sll (vector unsigned char,
6640 vector unsigned short);
6641 vector unsigned char vec_sll (vector unsigned char,
6642 vector unsigned char);
6644 vector float vec_slo (vector float, vector signed char);
6645 vector float vec_slo (vector float, vector unsigned char);
6646 vector signed int vec_slo (vector signed int, vector signed char);
6647 vector signed int vec_slo (vector signed int, vector unsigned char);
6648 vector unsigned int vec_slo (vector unsigned int, vector signed char);
6649 vector unsigned int vec_slo (vector unsigned int, vector unsigned char);
6651 vector signed short vec_slo (vector signed short, vector signed char);
6652 vector signed short vec_slo (vector signed short, vector unsigned char);
6654 vector unsigned short vec_slo (vector unsigned short,
6655 vector signed char);
6656 vector unsigned short vec_slo (vector unsigned short,
6657 vector unsigned char);
6658 vector signed char vec_slo (vector signed char, vector signed char);
6659 vector signed char vec_slo (vector signed char, vector unsigned char);
6660 vector unsigned char vec_slo (vector unsigned char, vector signed char);
6662 vector unsigned char vec_slo (vector unsigned char,
6663 vector unsigned char);
6665 vector signed char vec_splat (vector signed char, const char);
6666 vector unsigned char vec_splat (vector unsigned char, const char);
6667 vector signed short vec_splat (vector signed short, const char);
6668 vector unsigned short vec_splat (vector unsigned short, const char);
6669 vector float vec_splat (vector float, const char);
6670 vector signed int vec_splat (vector signed int, const char);
6671 vector unsigned int vec_splat (vector unsigned int, const char);
6673 vector signed char vec_splat_s8 (const char);
6675 vector signed short vec_splat_s16 (const char);
6677 vector signed int vec_splat_s32 (const char);
6679 vector unsigned char vec_splat_u8 (const char);
6681 vector unsigned short vec_splat_u16 (const char);
6683 vector unsigned int vec_splat_u32 (const char);
6685 vector signed char vec_sr (vector signed char, vector unsigned char);
6686 vector unsigned char vec_sr (vector unsigned char,
6687 vector unsigned char);
6688 vector signed short vec_sr (vector signed short, vector unsigned short);
6690 vector unsigned short vec_sr (vector unsigned short,
6691 vector unsigned short);
6692 vector signed int vec_sr (vector signed int, vector unsigned int);
6693 vector unsigned int vec_sr (vector unsigned int, vector unsigned int);
6695 vector signed char vec_sra (vector signed char, vector unsigned char);
6696 vector unsigned char vec_sra (vector unsigned char,
6697 vector unsigned char);
6698 vector signed short vec_sra (vector signed short,
6699 vector unsigned short);
6700 vector unsigned short vec_sra (vector unsigned short,
6701 vector unsigned short);
6702 vector signed int vec_sra (vector signed int, vector unsigned int);
6703 vector unsigned int vec_sra (vector unsigned int, vector unsigned int);
6705 vector signed int vec_srl (vector signed int, vector unsigned int);
6706 vector signed int vec_srl (vector signed int, vector unsigned short);
6707 vector signed int vec_srl (vector signed int, vector unsigned char);
6708 vector unsigned int vec_srl (vector unsigned int, vector unsigned int);
6709 vector unsigned int vec_srl (vector unsigned int,
6710 vector unsigned short);
6711 vector unsigned int vec_srl (vector unsigned int, vector unsigned char);
6713 vector signed short vec_srl (vector signed short, vector unsigned int);
6714 vector signed short vec_srl (vector signed short,
6715 vector unsigned short);
6716 vector signed short vec_srl (vector signed short, vector unsigned char);
6718 vector unsigned short vec_srl (vector unsigned short,
6719 vector unsigned int);
6720 vector unsigned short vec_srl (vector unsigned short,
6721 vector unsigned short);
6722 vector unsigned short vec_srl (vector unsigned short,
6723 vector unsigned char);
6724 vector signed char vec_srl (vector signed char, vector unsigned int);
6725 vector signed char vec_srl (vector signed char, vector unsigned short);
6726 vector signed char vec_srl (vector signed char, vector unsigned char);
6727 vector unsigned char vec_srl (vector unsigned char,
6728 vector unsigned int);
6729 vector unsigned char vec_srl (vector unsigned char,
6730 vector unsigned short);
6731 vector unsigned char vec_srl (vector unsigned char,
6732 vector unsigned char);
6734 vector float vec_sro (vector float, vector signed char);
6735 vector float vec_sro (vector float, vector unsigned char);
6736 vector signed int vec_sro (vector signed int, vector signed char);
6737 vector signed int vec_sro (vector signed int, vector unsigned char);
6738 vector unsigned int vec_sro (vector unsigned int, vector signed char);
6739 vector unsigned int vec_sro (vector unsigned int, vector unsigned char);
6741 vector signed short vec_sro (vector signed short, vector signed char);
6742 vector signed short vec_sro (vector signed short, vector unsigned char);
6744 vector unsigned short vec_sro (vector unsigned short,
6745 vector signed char);
6746 vector unsigned short vec_sro (vector unsigned short,
6747 vector unsigned char);
6748 vector signed char vec_sro (vector signed char, vector signed char);
6749 vector signed char vec_sro (vector signed char, vector unsigned char);
6750 vector unsigned char vec_sro (vector unsigned char, vector signed char);
6752 vector unsigned char vec_sro (vector unsigned char,
6753 vector unsigned char);
6755 void vec_st (vector float, int, float *);
6756 void vec_st (vector float, int, vector float *);
6757 void vec_st (vector signed int, int, int *);
6758 void vec_st (vector signed int, int, unsigned int *);
6759 void vec_st (vector unsigned int, int, unsigned int *);
6760 void vec_st (vector unsigned int, int, vector unsigned int *);
6761 void vec_st (vector signed short, int, short *);
6762 void vec_st (vector signed short, int, vector unsigned short *);
6763 void vec_st (vector signed short, int, vector signed short *);
6764 void vec_st (vector unsigned short, int, unsigned short *);
6765 void vec_st (vector unsigned short, int, vector unsigned short *);
6766 void vec_st (vector signed char, int, signed char *);
6767 void vec_st (vector signed char, int, unsigned char *);
6768 void vec_st (vector signed char, int, vector signed char *);
6769 void vec_st (vector unsigned char, int, unsigned char *);
6770 void vec_st (vector unsigned char, int, vector unsigned char *);
6772 void vec_ste (vector signed char, int, unsigned char *);
6773 void vec_ste (vector signed char, int, signed char *);
6774 void vec_ste (vector unsigned char, int, unsigned char *);
6775 void vec_ste (vector signed short, int, short *);
6776 void vec_ste (vector signed short, int, unsigned short *);
6777 void vec_ste (vector unsigned short, int, void *);
6778 void vec_ste (vector signed int, int, unsigned int *);
6779 void vec_ste (vector signed int, int, int *);
6780 void vec_ste (vector unsigned int, int, unsigned int *);
6781 void vec_ste (vector float, int, float *);
6783 void vec_stl (vector float, int, vector float *);
6784 void vec_stl (vector float, int, float *);
6785 void vec_stl (vector signed int, int, vector signed int *);
6786 void vec_stl (vector signed int, int, int *);
6787 void vec_stl (vector signed int, int, unsigned int *);
6788 void vec_stl (vector unsigned int, int, vector unsigned int *);
6789 void vec_stl (vector unsigned int, int, unsigned int *);
6790 void vec_stl (vector signed short, int, short *);
6791 void vec_stl (vector signed short, int, unsigned short *);
6792 void vec_stl (vector signed short, int, vector signed short *);
6793 void vec_stl (vector unsigned short, int, unsigned short *);
6794 void vec_stl (vector unsigned short, int, vector signed short *);
6795 void vec_stl (vector signed char, int, signed char *);
6796 void vec_stl (vector signed char, int, unsigned char *);
6797 void vec_stl (vector signed char, int, vector signed char *);
6798 void vec_stl (vector unsigned char, int, unsigned char *);
6799 void vec_stl (vector unsigned char, int, vector unsigned char *);
6801 vector signed char vec_sub (vector signed char, vector signed char);
6802 vector unsigned char vec_sub (vector signed char, vector unsigned char);
6804 vector unsigned char vec_sub (vector unsigned char, vector signed char);
6806 vector unsigned char vec_sub (vector unsigned char,
6807 vector unsigned char);
6808 vector signed short vec_sub (vector signed short, vector signed short);
6809 vector unsigned short vec_sub (vector signed short,
6810 vector unsigned short);
6811 vector unsigned short vec_sub (vector unsigned short,
6812 vector signed short);
6813 vector unsigned short vec_sub (vector unsigned short,
6814 vector unsigned short);
6815 vector signed int vec_sub (vector signed int, vector signed int);
6816 vector unsigned int vec_sub (vector signed int, vector unsigned int);
6817 vector unsigned int vec_sub (vector unsigned int, vector signed int);
6818 vector unsigned int vec_sub (vector unsigned int, vector unsigned int);
6819 vector float vec_sub (vector float, vector float);
6821 vector unsigned int vec_subc (vector unsigned int, vector unsigned int);
6823 vector unsigned char vec_subs (vector signed char,
6824 vector unsigned char);
6825 vector unsigned char vec_subs (vector unsigned char,
6826 vector signed char);
6827 vector unsigned char vec_subs (vector unsigned char,
6828 vector unsigned char);
6829 vector signed char vec_subs (vector signed char, vector signed char);
6830 vector unsigned short vec_subs (vector signed short,
6831 vector unsigned short);
6832 vector unsigned short vec_subs (vector unsigned short,
6833 vector signed short);
6834 vector unsigned short vec_subs (vector unsigned short,
6835 vector unsigned short);
6836 vector signed short vec_subs (vector signed short, vector signed short);
6838 vector unsigned int vec_subs (vector signed int, vector unsigned int);
6839 vector unsigned int vec_subs (vector unsigned int, vector signed int);
6840 vector unsigned int vec_subs (vector unsigned int, vector unsigned int);
6842 vector signed int vec_subs (vector signed int, vector signed int);
6844 vector unsigned int vec_sum4s (vector unsigned char,
6845 vector unsigned int);
6846 vector signed int vec_sum4s (vector signed char, vector signed int);
6847 vector signed int vec_sum4s (vector signed short, vector signed int);
6849 vector signed int vec_sum2s (vector signed int, vector signed int);
6851 vector signed int vec_sums (vector signed int, vector signed int);
6853 vector float vec_trunc (vector float);
6855 vector signed short vec_unpackh (vector signed char);
6856 vector unsigned int vec_unpackh (vector signed short);
6857 vector signed int vec_unpackh (vector signed short);
6859 vector signed short vec_unpackl (vector signed char);
6860 vector unsigned int vec_unpackl (vector signed short);
6861 vector signed int vec_unpackl (vector signed short);
6863 vector float vec_xor (vector float, vector float);
6864 vector float vec_xor (vector float, vector signed int);
6865 vector float vec_xor (vector signed int, vector float);
6866 vector signed int vec_xor (vector signed int, vector signed int);
6867 vector unsigned int vec_xor (vector signed int, vector unsigned int);
6868 vector unsigned int vec_xor (vector unsigned int, vector signed int);
6869 vector unsigned int vec_xor (vector unsigned int, vector unsigned int);
6870 vector signed short vec_xor (vector signed short, vector signed short);
6871 vector unsigned short vec_xor (vector signed short,
6872 vector unsigned short);
6873 vector unsigned short vec_xor (vector unsigned short,
6874 vector signed short);
6875 vector unsigned short vec_xor (vector unsigned short,
6876 vector unsigned short);
6877 vector signed char vec_xor (vector signed char, vector signed char);
6878 vector unsigned char vec_xor (vector signed char, vector unsigned char);
6880 vector unsigned char vec_xor (vector unsigned char, vector signed char);
6882 vector unsigned char vec_xor (vector unsigned char,
6883 vector unsigned char);
6885 vector signed int vec_all_eq (vector signed char, vector unsigned char);
6887 vector signed int vec_all_eq (vector signed char, vector signed char);
6888 vector signed int vec_all_eq (vector unsigned char, vector signed char);
6890 vector signed int vec_all_eq (vector unsigned char,
6891 vector unsigned char);
6892 vector signed int vec_all_eq (vector signed short,
6893 vector unsigned short);
6894 vector signed int vec_all_eq (vector signed short, vector signed short);
6896 vector signed int vec_all_eq (vector unsigned short,
6897 vector signed short);
6898 vector signed int vec_all_eq (vector unsigned short,
6899 vector unsigned short);
6900 vector signed int vec_all_eq (vector signed int, vector unsigned int);
6901 vector signed int vec_all_eq (vector signed int, vector signed int);
6902 vector signed int vec_all_eq (vector unsigned int, vector signed int);
6903 vector signed int vec_all_eq (vector unsigned int, vector unsigned int);
6905 vector signed int vec_all_eq (vector float, vector float);
6907 vector signed int vec_all_ge (vector signed char, vector unsigned char);
6909 vector signed int vec_all_ge (vector unsigned char, vector signed char);
6911 vector signed int vec_all_ge (vector unsigned char,
6912 vector unsigned char);
6913 vector signed int vec_all_ge (vector signed char, vector signed char);
6914 vector signed int vec_all_ge (vector signed short,
6915 vector unsigned short);
6916 vector signed int vec_all_ge (vector unsigned short,
6917 vector signed short);
6918 vector signed int vec_all_ge (vector unsigned short,
6919 vector unsigned short);
6920 vector signed int vec_all_ge (vector signed short, vector signed short);
6922 vector signed int vec_all_ge (vector signed int, vector unsigned int);
6923 vector signed int vec_all_ge (vector unsigned int, vector signed int);
6924 vector signed int vec_all_ge (vector unsigned int, vector unsigned int);
6926 vector signed int vec_all_ge (vector signed int, vector signed int);
6927 vector signed int vec_all_ge (vector float, vector float);
6929 vector signed int vec_all_gt (vector signed char, vector unsigned char);
6931 vector signed int vec_all_gt (vector unsigned char, vector signed char);
6933 vector signed int vec_all_gt (vector unsigned char,
6934 vector unsigned char);
6935 vector signed int vec_all_gt (vector signed char, vector signed char);
6936 vector signed int vec_all_gt (vector signed short,
6937 vector unsigned short);
6938 vector signed int vec_all_gt (vector unsigned short,
6939 vector signed short);
6940 vector signed int vec_all_gt (vector unsigned short,
6941 vector unsigned short);
6942 vector signed int vec_all_gt (vector signed short, vector signed short);
6944 vector signed int vec_all_gt (vector signed int, vector unsigned int);
6945 vector signed int vec_all_gt (vector unsigned int, vector signed int);
6946 vector signed int vec_all_gt (vector unsigned int, vector unsigned int);
6948 vector signed int vec_all_gt (vector signed int, vector signed int);
6949 vector signed int vec_all_gt (vector float, vector float);
6951 vector signed int vec_all_in (vector float, vector float);
6953 vector signed int vec_all_le (vector signed char, vector unsigned char);
6955 vector signed int vec_all_le (vector unsigned char, vector signed char);
6957 vector signed int vec_all_le (vector unsigned char,
6958 vector unsigned char);
6959 vector signed int vec_all_le (vector signed char, vector signed char);
6960 vector signed int vec_all_le (vector signed short,
6961 vector unsigned short);
6962 vector signed int vec_all_le (vector unsigned short,
6963 vector signed short);
6964 vector signed int vec_all_le (vector unsigned short,
6965 vector unsigned short);
6966 vector signed int vec_all_le (vector signed short, vector signed short);
6968 vector signed int vec_all_le (vector signed int, vector unsigned int);
6969 vector signed int vec_all_le (vector unsigned int, vector signed int);
6970 vector signed int vec_all_le (vector unsigned int, vector unsigned int);
6972 vector signed int vec_all_le (vector signed int, vector signed int);
6973 vector signed int vec_all_le (vector float, vector float);
6975 vector signed int vec_all_lt (vector signed char, vector unsigned char);
6977 vector signed int vec_all_lt (vector unsigned char, vector signed char);
6979 vector signed int vec_all_lt (vector unsigned char,
6980 vector unsigned char);
6981 vector signed int vec_all_lt (vector signed char, vector signed char);
6982 vector signed int vec_all_lt (vector signed short,
6983 vector unsigned short);
6984 vector signed int vec_all_lt (vector unsigned short,
6985 vector signed short);
6986 vector signed int vec_all_lt (vector unsigned short,
6987 vector unsigned short);
6988 vector signed int vec_all_lt (vector signed short, vector signed short);
6990 vector signed int vec_all_lt (vector signed int, vector unsigned int);
6991 vector signed int vec_all_lt (vector unsigned int, vector signed int);
6992 vector signed int vec_all_lt (vector unsigned int, vector unsigned int);
6994 vector signed int vec_all_lt (vector signed int, vector signed int);
6995 vector signed int vec_all_lt (vector float, vector float);
6997 vector signed int vec_all_nan (vector float);
6999 vector signed int vec_all_ne (vector signed char, vector unsigned char);
7001 vector signed int vec_all_ne (vector signed char, vector signed char);
7002 vector signed int vec_all_ne (vector unsigned char, vector signed char);
7004 vector signed int vec_all_ne (vector unsigned char,
7005 vector unsigned char);
7006 vector signed int vec_all_ne (vector signed short,
7007 vector unsigned short);
7008 vector signed int vec_all_ne (vector signed short, vector signed short);
7010 vector signed int vec_all_ne (vector unsigned short,
7011 vector signed short);
7012 vector signed int vec_all_ne (vector unsigned short,
7013 vector unsigned short);
7014 vector signed int vec_all_ne (vector signed int, vector unsigned int);
7015 vector signed int vec_all_ne (vector signed int, vector signed int);
7016 vector signed int vec_all_ne (vector unsigned int, vector signed int);
7017 vector signed int vec_all_ne (vector unsigned int, vector unsigned int);
7019 vector signed int vec_all_ne (vector float, vector float);
7021 vector signed int vec_all_nge (vector float, vector float);
7023 vector signed int vec_all_ngt (vector float, vector float);
7025 vector signed int vec_all_nle (vector float, vector float);
7027 vector signed int vec_all_nlt (vector float, vector float);
7029 vector signed int vec_all_numeric (vector float);
7031 vector signed int vec_any_eq (vector signed char, vector unsigned char);
7033 vector signed int vec_any_eq (vector signed char, vector signed char);
7034 vector signed int vec_any_eq (vector unsigned char, vector signed char);
7036 vector signed int vec_any_eq (vector unsigned char,
7037 vector unsigned char);
7038 vector signed int vec_any_eq (vector signed short,
7039 vector unsigned short);
7040 vector signed int vec_any_eq (vector signed short, vector signed short);
7042 vector signed int vec_any_eq (vector unsigned short,
7043 vector signed short);
7044 vector signed int vec_any_eq (vector unsigned short,
7045 vector unsigned short);
7046 vector signed int vec_any_eq (vector signed int, vector unsigned int);
7047 vector signed int vec_any_eq (vector signed int, vector signed int);
7048 vector signed int vec_any_eq (vector unsigned int, vector signed int);
7049 vector signed int vec_any_eq (vector unsigned int, vector unsigned int);
7051 vector signed int vec_any_eq (vector float, vector float);
7053 vector signed int vec_any_ge (vector signed char, vector unsigned char);
7055 vector signed int vec_any_ge (vector unsigned char, vector signed char);
7057 vector signed int vec_any_ge (vector unsigned char,
7058 vector unsigned char);
7059 vector signed int vec_any_ge (vector signed char, vector signed char);
7060 vector signed int vec_any_ge (vector signed short,
7061 vector unsigned short);
7062 vector signed int vec_any_ge (vector unsigned short,
7063 vector signed short);
7064 vector signed int vec_any_ge (vector unsigned short,
7065 vector unsigned short);
7066 vector signed int vec_any_ge (vector signed short, vector signed short);
7068 vector signed int vec_any_ge (vector signed int, vector unsigned int);
7069 vector signed int vec_any_ge (vector unsigned int, vector signed int);
7070 vector signed int vec_any_ge (vector unsigned int, vector unsigned int);
7072 vector signed int vec_any_ge (vector signed int, vector signed int);
7073 vector signed int vec_any_ge (vector float, vector float);
7075 vector signed int vec_any_gt (vector signed char, vector unsigned char);
7077 vector signed int vec_any_gt (vector unsigned char, vector signed char);
7079 vector signed int vec_any_gt (vector unsigned char,
7080 vector unsigned char);
7081 vector signed int vec_any_gt (vector signed char, vector signed char);
7082 vector signed int vec_any_gt (vector signed short,
7083 vector unsigned short);
7084 vector signed int vec_any_gt (vector unsigned short,
7085 vector signed short);
7086 vector signed int vec_any_gt (vector unsigned short,
7087 vector unsigned short);
7088 vector signed int vec_any_gt (vector signed short, vector signed short);
7090 vector signed int vec_any_gt (vector signed int, vector unsigned int);
7091 vector signed int vec_any_gt (vector unsigned int, vector signed int);
7092 vector signed int vec_any_gt (vector unsigned int, vector unsigned int);
7094 vector signed int vec_any_gt (vector signed int, vector signed int);
7095 vector signed int vec_any_gt (vector float, vector float);
7097 vector signed int vec_any_le (vector signed char, vector unsigned char);
7099 vector signed int vec_any_le (vector unsigned char, vector signed char);
7101 vector signed int vec_any_le (vector unsigned char,
7102 vector unsigned char);
7103 vector signed int vec_any_le (vector signed char, vector signed char);
7104 vector signed int vec_any_le (vector signed short,
7105 vector unsigned short);
7106 vector signed int vec_any_le (vector unsigned short,
7107 vector signed short);
7108 vector signed int vec_any_le (vector unsigned short,
7109 vector unsigned short);
7110 vector signed int vec_any_le (vector signed short, vector signed short);
7112 vector signed int vec_any_le (vector signed int, vector unsigned int);
7113 vector signed int vec_any_le (vector unsigned int, vector signed int);
7114 vector signed int vec_any_le (vector unsigned int, vector unsigned int);
7116 vector signed int vec_any_le (vector signed int, vector signed int);
7117 vector signed int vec_any_le (vector float, vector float);
7119 vector signed int vec_any_lt (vector signed char, vector unsigned char);
7121 vector signed int vec_any_lt (vector unsigned char, vector signed char);
7123 vector signed int vec_any_lt (vector unsigned char,
7124 vector unsigned char);
7125 vector signed int vec_any_lt (vector signed char, vector signed char);
7126 vector signed int vec_any_lt (vector signed short,
7127 vector unsigned short);
7128 vector signed int vec_any_lt (vector unsigned short,
7129 vector signed short);
7130 vector signed int vec_any_lt (vector unsigned short,
7131 vector unsigned short);
7132 vector signed int vec_any_lt (vector signed short, vector signed short);
7134 vector signed int vec_any_lt (vector signed int, vector unsigned int);
7135 vector signed int vec_any_lt (vector unsigned int, vector signed int);
7136 vector signed int vec_any_lt (vector unsigned int, vector unsigned int);
7138 vector signed int vec_any_lt (vector signed int, vector signed int);
7139 vector signed int vec_any_lt (vector float, vector float);
7141 vector signed int vec_any_nan (vector float);
7143 vector signed int vec_any_ne (vector signed char, vector unsigned char);
7145 vector signed int vec_any_ne (vector signed char, vector signed char);
7146 vector signed int vec_any_ne (vector unsigned char, vector signed char);
7148 vector signed int vec_any_ne (vector unsigned char,
7149 vector unsigned char);
7150 vector signed int vec_any_ne (vector signed short,
7151 vector unsigned short);
7152 vector signed int vec_any_ne (vector signed short, vector signed short);
7154 vector signed int vec_any_ne (vector unsigned short,
7155 vector signed short);
7156 vector signed int vec_any_ne (vector unsigned short,
7157 vector unsigned short);
7158 vector signed int vec_any_ne (vector signed int, vector unsigned int);
7159 vector signed int vec_any_ne (vector signed int, vector signed int);
7160 vector signed int vec_any_ne (vector unsigned int, vector signed int);
7161 vector signed int vec_any_ne (vector unsigned int, vector unsigned int);
7163 vector signed int vec_any_ne (vector float, vector float);
7165 vector signed int vec_any_nge (vector float, vector float);
7167 vector signed int vec_any_ngt (vector float, vector float);
7169 vector signed int vec_any_nle (vector float, vector float);
7171 vector signed int vec_any_nlt (vector float, vector float);
7173 vector signed int vec_any_numeric (vector float);
7175 vector signed int vec_any_out (vector float, vector float);
7179 @section Pragmas Accepted by GCC
7183 GCC supports several types of pragmas, primarily in order to compile
7184 code originally written for other compilers. Note that in general
7185 we do not recommend the use of pragmas; @xref{Function Attributes},
7186 for further explanation.
7190 * RS/6000 and PowerPC Pragmas::
7197 @subsection ARM Pragmas
7199 The ARM target defines pragmas for controlling the default addition of
7200 @code{long_call} and @code{short_call} attributes to functions.
7201 @xref{Function Attributes}, for information about the effects of these
7206 @cindex pragma, long_calls
7207 Set all subsequent functions to have the @code{long_call} attribute.
7210 @cindex pragma, no_long_calls
7211 Set all subsequent functions to have the @code{short_call} attribute.
7213 @item long_calls_off
7214 @cindex pragma, long_calls_off
7215 Do not affect the @code{long_call} or @code{short_call} attributes of
7216 subsequent functions.
7219 @node RS/6000 and PowerPC Pragmas
7220 @subsection RS/6000 and PowerPC Pragmas
7222 The RS/6000 and PowerPC targets define one pragma for controlling
7223 whether or not the @code{longcall} attribute is added to function
7224 declarations by default. This pragma overrides the @option{-mlongcall}
7225 option, but not the @code{longcall} and @code{shortcall} attributes.
7226 @xref{RS/6000 and PowerPC Options}, for more information about when long
7227 calls are and are not necessary.
7231 @cindex pragma, longcall
7232 Apply the @code{longcall} attribute to all subsequent function
7236 Do not apply the @code{longcall} attribute to subsequent function
7240 @c Describe c4x pragmas here.
7241 @c Describe h8300 pragmas here.
7242 @c Describe i370 pragmas here.
7243 @c Describe i960 pragmas here.
7244 @c Describe sh pragmas here.
7245 @c Describe v850 pragmas here.
7247 @node Darwin Pragmas
7248 @subsection Darwin Pragmas
7250 The following pragmas are available for all architectures running the
7251 Darwin operating system. These are useful for compatibility with other
7255 @item mark @var{tokens}@dots{}
7256 @cindex pragma, mark
7257 This pragma is accepted, but has no effect.
7259 @item options align=@var{alignment}
7260 @cindex pragma, options align
7261 This pragma sets the alignment of fields in structures. The values of
7262 @var{alignment} may be @code{mac68k}, to emulate m68k alignment, or
7263 @code{power}, to emulate PowerPC alignment. Uses of this pragma nest
7264 properly; to restore the previous setting, use @code{reset} for the
7267 @item segment @var{tokens}@dots{}
7268 @cindex pragma, segment
7269 This pragma is accepted, but has no effect.
7271 @item unused (@var{var} [, @var{var}]@dots{})
7272 @cindex pragma, unused
7273 This pragma declares variables to be possibly unused. GCC will not
7274 produce warnings for the listed variables. The effect is similar to
7275 that of the @code{unused} attribute, except that this pragma may appear
7276 anywhere within the variables' scopes.
7279 @node Solaris Pragmas
7280 @subsection Solaris Pragmas
7282 For compatibility with the SunPRO compiler, the following pragma
7286 @item redefine_extname @var{oldname} @var{newname}
7287 @cindex pragma, redefine_extname
7289 This pragma gives the C function @var{oldname} the assembler label
7290 @var{newname}. The pragma must appear before the function declaration.
7291 This pragma is equivalent to the asm labels extension (@pxref{Asm
7292 Labels}). The preprocessor defines @code{__PRAGMA_REDEFINE_EXTNAME}
7293 if the pragma is available.
7297 @subsection Tru64 Pragmas
7299 For compatibility with the Compaq C compiler, the following pragma
7303 @item extern_prefix @var{string}
7304 @cindex pragma, extern_prefix
7306 This pragma renames all subsequent function and variable declarations
7307 such that @var{string} is prepended to the name. This effect may be
7308 terminated by using another @code{extern_prefix} pragma with the
7311 This pragma is similar in intent to to the asm labels extension
7312 (@pxref{Asm Labels}) in that the system programmer wants to change
7313 the assembly-level ABI without changing the source-level API. The
7314 preprocessor defines @code{__PRAGMA_EXTERN_PREFIX} if the pragma is
7318 @node Unnamed Fields
7319 @section Unnamed struct/union fields within structs/unions.
7323 For compatibility with other compilers, GCC allows you to define
7324 a structure or union that contains, as fields, structures and unions
7325 without names. For example:
7338 In this example, the user would be able to access members of the unnamed
7339 union with code like @samp{foo.b}. Note that only unnamed structs and
7340 unions are allowed, you may not have, for example, an unnamed
7343 You must never create such structures that cause ambiguous field definitions.
7344 For example, this structure:
7355 It is ambiguous which @code{a} is being referred to with @samp{foo.a}.
7356 Such constructs are not supported and must be avoided. In the future,
7357 such constructs may be detected and treated as compilation errors.
7360 @section Thread-Local Storage
7361 @cindex Thread-Local Storage
7362 @cindex @acronym{TLS}
7365 Thread-local storage (@acronym{TLS}) is a mechanism by which variables
7366 are allocated such that there is one instance of the variable per extant
7367 thread. The run-time model GCC uses to implement this originates
7368 in the IA-64 processor-specific ABI, but has since been migrated
7369 to other processors as well. It requires significant support from
7370 the linker (@command{ld}), dynamic linker (@command{ld.so}), and
7371 system libraries (@file{libc.so} and @file{libpthread.so}), so it
7372 is not available everywhere.
7374 At the user level, the extension is visible with a new storage
7375 class keyword: @code{__thread}. For example:
7379 extern __thread struct state s;
7380 static __thread char *p;
7383 The @code{__thread} specifier may be used alone, with the @code{extern}
7384 or @code{static} specifiers, but with no other storage class specifier.
7385 When used with @code{extern} or @code{static}, @code{__thread} must appear
7386 immediately after the other storage class specifier.
7388 The @code{__thread} specifier may be applied to any global, file-scoped
7389 static, function-scoped static, or static data member of a class. It may
7390 not be applied to block-scoped automatic or non-static data member.
7392 When the address-of operator is applied to a thread-local variable, it is
7393 evaluated at run-time and returns the address of the current thread's
7394 instance of that variable. An address so obtained may be used by any
7395 thread. When a thread terminates, any pointers to thread-local variables
7396 in that thread become invalid.
7398 No static initialization may refer to the address of a thread-local variable.
7400 In C++, if an initializer is present for a thread-local variable, it must
7401 be a @var{constant-expression}, as defined in 5.19.2 of the ANSI/ISO C++
7404 See @uref{http://people.redhat.com/drepper/tls.pdf,
7405 ELF Handling For Thread-Local Storage} for a detailed explanation of
7406 the four thread-local storage addressing models, and how the run-time
7407 is expected to function.
7410 * C99 Thread-Local Edits::
7411 * C++98 Thread-Local Edits::
7414 @node C99 Thread-Local Edits
7415 @subsection ISO/IEC 9899:1999 Edits for Thread-Local Storage
7417 The following are a set of changes to ISO/IEC 9899:1999 (aka C99)
7418 that document the exact semantics of the language extension.
7422 @cite{5.1.2 Execution environments}
7424 Add new text after paragraph 1
7427 Within either execution environment, a @dfn{thread} is a flow of
7428 control within a program. It is implementation defined whether
7429 or not there may be more than one thread associated with a program.
7430 It is implementation defined how threads beyond the first are
7431 created, the name and type of the function called at thread
7432 startup, and how threads may be terminated. However, objects
7433 with thread storage duration shall be initialized before thread
7438 @cite{6.2.4 Storage durations of objects}
7440 Add new text before paragraph 3
7443 An object whose identifier is declared with the storage-class
7444 specifier @w{@code{__thread}} has @dfn{thread storage duration}.
7445 Its lifetime is the entire execution of the thread, and its
7446 stored value is initialized only once, prior to thread startup.
7450 @cite{6.4.1 Keywords}
7452 Add @code{__thread}.
7455 @cite{6.7.1 Storage-class specifiers}
7457 Add @code{__thread} to the list of storage class specifiers in
7460 Change paragraph 2 to
7463 With the exception of @code{__thread}, at most one storage-class
7464 specifier may be given [@dots{}]. The @code{__thread} specifier may
7465 be used alone, or immediately following @code{extern} or
7469 Add new text after paragraph 6
7472 The declaration of an identifier for a variable that has
7473 block scope that specifies @code{__thread} shall also
7474 specify either @code{extern} or @code{static}.
7476 The @code{__thread} specifier shall be used only with
7481 @node C++98 Thread-Local Edits
7482 @subsection ISO/IEC 14882:1998 Edits for Thread-Local Storage
7484 The following are a set of changes to ISO/IEC 14882:1998 (aka C++98)
7485 that document the exact semantics of the language extension.
7489 @b{[intro.execution]}
7491 New text after paragraph 4
7494 A @dfn{thread} is a flow of control within the abstract machine.
7495 It is implementation defined whether or not there may be more than
7499 New text after paragraph 7
7502 It is unspecified whether additional action must be taken to
7503 ensure when and whether side effects are visible to other threads.
7509 Add @code{__thread}.
7512 @b{[basic.start.main]}
7514 Add after paragraph 5
7517 The thread that begins execution at the @code{main} function is called
7518 the @dfn{main thread}. It is implementation defined how functions
7519 beginning threads other than the main thread are designated or typed.
7520 A function so designated, as well as the @code{main} function, is called
7521 a @dfn{thread startup function}. It is implementation defined what
7522 happens if a thread startup function returns. It is implementation
7523 defined what happens to other threads when any thread calls @code{exit}.
7527 @b{[basic.start.init]}
7529 Add after paragraph 4
7532 The storage for an object of thread storage duration shall be
7533 statically initialized before the first statement of the thread startup
7534 function. An object of thread storage duration shall not require
7535 dynamic initialization.
7539 @b{[basic.start.term]}
7541 Add after paragraph 3
7544 The type of an object with thread storage duration shall not have a
7545 non-trivial destructor, nor shall it be an array type whose elements
7546 (directly or indirectly) have non-trivial destructors.
7552 Add ``thread storage duration'' to the list in paragraph 1.
7557 Thread, static, and automatic storage durations are associated with
7558 objects introduced by declarations [@dots{}].
7561 Add @code{__thread} to the list of specifiers in paragraph 3.
7564 @b{[basic.stc.thread]}
7566 New section before @b{[basic.stc.static]}
7569 The keyword @code{__thread} applied to a non-local object gives the
7570 object thread storage duration.
7572 A local variable or class data member declared both @code{static}
7573 and @code{__thread} gives the variable or member thread storage
7578 @b{[basic.stc.static]}
7583 All objects which have neither thread storage duration, dynamic
7584 storage duration nor are local [@dots{}].
7590 Add @code{__thread} to the list in paragraph 1.
7595 With the exception of @code{__thread}, at most one
7596 @var{storage-class-specifier} shall appear in a given
7597 @var{decl-specifier-seq}. The @code{__thread} specifier may
7598 be used alone, or immediately following the @code{extern} or
7599 @code{static} specifiers. [@dots{}]
7602 Add after paragraph 5
7605 The @code{__thread} specifier can be applied only to the names of objects
7606 and to anonymous unions.
7612 Add after paragraph 6
7615 Non-@code{static} members shall not be @code{__thread}.
7619 @node C++ Extensions
7620 @chapter Extensions to the C++ Language
7621 @cindex extensions, C++ language
7622 @cindex C++ language extensions
7624 The GNU compiler provides these extensions to the C++ language (and you
7625 can also use most of the C language extensions in your C++ programs). If you
7626 want to write code that checks whether these features are available, you can
7627 test for the GNU compiler the same way as for C programs: check for a
7628 predefined macro @code{__GNUC__}. You can also use @code{__GNUG__} to
7629 test specifically for GNU C++ (@pxref{Common Predefined Macros,,
7630 Predefined Macros,cpp,The GNU C Preprocessor}).
7633 * Min and Max:: C++ Minimum and maximum operators.
7634 * Volatiles:: What constitutes an access to a volatile object.
7635 * Restricted Pointers:: C99 restricted pointers and references.
7636 * Vague Linkage:: Where G++ puts inlines, vtables and such.
7637 * C++ Interface:: You can use a single C++ header file for both
7638 declarations and definitions.
7639 * Template Instantiation:: Methods for ensuring that exactly one copy of
7640 each needed template instantiation is emitted.
7641 * Bound member functions:: You can extract a function pointer to the
7642 method denoted by a @samp{->*} or @samp{.*} expression.
7643 * C++ Attributes:: Variable, function, and type attributes for C++ only.
7644 * Java Exceptions:: Tweaking exception handling to work with Java.
7645 * Deprecated Features:: Things will disappear from g++.
7646 * Backwards Compatibility:: Compatibilities with earlier definitions of C++.
7650 @section Minimum and Maximum Operators in C++
7652 It is very convenient to have operators which return the ``minimum'' or the
7653 ``maximum'' of two arguments. In GNU C++ (but not in GNU C),
7656 @item @var{a} <? @var{b}
7658 @cindex minimum operator
7659 is the @dfn{minimum}, returning the smaller of the numeric values
7660 @var{a} and @var{b};
7662 @item @var{a} >? @var{b}
7664 @cindex maximum operator
7665 is the @dfn{maximum}, returning the larger of the numeric values @var{a}
7669 These operations are not primitive in ordinary C++, since you can
7670 use a macro to return the minimum of two things in C++, as in the
7674 #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))
7678 You might then use @w{@samp{int min = MIN (i, j);}} to set @var{min} to
7679 the minimum value of variables @var{i} and @var{j}.
7681 However, side effects in @code{X} or @code{Y} may cause unintended
7682 behavior. For example, @code{MIN (i++, j++)} will fail, incrementing
7683 the smaller counter twice. The GNU C @code{typeof} extension allows you
7684 to write safe macros that avoid this kind of problem (@pxref{Typeof}).
7685 However, writing @code{MIN} and @code{MAX} as macros also forces you to
7686 use function-call notation for a fundamental arithmetic operation.
7687 Using GNU C++ extensions, you can write @w{@samp{int min = i <? j;}}
7690 Since @code{<?} and @code{>?} are built into the compiler, they properly
7691 handle expressions with side-effects; @w{@samp{int min = i++ <? j++;}}
7695 @section When is a Volatile Object Accessed?
7696 @cindex accessing volatiles
7697 @cindex volatile read
7698 @cindex volatile write
7699 @cindex volatile access
7701 Both the C and C++ standard have the concept of volatile objects. These
7702 are normally accessed by pointers and used for accessing hardware. The
7703 standards encourage compilers to refrain from optimizations
7704 concerning accesses to volatile objects that it might perform on
7705 non-volatile objects. The C standard leaves it implementation defined
7706 as to what constitutes a volatile access. The C++ standard omits to
7707 specify this, except to say that C++ should behave in a similar manner
7708 to C with respect to volatiles, where possible. The minimum either
7709 standard specifies is that at a sequence point all previous accesses to
7710 volatile objects have stabilized and no subsequent accesses have
7711 occurred. Thus an implementation is free to reorder and combine
7712 volatile accesses which occur between sequence points, but cannot do so
7713 for accesses across a sequence point. The use of volatiles does not
7714 allow you to violate the restriction on updating objects multiple times
7715 within a sequence point.
7717 In most expressions, it is intuitively obvious what is a read and what is
7718 a write. For instance
7721 volatile int *dst = @var{somevalue};
7722 volatile int *src = @var{someothervalue};
7727 will cause a read of the volatile object pointed to by @var{src} and stores the
7728 value into the volatile object pointed to by @var{dst}. There is no
7729 guarantee that these reads and writes are atomic, especially for objects
7730 larger than @code{int}.
7732 Less obvious expressions are where something which looks like an access
7733 is used in a void context. An example would be,
7736 volatile int *src = @var{somevalue};
7740 With C, such expressions are rvalues, and as rvalues cause a read of
7741 the object, GCC interprets this as a read of the volatile being pointed
7742 to. The C++ standard specifies that such expressions do not undergo
7743 lvalue to rvalue conversion, and that the type of the dereferenced
7744 object may be incomplete. The C++ standard does not specify explicitly
7745 that it is this lvalue to rvalue conversion which is responsible for
7746 causing an access. However, there is reason to believe that it is,
7747 because otherwise certain simple expressions become undefined. However,
7748 because it would surprise most programmers, G++ treats dereferencing a
7749 pointer to volatile object of complete type in a void context as a read
7750 of the object. When the object has incomplete type, G++ issues a
7755 struct T @{int m;@};
7756 volatile S *ptr1 = @var{somevalue};
7757 volatile T *ptr2 = @var{somevalue};
7762 In this example, a warning is issued for @code{*ptr1}, and @code{*ptr2}
7763 causes a read of the object pointed to. If you wish to force an error on
7764 the first case, you must force a conversion to rvalue with, for instance
7765 a static cast, @code{static_cast<S>(*ptr1)}.
7767 When using a reference to volatile, G++ does not treat equivalent
7768 expressions as accesses to volatiles, but instead issues a warning that
7769 no volatile is accessed. The rationale for this is that otherwise it
7770 becomes difficult to determine where volatile access occur, and not
7771 possible to ignore the return value from functions returning volatile
7772 references. Again, if you wish to force a read, cast the reference to
7775 @node Restricted Pointers
7776 @section Restricting Pointer Aliasing
7777 @cindex restricted pointers
7778 @cindex restricted references
7779 @cindex restricted this pointer
7781 As with gcc, g++ understands the C99 feature of restricted pointers,
7782 specified with the @code{__restrict__}, or @code{__restrict} type
7783 qualifier. Because you cannot compile C++ by specifying the @option{-std=c99}
7784 language flag, @code{restrict} is not a keyword in C++.
7786 In addition to allowing restricted pointers, you can specify restricted
7787 references, which indicate that the reference is not aliased in the local
7791 void fn (int *__restrict__ rptr, int &__restrict__ rref)
7798 In the body of @code{fn}, @var{rptr} points to an unaliased integer and
7799 @var{rref} refers to a (different) unaliased integer.
7801 You may also specify whether a member function's @var{this} pointer is
7802 unaliased by using @code{__restrict__} as a member function qualifier.
7805 void T::fn () __restrict__
7812 Within the body of @code{T::fn}, @var{this} will have the effective
7813 definition @code{T *__restrict__ const this}. Notice that the
7814 interpretation of a @code{__restrict__} member function qualifier is
7815 different to that of @code{const} or @code{volatile} qualifier, in that it
7816 is applied to the pointer rather than the object. This is consistent with
7817 other compilers which implement restricted pointers.
7819 As with all outermost parameter qualifiers, @code{__restrict__} is
7820 ignored in function definition matching. This means you only need to
7821 specify @code{__restrict__} in a function definition, rather than
7822 in a function prototype as well.
7825 @section Vague Linkage
7826 @cindex vague linkage
7828 There are several constructs in C++ which require space in the object
7829 file but are not clearly tied to a single translation unit. We say that
7830 these constructs have ``vague linkage''. Typically such constructs are
7831 emitted wherever they are needed, though sometimes we can be more
7835 @item Inline Functions
7836 Inline functions are typically defined in a header file which can be
7837 included in many different compilations. Hopefully they can usually be
7838 inlined, but sometimes an out-of-line copy is necessary, if the address
7839 of the function is taken or if inlining fails. In general, we emit an
7840 out-of-line copy in all translation units where one is needed. As an
7841 exception, we only emit inline virtual functions with the vtable, since
7842 it will always require a copy.
7844 Local static variables and string constants used in an inline function
7845 are also considered to have vague linkage, since they must be shared
7846 between all inlined and out-of-line instances of the function.
7850 C++ virtual functions are implemented in most compilers using a lookup
7851 table, known as a vtable. The vtable contains pointers to the virtual
7852 functions provided by a class, and each object of the class contains a
7853 pointer to its vtable (or vtables, in some multiple-inheritance
7854 situations). If the class declares any non-inline, non-pure virtual
7855 functions, the first one is chosen as the ``key method'' for the class,
7856 and the vtable is only emitted in the translation unit where the key
7859 @emph{Note:} If the chosen key method is later defined as inline, the
7860 vtable will still be emitted in every translation unit which defines it.
7861 Make sure that any inline virtuals are declared inline in the class
7862 body, even if they are not defined there.
7864 @item type_info objects
7867 C++ requires information about types to be written out in order to
7868 implement @samp{dynamic_cast}, @samp{typeid} and exception handling.
7869 For polymorphic classes (classes with virtual functions), the type_info
7870 object is written out along with the vtable so that @samp{dynamic_cast}
7871 can determine the dynamic type of a class object at runtime. For all
7872 other types, we write out the type_info object when it is used: when
7873 applying @samp{typeid} to an expression, throwing an object, or
7874 referring to a type in a catch clause or exception specification.
7876 @item Template Instantiations
7877 Most everything in this section also applies to template instantiations,
7878 but there are other options as well.
7879 @xref{Template Instantiation,,Where's the Template?}.
7883 When used with GNU ld version 2.8 or later on an ELF system such as
7884 Linux/GNU or Solaris 2, or on Microsoft Windows, duplicate copies of
7885 these constructs will be discarded at link time. This is known as
7888 On targets that don't support COMDAT, but do support weak symbols, GCC
7889 will use them. This way one copy will override all the others, but
7890 the unused copies will still take up space in the executable.
7892 For targets which do not support either COMDAT or weak symbols,
7893 most entities with vague linkage will be emitted as local symbols to
7894 avoid duplicate definition errors from the linker. This will not happen
7895 for local statics in inlines, however, as having multiple copies will
7896 almost certainly break things.
7898 @xref{C++ Interface,,Declarations and Definitions in One Header}, for
7899 another way to control placement of these constructs.
7902 @section Declarations and Definitions in One Header
7904 @cindex interface and implementation headers, C++
7905 @cindex C++ interface and implementation headers
7906 C++ object definitions can be quite complex. In principle, your source
7907 code will need two kinds of things for each object that you use across
7908 more than one source file. First, you need an @dfn{interface}
7909 specification, describing its structure with type declarations and
7910 function prototypes. Second, you need the @dfn{implementation} itself.
7911 It can be tedious to maintain a separate interface description in a
7912 header file, in parallel to the actual implementation. It is also
7913 dangerous, since separate interface and implementation definitions may
7914 not remain parallel.
7916 @cindex pragmas, interface and implementation
7917 With GNU C++, you can use a single header file for both purposes.
7920 @emph{Warning:} The mechanism to specify this is in transition. For the
7921 nonce, you must use one of two @code{#pragma} commands; in a future
7922 release of GNU C++, an alternative mechanism will make these
7923 @code{#pragma} commands unnecessary.
7926 The header file contains the full definitions, but is marked with
7927 @samp{#pragma interface} in the source code. This allows the compiler
7928 to use the header file only as an interface specification when ordinary
7929 source files incorporate it with @code{#include}. In the single source
7930 file where the full implementation belongs, you can use either a naming
7931 convention or @samp{#pragma implementation} to indicate this alternate
7932 use of the header file.
7935 @item #pragma interface
7936 @itemx #pragma interface "@var{subdir}/@var{objects}.h"
7937 @kindex #pragma interface
7938 Use this directive in @emph{header files} that define object classes, to save
7939 space in most of the object files that use those classes. Normally,
7940 local copies of certain information (backup copies of inline member
7941 functions, debugging information, and the internal tables that implement
7942 virtual functions) must be kept in each object file that includes class
7943 definitions. You can use this pragma to avoid such duplication. When a
7944 header file containing @samp{#pragma interface} is included in a
7945 compilation, this auxiliary information will not be generated (unless
7946 the main input source file itself uses @samp{#pragma implementation}).
7947 Instead, the object files will contain references to be resolved at link
7950 The second form of this directive is useful for the case where you have
7951 multiple headers with the same name in different directories. If you
7952 use this form, you must specify the same string to @samp{#pragma
7955 @item #pragma implementation
7956 @itemx #pragma implementation "@var{objects}.h"
7957 @kindex #pragma implementation
7958 Use this pragma in a @emph{main input file}, when you want full output from
7959 included header files to be generated (and made globally visible). The
7960 included header file, in turn, should use @samp{#pragma interface}.
7961 Backup copies of inline member functions, debugging information, and the
7962 internal tables used to implement virtual functions are all generated in
7963 implementation files.
7965 @cindex implied @code{#pragma implementation}
7966 @cindex @code{#pragma implementation}, implied
7967 @cindex naming convention, implementation headers
7968 If you use @samp{#pragma implementation} with no argument, it applies to
7969 an include file with the same basename@footnote{A file's @dfn{basename}
7970 was the name stripped of all leading path information and of trailing
7971 suffixes, such as @samp{.h} or @samp{.C} or @samp{.cc}.} as your source
7972 file. For example, in @file{allclass.cc}, giving just
7973 @samp{#pragma implementation}
7974 by itself is equivalent to @samp{#pragma implementation "allclass.h"}.
7976 In versions of GNU C++ prior to 2.6.0 @file{allclass.h} was treated as
7977 an implementation file whenever you would include it from
7978 @file{allclass.cc} even if you never specified @samp{#pragma
7979 implementation}. This was deemed to be more trouble than it was worth,
7980 however, and disabled.
7982 If you use an explicit @samp{#pragma implementation}, it must appear in
7983 your source file @emph{before} you include the affected header files.
7985 Use the string argument if you want a single implementation file to
7986 include code from multiple header files. (You must also use
7987 @samp{#include} to include the header file; @samp{#pragma
7988 implementation} only specifies how to use the file---it doesn't actually
7991 There is no way to split up the contents of a single header file into
7992 multiple implementation files.
7995 @cindex inlining and C++ pragmas
7996 @cindex C++ pragmas, effect on inlining
7997 @cindex pragmas in C++, effect on inlining
7998 @samp{#pragma implementation} and @samp{#pragma interface} also have an
7999 effect on function inlining.
8001 If you define a class in a header file marked with @samp{#pragma
8002 interface}, the effect on a function defined in that class is similar to
8003 an explicit @code{extern} declaration---the compiler emits no code at
8004 all to define an independent version of the function. Its definition
8005 is used only for inlining with its callers.
8007 @opindex fno-implement-inlines
8008 Conversely, when you include the same header file in a main source file
8009 that declares it as @samp{#pragma implementation}, the compiler emits
8010 code for the function itself; this defines a version of the function
8011 that can be found via pointers (or by callers compiled without
8012 inlining). If all calls to the function can be inlined, you can avoid
8013 emitting the function by compiling with @option{-fno-implement-inlines}.
8014 If any calls were not inlined, you will get linker errors.
8016 @node Template Instantiation
8017 @section Where's the Template?
8018 @cindex template instantiation
8020 C++ templates are the first language feature to require more
8021 intelligence from the environment than one usually finds on a UNIX
8022 system. Somehow the compiler and linker have to make sure that each
8023 template instance occurs exactly once in the executable if it is needed,
8024 and not at all otherwise. There are two basic approaches to this
8025 problem, which I will refer to as the Borland model and the Cfront model.
8029 Borland C++ solved the template instantiation problem by adding the code
8030 equivalent of common blocks to their linker; the compiler emits template
8031 instances in each translation unit that uses them, and the linker
8032 collapses them together. The advantage of this model is that the linker
8033 only has to consider the object files themselves; there is no external
8034 complexity to worry about. This disadvantage is that compilation time
8035 is increased because the template code is being compiled repeatedly.
8036 Code written for this model tends to include definitions of all
8037 templates in the header file, since they must be seen to be
8041 The AT&T C++ translator, Cfront, solved the template instantiation
8042 problem by creating the notion of a template repository, an
8043 automatically maintained place where template instances are stored. A
8044 more modern version of the repository works as follows: As individual
8045 object files are built, the compiler places any template definitions and
8046 instantiations encountered in the repository. At link time, the link
8047 wrapper adds in the objects in the repository and compiles any needed
8048 instances that were not previously emitted. The advantages of this
8049 model are more optimal compilation speed and the ability to use the
8050 system linker; to implement the Borland model a compiler vendor also
8051 needs to replace the linker. The disadvantages are vastly increased
8052 complexity, and thus potential for error; for some code this can be
8053 just as transparent, but in practice it can been very difficult to build
8054 multiple programs in one directory and one program in multiple
8055 directories. Code written for this model tends to separate definitions
8056 of non-inline member templates into a separate file, which should be
8057 compiled separately.
8060 When used with GNU ld version 2.8 or later on an ELF system such as
8061 Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the
8062 Borland model. On other systems, g++ implements neither automatic
8065 A future version of g++ will support a hybrid model whereby the compiler
8066 will emit any instantiations for which the template definition is
8067 included in the compile, and store template definitions and
8068 instantiation context information into the object file for the rest.
8069 The link wrapper will extract that information as necessary and invoke
8070 the compiler to produce the remaining instantiations. The linker will
8071 then combine duplicate instantiations.
8073 In the mean time, you have the following options for dealing with
8074 template instantiations:
8079 Compile your template-using code with @option{-frepo}. The compiler will
8080 generate files with the extension @samp{.rpo} listing all of the
8081 template instantiations used in the corresponding object files which
8082 could be instantiated there; the link wrapper, @samp{collect2}, will
8083 then update the @samp{.rpo} files to tell the compiler where to place
8084 those instantiations and rebuild any affected object files. The
8085 link-time overhead is negligible after the first pass, as the compiler
8086 will continue to place the instantiations in the same files.
8088 This is your best option for application code written for the Borland
8089 model, as it will just work. Code written for the Cfront model will
8090 need to be modified so that the template definitions are available at
8091 one or more points of instantiation; usually this is as simple as adding
8092 @code{#include <tmethods.cc>} to the end of each template header.
8094 For library code, if you want the library to provide all of the template
8095 instantiations it needs, just try to link all of its object files
8096 together; the link will fail, but cause the instantiations to be
8097 generated as a side effect. Be warned, however, that this may cause
8098 conflicts if multiple libraries try to provide the same instantiations.
8099 For greater control, use explicit instantiation as described in the next
8103 @opindex fno-implicit-templates
8104 Compile your code with @option{-fno-implicit-templates} to disable the
8105 implicit generation of template instances, and explicitly instantiate
8106 all the ones you use. This approach requires more knowledge of exactly
8107 which instances you need than do the others, but it's less
8108 mysterious and allows greater control. You can scatter the explicit
8109 instantiations throughout your program, perhaps putting them in the
8110 translation units where the instances are used or the translation units
8111 that define the templates themselves; you can put all of the explicit
8112 instantiations you need into one big file; or you can create small files
8119 template class Foo<int>;
8120 template ostream& operator <<
8121 (ostream&, const Foo<int>&);
8124 for each of the instances you need, and create a template instantiation
8127 If you are using Cfront-model code, you can probably get away with not
8128 using @option{-fno-implicit-templates} when compiling files that don't
8129 @samp{#include} the member template definitions.
8131 If you use one big file to do the instantiations, you may want to
8132 compile it without @option{-fno-implicit-templates} so you get all of the
8133 instances required by your explicit instantiations (but not by any
8134 other files) without having to specify them as well.
8136 g++ has extended the template instantiation syntax given in the ISO
8137 standard to allow forward declaration of explicit instantiations
8138 (with @code{extern}), instantiation of the compiler support data for a
8139 template class (i.e.@: the vtable) without instantiating any of its
8140 members (with @code{inline}), and instantiation of only the static data
8141 members of a template class, without the support data or member
8142 functions (with (@code{static}):
8145 extern template int max (int, int);
8146 inline template class Foo<int>;
8147 static template class Foo<int>;
8151 Do nothing. Pretend g++ does implement automatic instantiation
8152 management. Code written for the Borland model will work fine, but
8153 each translation unit will contain instances of each of the templates it
8154 uses. In a large program, this can lead to an unacceptable amount of code
8157 @xref{C++ Interface,,Declarations and Definitions in One Header}, for
8158 more discussion of these pragmas.
8161 @node Bound member functions
8162 @section Extracting the function pointer from a bound pointer to member function
8164 @cindex pointer to member function
8165 @cindex bound pointer to member function
8167 In C++, pointer to member functions (PMFs) are implemented using a wide
8168 pointer of sorts to handle all the possible call mechanisms; the PMF
8169 needs to store information about how to adjust the @samp{this} pointer,
8170 and if the function pointed to is virtual, where to find the vtable, and
8171 where in the vtable to look for the member function. If you are using
8172 PMFs in an inner loop, you should really reconsider that decision. If
8173 that is not an option, you can extract the pointer to the function that
8174 would be called for a given object/PMF pair and call it directly inside
8175 the inner loop, to save a bit of time.
8177 Note that you will still be paying the penalty for the call through a
8178 function pointer; on most modern architectures, such a call defeats the
8179 branch prediction features of the CPU@. This is also true of normal
8180 virtual function calls.
8182 The syntax for this extension is
8186 extern int (A::*fp)();
8187 typedef int (*fptr)(A *);
8189 fptr p = (fptr)(a.*fp);
8192 For PMF constants (i.e.@: expressions of the form @samp{&Klasse::Member}),
8193 no object is needed to obtain the address of the function. They can be
8194 converted to function pointers directly:
8197 fptr p1 = (fptr)(&A::foo);
8200 @opindex Wno-pmf-conversions
8201 You must specify @option{-Wno-pmf-conversions} to use this extension.
8203 @node C++ Attributes
8204 @section C++-Specific Variable, Function, and Type Attributes
8206 Some attributes only make sense for C++ programs.
8209 @item init_priority (@var{priority})
8210 @cindex init_priority attribute
8213 In Standard C++, objects defined at namespace scope are guaranteed to be
8214 initialized in an order in strict accordance with that of their definitions
8215 @emph{in a given translation unit}. No guarantee is made for initializations
8216 across translation units. However, GNU C++ allows users to control the
8217 order of initialization of objects defined at namespace scope with the
8218 @code{init_priority} attribute by specifying a relative @var{priority},
8219 a constant integral expression currently bounded between 101 and 65535
8220 inclusive. Lower numbers indicate a higher priority.
8222 In the following example, @code{A} would normally be created before
8223 @code{B}, but the @code{init_priority} attribute has reversed that order:
8226 Some_Class A __attribute__ ((init_priority (2000)));
8227 Some_Class B __attribute__ ((init_priority (543)));
8231 Note that the particular values of @var{priority} do not matter; only their
8234 @item java_interface
8235 @cindex java_interface attribute
8237 This type attribute informs C++ that the class is a Java interface. It may
8238 only be applied to classes declared within an @code{extern "Java"} block.
8239 Calls to methods declared in this interface will be dispatched using GCJ's
8240 interface table mechanism, instead of regular virtual table dispatch.
8244 @node Java Exceptions
8245 @section Java Exceptions
8247 The Java language uses a slightly different exception handling model
8248 from C++. Normally, GNU C++ will automatically detect when you are
8249 writing C++ code that uses Java exceptions, and handle them
8250 appropriately. However, if C++ code only needs to execute destructors
8251 when Java exceptions are thrown through it, GCC will guess incorrectly.
8252 Sample problematic code is:
8255 struct S @{ ~S(); @};
8256 extern void bar(); // is written in Java, and may throw exceptions
8265 The usual effect of an incorrect guess is a link failure, complaining of
8266 a missing routine called @samp{__gxx_personality_v0}.
8268 You can inform the compiler that Java exceptions are to be used in a
8269 translation unit, irrespective of what it might think, by writing
8270 @samp{@w{#pragma GCC java_exceptions}} at the head of the file. This
8271 @samp{#pragma} must appear before any functions that throw or catch
8272 exceptions, or run destructors when exceptions are thrown through them.
8274 You cannot mix Java and C++ exceptions in the same translation unit. It
8275 is believed to be safe to throw a C++ exception from one file through
8276 another file compiled for the Java exception model, or vice versa, but
8277 there may be bugs in this area.
8279 @node Deprecated Features
8280 @section Deprecated Features
8282 In the past, the GNU C++ compiler was extended to experiment with new
8283 features, at a time when the C++ language was still evolving. Now that
8284 the C++ standard is complete, some of those features are superseded by
8285 superior alternatives. Using the old features might cause a warning in
8286 some cases that the feature will be dropped in the future. In other
8287 cases, the feature might be gone already.
8289 While the list below is not exhaustive, it documents some of the options
8290 that are now deprecated:
8293 @item -fexternal-templates
8294 @itemx -falt-external-templates
8295 These are two of the many ways for g++ to implement template
8296 instantiation. @xref{Template Instantiation}. The C++ standard clearly
8297 defines how template definitions have to be organized across
8298 implementation units. g++ has an implicit instantiation mechanism that
8299 should work just fine for standard-conforming code.
8301 @item -fstrict-prototype
8302 @itemx -fno-strict-prototype
8303 Previously it was possible to use an empty prototype parameter list to
8304 indicate an unspecified number of parameters (like C), rather than no
8305 parameters, as C++ demands. This feature has been removed, except where
8306 it is required for backwards compatibility @xref{Backwards Compatibility}.
8309 The named return value extension has been deprecated, and is now
8312 The use of initializer lists with new expressions has been deprecated,
8313 and is now removed from g++.
8315 Floating and complex non-type template parameters have been deprecated,
8316 and are now removed from g++.
8318 The implicit typename extension has been deprecated and is now
8321 The use of default arguments in function pointers, function typedefs and
8322 and other places where they are not permitted by the standard is
8323 deprecated and will be removed from a future version of g++.
8325 @node Backwards Compatibility
8326 @section Backwards Compatibility
8327 @cindex Backwards Compatibility
8328 @cindex ARM [Annotated C++ Reference Manual]
8330 Now that there is a definitive ISO standard C++, G++ has a specification
8331 to adhere to. The C++ language evolved over time, and features that
8332 used to be acceptable in previous drafts of the standard, such as the ARM
8333 [Annotated C++ Reference Manual], are no longer accepted. In order to allow
8334 compilation of C++ written to such drafts, G++ contains some backwards
8335 compatibilities. @emph{All such backwards compatibility features are
8336 liable to disappear in future versions of G++.} They should be considered
8337 deprecated @xref{Deprecated Features}.
8341 If a variable is declared at for scope, it used to remain in scope until
8342 the end of the scope which contained the for statement (rather than just
8343 within the for scope). G++ retains this, but issues a warning, if such a
8344 variable is accessed outside the for scope.
8346 @item Implicit C language
8347 Old C system header files did not contain an @code{extern "C" @{@dots{}@}}
8348 scope to set the language. On such systems, all header files are
8349 implicitly scoped inside a C language scope. Also, an empty prototype
8350 @code{()} will be treated as an unspecified number of arguments, rather
8351 than no arguments, as C++ demands.