1 This is ../libffi/doc/libffi.info, produced by makeinfo version 4.13
2 from ../libffi/doc/libffi.texi.
4 This manual is for Libffi, a portable foreign-function interface
7 Copyright (C) 2008, 2010 Red Hat, Inc.
9 Permission is granted to copy, distribute and/or modify this
10 document under the terms of the GNU General Public License as
11 published by the Free Software Foundation; either version 2, or
12 (at your option) any later version. A copy of the license is
13 included in the section entitled "GNU General Public License".
16 INFO-DIR-SECTION Development
18 * libffi: (libffi). Portable foreign-function interface library.
22 File: libffi.info, Node: Top, Next: Introduction, Up: (dir)
27 This manual is for Libffi, a portable foreign-function interface
30 Copyright (C) 2008, 2010 Red Hat, Inc.
32 Permission is granted to copy, distribute and/or modify this
33 document under the terms of the GNU General Public License as
34 published by the Free Software Foundation; either version 2, or
35 (at your option) any later version. A copy of the license is
36 included in the section entitled "GNU General Public License".
41 * Introduction:: What is libffi?
42 * Using libffi:: How to use libffi.
43 * Missing Features:: Things libffi can't do.
47 File: libffi.info, Node: Introduction, Next: Using libffi, Prev: Top, Up: Top
52 Compilers for high level languages generate code that follow certain
53 conventions. These conventions are necessary, in part, for separate
54 compilation to work. One such convention is the "calling convention".
55 The calling convention is a set of assumptions made by the compiler
56 about where function arguments will be found on entry to a function. A
57 calling convention also specifies where the return value for a function
58 is found. The calling convention is also sometimes called the "ABI" or
59 "Application Binary Interface".
61 Some programs may not know at the time of compilation what arguments
62 are to be passed to a function. For instance, an interpreter may be
63 told at run-time about the number and types of arguments used to call a
64 given function. `Libffi' can be used in such programs to provide a
65 bridge from the interpreter program to compiled code.
67 The `libffi' library provides a portable, high level programming
68 interface to various calling conventions. This allows a programmer to
69 call any function specified by a call interface description at run time.
71 FFI stands for Foreign Function Interface. A foreign function
72 interface is the popular name for the interface that allows code
73 written in one language to call code written in another language. The
74 `libffi' library really only provides the lowest, machine dependent
75 layer of a fully featured foreign function interface. A layer must
76 exist above `libffi' that handles type conversions for values passed
77 between the two languages.
80 File: libffi.info, Node: Using libffi, Next: Missing Features, Prev: Introduction, Up: Top
87 * The Basics:: The basic libffi API.
88 * Simple Example:: A simple example.
89 * Types:: libffi type descriptions.
90 * Multiple ABIs:: Different passing styles on one platform.
91 * The Closure API:: Writing a generic function.
92 * Closure Example:: A closure example.
95 File: libffi.info, Node: The Basics, Next: Simple Example, Up: Using libffi
100 `Libffi' assumes that you have a pointer to the function you wish to
101 call and that you know the number and types of arguments to pass it, as
102 well as the return type of the function.
104 The first thing you must do is create an `ffi_cif' object that
105 matches the signature of the function you wish to call. This is a
106 separate step because it is common to make multiple calls using a
107 single `ffi_cif'. The "cif" in `ffi_cif' stands for Call InterFace.
108 To prepare a call interface object, use the function `ffi_prep_cif'.
110 -- Function: ffi_status ffi_prep_cif (ffi_cif *CIF, ffi_abi ABI,
111 unsigned int NARGS, ffi_type *RTYPE, ffi_type **ARGTYPES)
112 This initializes CIF according to the given parameters.
114 ABI is the ABI to use; normally `FFI_DEFAULT_ABI' is what you
115 want. *note Multiple ABIs:: for more information.
117 NARGS is the number of arguments that this function accepts.
118 `libffi' does not yet handle varargs functions; see *note Missing
119 Features:: for more information.
121 RTYPE is a pointer to an `ffi_type' structure that describes the
122 return type of the function. *Note Types::.
124 ARGTYPES is a vector of `ffi_type' pointers. ARGTYPES must have
125 NARGS elements. If NARGS is 0, this argument is ignored.
127 `ffi_prep_cif' returns a `libffi' status code, of type
128 `ffi_status'. This will be either `FFI_OK' if everything worked
129 properly; `FFI_BAD_TYPEDEF' if one of the `ffi_type' objects is
130 incorrect; or `FFI_BAD_ABI' if the ABI parameter is invalid.
132 To call a function using an initialized `ffi_cif', use the
135 -- Function: void ffi_call (ffi_cif *CIF, void *FN, void *RVALUE, void
137 This calls the function FN according to the description given in
138 CIF. CIF must have already been prepared using `ffi_prep_cif'.
140 RVALUE is a pointer to a chunk of memory that will hold the result
141 of the function call. This must be large enough to hold the
142 result and must be suitably aligned; it is the caller's
143 responsibility to ensure this. If CIF declares that the function
144 returns `void' (using `ffi_type_void'), then RVALUE is ignored.
145 If RVALUE is `NULL', then the return value is discarded.
147 AVALUES is a vector of `void *' pointers that point to the memory
148 locations holding the argument values for a call. If CIF declares
149 that the function has no arguments (i.e., NARGS was 0), then
150 AVALUES is ignored. Note that argument values may be modified by
151 the callee (for instance, structs passed by value); the burden of
152 copying pass-by-value arguments is placed on the caller.
155 File: libffi.info, Node: Simple Example, Next: Types, Prev: The Basics, Up: Using libffi
160 Here is a trivial example that calls `puts' a few times.
173 /* Initialize the argument info vectors */
174 args[0] = &ffi_type_pointer;
177 /* Initialize the cif */
178 if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
179 &ffi_type_uint, args) == FFI_OK)
182 ffi_call(&cif, puts, &rc, values);
183 /* rc now holds the result of the call to puts */
185 /* values holds a pointer to the function's arg, so to
186 call puts() again all we need to do is change the
189 ffi_call(&cif, puts, &rc, values);
196 File: libffi.info, Node: Types, Next: Multiple ABIs, Prev: Simple Example, Up: Using libffi
203 * Primitive Types:: Built-in types.
204 * Structures:: Structure types.
205 * Type Example:: Structure type example.
208 File: libffi.info, Node: Primitive Types, Next: Structures, Up: Types
210 2.3.1 Primitive Types
211 ---------------------
213 `Libffi' provides a number of built-in type descriptors that can be
214 used to describe argument and return types:
217 The type `void'. This cannot be used for argument types, only for
221 An unsigned, 8-bit integer type.
224 A signed, 8-bit integer type.
227 An unsigned, 16-bit integer type.
230 A signed, 16-bit integer type.
233 An unsigned, 32-bit integer type.
236 A signed, 32-bit integer type.
239 An unsigned, 64-bit integer type.
242 A signed, 64-bit integer type.
251 The C `unsigned char' type.
254 The C `signed char' type. (Note that there is not an exact
255 equivalent to the C `char' type in `libffi'; ordinarily you should
256 either use `ffi_type_schar' or `ffi_type_uchar' depending on
257 whether `char' is signed.)
260 The C `unsigned short' type.
266 The C `unsigned int' type.
272 The C `unsigned long' type.
277 `ffi_type_longdouble'
278 On platforms that have a C `long double' type, this is defined.
279 On other platforms, it is not.
282 A generic `void *' pointer. You should use this for all pointers,
283 regardless of their real type.
285 Each of these is of type `ffi_type', so you must take the address
286 when passing to `ffi_prep_cif'.
289 File: libffi.info, Node: Structures, Next: Type Example, Prev: Primitive Types, Up: Types
294 Although `libffi' has no special support for unions or bit-fields, it
295 is perfectly happy passing structures back and forth. You must first
296 describe the structure to `libffi' by creating a new `ffi_type' object
300 The `ffi_type' has the following members:
302 This is set by `libffi'; you should initialize it to zero.
304 `unsigned short alignment'
305 This is set by `libffi'; you should initialize it to zero.
307 `unsigned short type'
308 For a structure, this should be set to `FFI_TYPE_STRUCT'.
310 `ffi_type **elements'
311 This is a `NULL'-terminated array of pointers to `ffi_type'
312 objects. There is one element per field of the struct.
315 File: libffi.info, Node: Type Example, Prev: Structures, Up: Types
320 The following example initializes a `ffi_type' object representing the
321 `tm' struct from Linux's `time.h'.
323 Here is how the struct is defined:
335 /* Those are for future use. */
336 long int __tm_gmtoff__;
337 __const char *__tm_zone__;
340 Here is the corresponding code to describe this struct to `libffi':
344 ffi_type *tm_type_elements[12];
347 tm_type.size = tm_type.alignment = 0;
348 tm_type.elements = &tm_type_elements;
350 for (i = 0; i < 9; i++)
351 tm_type_elements[i] = &ffi_type_sint;
353 tm_type_elements[9] = &ffi_type_slong;
354 tm_type_elements[10] = &ffi_type_pointer;
355 tm_type_elements[11] = NULL;
357 /* tm_type can now be used to represent tm argument types and
358 return types for ffi_prep_cif() */
362 File: libffi.info, Node: Multiple ABIs, Next: The Closure API, Prev: Types, Up: Using libffi
367 A given platform may provide multiple different ABIs at once. For
368 instance, the x86 platform has both `stdcall' and `fastcall' functions.
370 `libffi' provides some support for this. However, this is
371 necessarily platform-specific.
374 File: libffi.info, Node: The Closure API, Next: Closure Example, Prev: Multiple ABIs, Up: Using libffi
379 `libffi' also provides a way to write a generic function - a function
380 that can accept and decode any combination of arguments. This can be
381 useful when writing an interpreter, or to provide wrappers for
384 This facility is called the "closure API". Closures are not
385 supported on all platforms; you can check the `FFI_CLOSURES' define to
386 determine whether they are supported on the current platform.
388 Because closures work by assembling a tiny function at runtime, they
389 require special allocation on platforms that have a non-executable
390 heap. Memory management for closures is handled by a pair of functions:
392 -- Function: void *ffi_closure_alloc (size_t SIZE, void **CODE)
393 Allocate a chunk of memory holding SIZE bytes. This returns a
394 pointer to the writable address, and sets *CODE to the
395 corresponding executable address.
397 SIZE should be sufficient to hold a `ffi_closure' object.
399 -- Function: void ffi_closure_free (void *WRITABLE)
400 Free memory allocated using `ffi_closure_alloc'. The argument is
401 the writable address that was returned.
403 Once you have allocated the memory for a closure, you must construct
404 a `ffi_cif' describing the function call. Finally you can prepare the
407 -- Function: ffi_status ffi_prep_closure_loc (ffi_closure *CLOSURE,
408 ffi_cif *CIF, void (*FUN) (ffi_cif *CIF, void *RET, void
409 **ARGS, void *USER_DATA), void *USER_DATA, void *CODELOC)
410 Prepare a closure function.
412 CLOSURE is the address of a `ffi_closure' object; this is the
413 writable address returned by `ffi_closure_alloc'.
415 CIF is the `ffi_cif' describing the function parameters.
417 USER_DATA is an arbitrary datum that is passed, uninterpreted, to
418 your closure function.
420 CODELOC is the executable address returned by `ffi_closure_alloc'.
422 FUN is the function which will be called when the closure is
423 invoked. It is called with the arguments:
425 The `ffi_cif' passed to `ffi_prep_closure_loc'.
428 A pointer to the memory used for the function's return value.
429 FUN must fill this, unless the function is declared as
433 A vector of pointers to memory holding the arguments to the
437 The same USER_DATA that was passed to `ffi_prep_closure_loc'.
439 `ffi_prep_closure_loc' will return `FFI_OK' if everything went ok,
440 and something else on error.
442 After calling `ffi_prep_closure_loc', you can cast CODELOC to the
443 appropriate pointer-to-function type.
445 You may see old code referring to `ffi_prep_closure'. This function
446 is deprecated, as it cannot handle the need for separate writable and
447 executable addresses.
450 File: libffi.info, Node: Closure Example, Prev: The Closure API, Up: Using libffi
455 A trivial example that creates a new `puts' by binding `fputs' with
461 /* Acts like puts with the file given at time of enclosure. */
462 void puts_binding(ffi_cif *cif, unsigned int *ret, void* args[],
465 *ret = fputs(*(char **)args[0], stream);
472 ffi_closure *closure;
474 int (*bound_puts)(char *);
477 /* Allocate closure and bound_puts */
478 closure = ffi_closure_alloc(sizeof(ffi_closure), &bound_puts);
482 /* Initialize the argument info vectors */
483 args[0] = &ffi_type_pointer;
485 /* Initialize the cif */
486 if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
487 &ffi_type_uint, args) == FFI_OK)
489 /* Initialize the closure, setting stream to stdout */
490 if (ffi_prep_closure_loc(closure, &cif, puts_binding,
491 stdout, bound_puts) == FFI_OK)
493 rc = bound_puts("Hello World!");
494 /* rc now holds the result of the call to fputs */
499 /* Deallocate both closure, and bound_puts */
500 ffi_closure_free(closure);
506 File: libffi.info, Node: Missing Features, Next: Index, Prev: Using libffi, Up: Top
511 `libffi' is missing a few features. We welcome patches to add support
514 * There is no support for calling varargs functions. This may work
515 on some platforms, depending on how the ABI is defined, but it is
518 * There is no support for bit fields in structures.
522 * The "raw" API is undocumented.
525 File: libffi.info, Node: Index, Prev: Missing Features, Up: Top
533 * : Structures. (line 12)
534 * ABI: Introduction. (line 13)
535 * Application Binary Interface: Introduction. (line 13)
536 * calling convention: Introduction. (line 13)
537 * cif: The Basics. (line 14)
538 * closure API: The Closure API. (line 13)
539 * closures: The Closure API. (line 13)
540 * FFI: Introduction. (line 31)
541 * ffi_call: The Basics. (line 41)
542 * ffi_closure_alloc: The Closure API. (line 19)
543 * ffi_closure_free: The Closure API. (line 26)
544 * FFI_CLOSURES: The Closure API. (line 13)
545 * ffi_prep_cif: The Basics. (line 16)
546 * ffi_prep_closure_loc: The Closure API. (line 34)
547 * ffi_status <1>: The Closure API. (line 37)
548 * ffi_status: The Basics. (line 18)
549 * ffi_type: Structures. (line 11)
550 * ffi_type_double: Primitive Types. (line 41)
551 * ffi_type_float: Primitive Types. (line 38)
552 * ffi_type_longdouble: Primitive Types. (line 71)
553 * ffi_type_pointer: Primitive Types. (line 75)
554 * ffi_type_schar: Primitive Types. (line 47)
555 * ffi_type_sint: Primitive Types. (line 62)
556 * ffi_type_sint16: Primitive Types. (line 23)
557 * ffi_type_sint32: Primitive Types. (line 29)
558 * ffi_type_sint64: Primitive Types. (line 35)
559 * ffi_type_sint8: Primitive Types. (line 17)
560 * ffi_type_slong: Primitive Types. (line 68)
561 * ffi_type_sshort: Primitive Types. (line 56)
562 * ffi_type_uchar: Primitive Types. (line 44)
563 * ffi_type_uint: Primitive Types. (line 59)
564 * ffi_type_uint16: Primitive Types. (line 20)
565 * ffi_type_uint32: Primitive Types. (line 26)
566 * ffi_type_uint64: Primitive Types. (line 32)
567 * ffi_type_uint8: Primitive Types. (line 14)
568 * ffi_type_ulong: Primitive Types. (line 65)
569 * ffi_type_ushort: Primitive Types. (line 53)
570 * ffi_type_void: Primitive Types. (line 10)
571 * Foreign Function Interface: Introduction. (line 31)
572 * void <1>: The Closure API. (line 20)
573 * void: The Basics. (line 43)
579 Node: Introduction
\7f1448
580 Node: Using libffi
\7f3084
581 Node: The Basics
\7f3570
582 Node: Simple Example
\7f6356
584 Node: Primitive Types
\7f7666
585 Node: Structures
\7f9486
586 Node: Type Example
\7f10346
587 Node: Multiple ABIs
\7f11569
588 Node: The Closure API
\7f11940
589 Node: Closure Example
\7f14884
590 Node: Missing Features
\7f16443