* Multiple ABIs:: Different passing styles on one platform.
* The Closure API:: Writing a generic function.
* Closure Example:: A closure example.
+* Thread Safety:: Thread safety.
@end menu
@end defun
+Note that the resulting @code{ffi_cif} holds pointers to all the
+@code{ffi_type} objects that were used durin initialization. You must
+ensure that these type objects have a lifetime at least as long as
+that of the @code{ffi_cif}.
To call a function using an initialized @code{ffi_cif}, use the
@code{ffi_call} function:
@code{void} (using @code{ffi_type_void}), then @var{rvalue} is
ignored.
+In most situations, @samp{libffi} will handle promotion according to
+the ABI. However, for historical reasons, there is a special case
+with return values that must be handled by your code. In particular,
+for integral (not @code{struct}) types that are narrower than the
+system register size, the return value will be widened by
+@samp{libffi}. @samp{libffi} provides a type, @code{ffi_arg}, that
+can be used as the return type. For example, if the CIF was defined
+with a return type of @code{char}, @samp{libffi} will try to store a
+full @code{ffi_arg} into the return value.
+
@var{avalues} is a vector of @code{void *} pointers that point to the
memory locations holding the argument values for a call. If @var{cif}
declares that the function has no arguments (i.e., @var{nargs} was 0),
then @var{avalues} is ignored. Note that argument values may be
modified by the callee (for instance, structs passed by value); the
burden of copying pass-by-value arguments is placed on the caller.
+
+Note that while the return value must be register-sized, arguments
+should exactly match their declared type. For example, if an argument
+is a @code{short}, then the entry is @var{avalues} should point to an
+object declared as @code{short}; but if the return type is
+@code{short}, then @var{rvalue} should point to an object declared as
+a larger type -- usually @code{ffi_arg}.
@end defun
@menu
* Primitive Types:: Built-in types.
* Structures:: Structure types.
+* Size and Alignment:: Size and alignment of types.
+* Arrays and Unions:: Arrays and unions.
* Type Example:: Structure type example.
* Complex:: Complex types.
* Complex Type Example:: Complex type example.
@node Structures
@subsection Structures
-Although @samp{libffi} has no special support for unions or
-bit-fields, it is perfectly happy passing structures back and forth.
+@samp{libffi} is perfectly happy passing structures back and forth.
You must first describe the structure to @samp{libffi} by creating a
new @code{ffi_type} object for it.
@item ffi_type **elements
This is a @samp{NULL}-terminated array of pointers to @code{ffi_type}
objects. There is one element per field of the struct.
+
+Note that @samp{libffi} has no special support for bit-fields. You
+must manage these manually.
@end table
@end deftp
+The @code{size} and @code{alignment} fields will be filled in by
+@code{ffi_prep_cif} or @code{ffi_prep_cif_var}, as needed.
+
+@node Size and Alignment
+@subsection Size and Alignment
+
+@code{libffi} will set the @code{size} and @code{alignment} fields of
+an @code{ffi_type} object for you. It does so using its knowledge of
+the ABI.
+
+You might expect that you can simply read these fields for a type that
+has been laid out by @code{libffi}. However, there are some caveats.
+
+@itemize @bullet
+@item
+The size or alignment of some of the built-in types may vary depending
+on the chosen ABI.
+
+@item
+The size and alignment of a new structure type will not be set by
+@code{libffi} until it has been passed to @code{ffi_prep_cif}.
+
+@item
+A structure type cannot be shared across ABIs. Instead each ABI needs
+its own copy of the structure type.
+@end itemize
+
+So, before examining these fields, it is safest to pass the
+@code{ffi_type} object to @code{ffi_prep_cif} first. This function
+will do all the needed setup.
+
+@example
+ffi_type *desired_type;
+ffi_abi desired_abi;
+@dots{}
+ffi_cif cif;
+if (ffi_prep_cif (&cif, desired_abi, 0, desired_type, NULL) == FFI_OK)
+ @{
+ size_t size = desired_type->size;
+ unsigned short alignment = desired_type->alignment;
+ @}
+@end example
+
+@node Arrays and Unions
+@subsection Arrays and Unions
+
+@subsubsection Arrays
+
+@samp{libffi} does not have direct support for arrays or unions.
+However, they can be emulated using structures.
+
+To emulate an array, simply create an @code{ffi_type} using
+@code{FFI_TYPE_STRUCT} with as many members as there are elements in
+the array.
+
+@example
+ffi_type array_type;
+ffi_type **elements
+int i;
+
+elements = malloc ((n + 1) * sizeof (ffi_type *));
+for (i = 0; i < n; ++i)
+ elements[i] = array_element_type;
+elements[n] = NULL;
+
+array_type.size = array_type.alignment = 0;
+array_type.type = FFI_TYPE_STRUCT;
+array_type.elements = elements;
+@end example
+
+Note that arrays cannot be passed or returned by value in C --
+structure types created like this should only be used to refer to
+members of real @code{FFI_TYPE_STRUCT} objects.
+
+However, a phony array type like this will not cause any errors from
+@samp{libffi} if you use it as an argument or return type. This may
+be confusing.
+
+@subsubsection Unions
+
+A union can also be emulated using @code{FFI_TYPE_STRUCT}. In this
+case, however, you must make sure that the size and alignment match
+the real requirements of the union.
+
+One simple way to do this is to ensue that each element type is laid
+out. Then, give the new structure type a single element; the size of
+the largest element; and the largest alignment seen as well.
+
+This example uses the @code{ffi_prep_cif} trick to ensure that each
+element type is laid out.
+
+@example
+ffi_abi desired_abi;
+ffi_type union_type;
+ffi_type **union_elements;
+
+int i;
+ffi_type element_types[2];
+
+element_types[1] = NULL;
+
+union_type.size = union_type.alignment = 0;
+union_type.type = FFI_TYPE_STRUCT;
+union_type.elements = element_types;
+
+for (i = 0; union_elements[i]; ++i)
+ @{
+ ffi_cif cif;
+ if (ffi_prep_cif (&cif, desired_abi, 0, union_elements[i], NULL) == FFI_OK)
+ @{
+ if (union_elements[i]->size > union_type.size)
+ @{
+ union_type.size = union_elements[i];
+ size = union_elements[i]->size;
+ @}
+ if (union_elements[i]->alignment > union_type.alignment)
+ union_type.alignment = union_elements[i]->alignment;
+ @}
+ @}
+@end example
@node Type Example
@subsection Type Example
the writable address returned by @code{ffi_closure_alloc}.
@var{cif} is the @code{ffi_cif} describing the function parameters.
+Note that this object, and the types to which it refers, must be kept
+alive until the closure itself is freed.
@var{user_data} is an arbitrary datum that is passed, uninterpreted,
to your closure function.
@item ret
A pointer to the memory used for the function's return value.
@var{fun} must fill this, unless the function is declared as returning
-@code{void}.
+@code{void}. Note that this points to memory that is exactly the size
+of the type given as the return type when initializing the CIF. In
+particular, closures do not have the special promotion behavior of
+@code{ffi_call}.
@c FIXME: is this NULL for void-returning functions?
+@c (experimentally it is not, but it seems like a good idea)
@item args
A vector of pointers to memory holding the arguments to the function.
@end table
@code{ffi_prep_closure_loc} will return @code{FFI_OK} if everything
-went ok, and something else on error.
-@c FIXME: what?
+went ok, and one of the other @code{ffi_status} values on error.
After calling @code{ffi_prep_closure_loc}, you can cast @var{codeloc}
to the appropriate pointer-to-function type.
@end example
+@node Thread Safety
+@section Thread Safety
+
+@code{libffi} is not completely thread-safe. However, many parts are,
+and if you follow some simple rules, you can use it safely in a
+multi-threaded program.
+
+@itemize @bullet
+@item
+@code{ffi_prep_cif} may modify the @code{ffi_type} objects passed to
+it. It is best to ensure that only a single thread prepares a given
+@code{ffi_cif} at a time.
+
+@item
+On some platforms, @code{ffi_prep_cif} may modify the size and
+alignment of some types, depending on the chosen ABI. On these
+platforms, if you switch between ABIs, you must ensure that there is
+only one call to @code{ffi_prep_cif} at a time.
+
+Currently the only affected platform is PowerPC and the only affected
+type is @code{long double}.
+@end itemize
@node Missing Features
@chapter Missing Features
@item
The ``raw'' API is undocumented.
-@c argument promotion?
-@c unions?
@c anything else?
+
+@item
+The Go API is undocumented.
@end itemize
Note that variadic support is very new and tested on a relatively