1 \input texinfo @c -*-texinfo-*-
3 @setfilename libffi.info
8 @c Merge the standard indexes into a single one.
19 This manual is for Libffi, a portable foreign-function interface
22 Copyright @copyright{} 2008, 2010 Red Hat, Inc.
25 Permission is granted to copy, distribute and/or modify this document
26 under the terms of the GNU General Public License as published by the
27 Free Software Foundation; either version 2, or (at your option) any
28 later version. A copy of the license is included in the
29 section entitled ``GNU General Public License''.
34 @dircategory Development
36 * libffi: (libffi). Portable foreign-function interface library.
42 @vskip 0pt plus 1filll
54 * Introduction:: What is libffi?
55 * Using libffi:: How to use libffi.
56 * Missing Features:: Things libffi can't do.
64 @chapter What is libffi?
66 Compilers for high level languages generate code that follow certain
67 conventions. These conventions are necessary, in part, for separate
68 compilation to work. One such convention is the @dfn{calling
69 convention}. The calling convention is a set of assumptions made by
70 the compiler about where function arguments will be found on entry to
71 a function. A calling convention also specifies where the return
72 value for a function is found. The calling convention is also
73 sometimes called the @dfn{ABI} or @dfn{Application Binary Interface}.
74 @cindex calling convention
76 @cindex Application Binary Interface
78 Some programs may not know at the time of compilation what arguments
79 are to be passed to a function. For instance, an interpreter may be
80 told at run-time about the number and types of arguments used to call
81 a given function. @samp{Libffi} can be used in such programs to
82 provide a bridge from the interpreter program to compiled code.
84 The @samp{libffi} library provides a portable, high level programming
85 interface to various calling conventions. This allows a programmer to
86 call any function specified by a call interface description at run
89 @acronym{FFI} stands for Foreign Function Interface. A foreign
90 function interface is the popular name for the interface that allows
91 code written in one language to call code written in another language.
92 The @samp{libffi} library really only provides the lowest, machine
93 dependent layer of a fully featured foreign function interface. A
94 layer must exist above @samp{libffi} that handles type conversions for
95 values passed between the two languages.
97 @cindex Foreign Function Interface
101 @chapter Using libffi
104 * The Basics:: The basic libffi API.
105 * Simple Example:: A simple example.
106 * Types:: libffi type descriptions.
107 * Multiple ABIs:: Different passing styles on one platform.
108 * The Closure API:: Writing a generic function.
109 * Closure Example:: A closure example.
116 @samp{Libffi} assumes that you have a pointer to the function you wish
117 to call and that you know the number and types of arguments to pass
118 it, as well as the return type of the function.
120 The first thing you must do is create an @code{ffi_cif} object that
121 matches the signature of the function you wish to call. This is a
122 separate step because it is common to make multiple calls using a
123 single @code{ffi_cif}. The @dfn{cif} in @code{ffi_cif} stands for
124 Call InterFace. To prepare a call interface object, use the function
129 @defun ffi_status ffi_prep_cif (ffi_cif *@var{cif}, ffi_abi @var{abi}, unsigned int @var{nargs}, ffi_type *@var{rtype}, ffi_type **@var{argtypes})
130 This initializes @var{cif} according to the given parameters.
132 @var{abi} is the ABI to use; normally @code{FFI_DEFAULT_ABI} is what
133 you want. @ref{Multiple ABIs} for more information.
135 @var{nargs} is the number of arguments that this function accepts.
136 @samp{libffi} does not yet handle varargs functions; see @ref{Missing
137 Features} for more information.
139 @var{rtype} is a pointer to an @code{ffi_type} structure that
140 describes the return type of the function. @xref{Types}.
142 @var{argtypes} is a vector of @code{ffi_type} pointers.
143 @var{argtypes} must have @var{nargs} elements. If @var{nargs} is 0,
144 this argument is ignored.
146 @code{ffi_prep_cif} returns a @code{libffi} status code, of type
147 @code{ffi_status}. This will be either @code{FFI_OK} if everything
148 worked properly; @code{FFI_BAD_TYPEDEF} if one of the @code{ffi_type}
149 objects is incorrect; or @code{FFI_BAD_ABI} if the @var{abi} parameter
154 To call a function using an initialized @code{ffi_cif}, use the
155 @code{ffi_call} function:
158 @defun void ffi_call (ffi_cif *@var{cif}, void *@var{fn}, void *@var{rvalue}, void **@var{avalues})
159 This calls the function @var{fn} according to the description given in
160 @var{cif}. @var{cif} must have already been prepared using
163 @var{rvalue} is a pointer to a chunk of memory that will hold the
164 result of the function call. This must be large enough to hold the
165 result and must be suitably aligned; it is the caller's responsibility
166 to ensure this. If @var{cif} declares that the function returns
167 @code{void} (using @code{ffi_type_void}), then @var{rvalue} is
168 ignored. If @var{rvalue} is @samp{NULL}, then the return value is
171 @var{avalues} is a vector of @code{void *} pointers that point to the
172 memory locations holding the argument values for a call. If @var{cif}
173 declares that the function has no arguments (i.e., @var{nargs} was 0),
174 then @var{avalues} is ignored.
179 @section Simple Example
181 Here is a trivial example that calls @code{puts} a few times.
195 /* Initialize the argument info vectors */
196 args[0] = &ffi_type_pointer;
199 /* Initialize the cif */
200 if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
201 &ffi_type_uint, args) == FFI_OK)
204 ffi_call(&cif, puts, &rc, values);
205 /* rc now holds the result of the call to puts */
207 /* values holds a pointer to the function's arg, so to
208 call puts() again all we need to do is change the
211 ffi_call(&cif, puts, &rc, values);
223 * Primitive Types:: Built-in types.
224 * Structures:: Structure types.
225 * Type Example:: Structure type example.
228 @node Primitive Types
229 @subsection Primitive Types
231 @code{Libffi} provides a number of built-in type descriptors that can
232 be used to describe argument and return types:
236 @tindex ffi_type_void
237 The type @code{void}. This cannot be used for argument types, only
241 @tindex ffi_type_uint8
242 An unsigned, 8-bit integer type.
245 @tindex ffi_type_sint8
246 A signed, 8-bit integer type.
248 @item ffi_type_uint16
249 @tindex ffi_type_uint16
250 An unsigned, 16-bit integer type.
252 @item ffi_type_sint16
253 @tindex ffi_type_sint16
254 A signed, 16-bit integer type.
256 @item ffi_type_uint32
257 @tindex ffi_type_uint32
258 An unsigned, 32-bit integer type.
260 @item ffi_type_sint32
261 @tindex ffi_type_sint32
262 A signed, 32-bit integer type.
264 @item ffi_type_uint64
265 @tindex ffi_type_uint64
266 An unsigned, 64-bit integer type.
268 @item ffi_type_sint64
269 @tindex ffi_type_sint64
270 A signed, 64-bit integer type.
273 @tindex ffi_type_float
274 The C @code{float} type.
276 @item ffi_type_double
277 @tindex ffi_type_double
278 The C @code{double} type.
281 @tindex ffi_type_uchar
282 The C @code{unsigned char} type.
285 @tindex ffi_type_schar
286 The C @code{signed char} type. (Note that there is not an exact
287 equivalent to the C @code{char} type in @code{libffi}; ordinarily you
288 should either use @code{ffi_type_schar} or @code{ffi_type_uchar}
289 depending on whether @code{char} is signed.)
291 @item ffi_type_ushort
292 @tindex ffi_type_ushort
293 The C @code{unsigned short} type.
295 @item ffi_type_sshort
296 @tindex ffi_type_sshort
297 The C @code{short} type.
300 @tindex ffi_type_uint
301 The C @code{unsigned int} type.
304 @tindex ffi_type_sint
305 The C @code{int} type.
308 @tindex ffi_type_ulong
309 The C @code{unsigned long} type.
312 @tindex ffi_type_slong
313 The C @code{long} type.
315 @item ffi_type_longdouble
316 @tindex ffi_type_longdouble
317 On platforms that have a C @code{long double} type, this is defined.
318 On other platforms, it is not.
320 @item ffi_type_pointer
321 @tindex ffi_type_pointer
322 A generic @code{void *} pointer. You should use this for all
323 pointers, regardless of their real type.
326 Each of these is of type @code{ffi_type}, so you must take the address
327 when passing to @code{ffi_prep_cif}.
331 @subsection Structures
333 Although @samp{libffi} has no special support for unions or
334 bit-fields, it is perfectly happy passing structures back and forth.
335 You must first describe the structure to @samp{libffi} by creating a
336 new @code{ffi_type} object for it.
340 The @code{ffi_type} has the following members:
343 This is set by @code{libffi}; you should initialize it to zero.
345 @item unsigned short alignment
346 This is set by @code{libffi}; you should initialize it to zero.
348 @item unsigned short type
349 For a structure, this should be set to @code{FFI_TYPE_STRUCT}.
351 @item ffi_type **elements
352 This is a @samp{NULL}-terminated array of pointers to @code{ffi_type}
353 objects. There is one element per field of the struct.
359 @subsection Type Example
361 The following example initializes a @code{ffi_type} object
362 representing the @code{tm} struct from Linux's @file{time.h}.
364 Here is how the struct is defined:
377 /* Those are for future use. */
378 long int __tm_gmtoff__;
379 __const char *__tm_zone__;
383 Here is the corresponding code to describe this struct to
389 ffi_type *tm_type_elements[12];
392 tm_type.size = tm_type.alignment = 0;
393 tm_type.elements = &tm_type_elements;
395 for (i = 0; i < 9; i++)
396 tm_type_elements[i] = &ffi_type_sint;
398 tm_type_elements[9] = &ffi_type_slong;
399 tm_type_elements[10] = &ffi_type_pointer;
400 tm_type_elements[11] = NULL;
402 /* tm_type can now be used to represent tm argument types and
403 return types for ffi_prep_cif() */
409 @section Multiple ABIs
411 A given platform may provide multiple different ABIs at once. For
412 instance, the x86 platform has both @samp{stdcall} and @samp{fastcall}
415 @code{libffi} provides some support for this. However, this is
416 necessarily platform-specific.
418 @c FIXME: document the platforms
420 @node The Closure API
421 @section The Closure API
423 @code{libffi} also provides a way to write a generic function -- a
424 function that can accept and decode any combination of arguments.
425 This can be useful when writing an interpreter, or to provide wrappers
426 for arbitrary functions.
428 This facility is called the @dfn{closure API}. Closures are not
429 supported on all platforms; you can check the @code{FFI_CLOSURES}
430 define to determine whether they are supported on the current
436 Because closures work by assembling a tiny function at runtime, they
437 require special allocation on platforms that have a non-executable
438 heap. Memory management for closures is handled by a pair of
441 @findex ffi_closure_alloc
442 @defun void *ffi_closure_alloc (size_t @var{size}, void **@var{code})
443 Allocate a chunk of memory holding @var{size} bytes. This returns a
444 pointer to the writable address, and sets *@var{code} to the
445 corresponding executable address.
447 @var{size} should be sufficient to hold a @code{ffi_closure} object.
450 @findex ffi_closure_free
451 @defun void ffi_closure_free (void *@var{writable})
452 Free memory allocated using @code{ffi_closure_alloc}. The argument is
453 the writable address that was returned.
457 Once you have allocated the memory for a closure, you must construct a
458 @code{ffi_cif} describing the function call. Finally you can prepare
459 the closure function:
461 @findex ffi_prep_closure_loc
462 @defun ffi_status ffi_prep_closure_loc (ffi_closure *@var{closure}, ffi_cif *@var{cif}, void (*@var{fun}) (ffi_cif *@var{cif}, void *@var{ret}, void **@var{args}, void *@var{user_data}), void *@var{user_data}, void *@var{codeloc})
463 Prepare a closure function.
465 @var{closure} is the address of a @code{ffi_closure} object; this is
466 the writable address returned by @code{ffi_closure_alloc}.
468 @var{cif} is the @code{ffi_cif} describing the function parameters.
470 @var{user_data} is an arbitrary datum that is passed, uninterpreted,
471 to your closure function.
473 @var{codeloc} is the executable address returned by
474 @code{ffi_closure_alloc}.
476 @var{fun} is the function which will be called when the closure is
477 invoked. It is called with the arguments:
480 The @code{ffi_cif} passed to @code{ffi_prep_closure_loc}.
483 A pointer to the memory used for the function's return value.
484 @var{fun} must fill this, unless the function is declared as returning
486 @c FIXME: is this NULL for void-returning functions?
489 A vector of pointers to memory holding the arguments to the function.
492 The same @var{user_data} that was passed to
493 @code{ffi_prep_closure_loc}.
496 @code{ffi_prep_closure_loc} will return @code{FFI_OK} if everything
497 went ok, and something else on error.
500 After calling @code{ffi_prep_closure_loc}, you can cast @var{codeloc}
501 to the appropriate pointer-to-function type.
504 You may see old code referring to @code{ffi_prep_closure}. This
505 function is deprecated, as it cannot handle the need for separate
506 writable and executable addresses.
508 @node Closure Example
509 @section Closure Example
511 A trivial example that creates a new @code{puts} by binding
512 @code{fputs} with @code{stdin}.
518 /* Acts like puts with the file given at time of enclosure. */
519 void puts_binding(ffi_cif *cif, unsigned int *ret, void* args[],
522 *ret = fputs(*(char **)args[0], stream);
529 ffi_closure *closure;
531 int (*bound_puts)(char *);
534 /* Allocate closure and bound_puts */
535 closure = ffi_closure_alloc(sizeof(ffi_closure), &bound_puts);
539 /* Initialize the argument info vectors */
540 args[0] = &ffi_type_pointer;
542 /* Initialize the cif */
543 if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
544 &ffi_type_uint, args) == FFI_OK)
546 /* Initialize the closure, setting stream to stdout */
547 if (ffi_prep_closure_loc(closure, &cif, puts_binding,
548 stdout, bound_puts) == FFI_OK)
550 rc = bound_puts("Hello World!");
551 /* rc now holds the result of the call to fputs */
556 /* Deallocate both closure, and bound_puts */
557 ffi_closure_free(closure);
565 @node Missing Features
566 @chapter Missing Features
568 @code{libffi} is missing a few features. We welcome patches to add
573 There is no support for calling varargs functions. This may work on
574 some platforms, depending on how the ABI is defined, but it is not
578 There is no support for bit fields in structures.
586 The ``raw'' API is undocumented.
587 @c argument promotion?