1 This is doc/libffi.info, produced by makeinfo version 4.12 from
4 This manual is for Libffi, a portable foreign-function interface
7 Copyright (C) 2008 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".
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 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.
94 File: libffi.info, Node: The Basics, Next: Simple Example, Up: Using libffi
99 `Libffi' assumes that you have a pointer to the function you wish to
100 call and that you know the number and types of arguments to pass it, as
101 well as the return type of the function.
103 The first thing you must do is create an `ffi_cif' object that
104 matches the signature of the function you wish to call. This is a
105 separate step because it is common to make multiple calls using a
106 single `ffi_cif'. The "cif" in `ffi_cif' stands for Call InterFace.
107 To prepare a call interface object, use the function `ffi_prep_cif'.
109 -- Function: ffi_status ffi_prep_cif (ffi_cif *CIF, ffi_abi ABI,
110 unsigned int NARGS, ffi_type *RTYPE, ffi_type **ARGTYPES)
111 This initializes CIF according to the given parameters.
113 ABI is the ABI to use; normally `FFI_DEFAULT_ABI' is what you
114 want. *note Multiple ABIs:: for more information.
116 NARGS is the number of arguments that this function accepts.
117 `libffi' does not yet handle varargs functions; see *note Missing
118 Features:: for more information.
120 RTYPE is a pointer to an `ffi_type' structure that describes the
121 return type of the function. *Note Types::.
123 ARGTYPES is a vector of `ffi_type' pointers. ARGTYPES must have
124 NARGS elements. If NARGS is 0, this argument is ignored.
126 `ffi_prep_cif' returns a `libffi' status code, of type
127 `ffi_status'. This will be either `FFI_OK' if everything worked
128 properly; `FFI_BAD_TYPEDEF' if one of the `ffi_type' objects is
129 incorrect; or `FFI_BAD_ABI' if the ABI parameter is invalid.
131 To call a function using an initialized `ffi_cif', use the
134 -- Function: void ffi_call (ffi_cif *CIF, void *FN, void *RVALUE, void
136 This calls the function FN according to the description given in
137 CIF. CIF must have already been prepared using `ffi_prep_cif'.
139 RVALUE is a pointer to a chunk of memory that will hold the result
140 of the function call. This must be large enough to hold the
141 result and must be suitably aligned; it is the caller's
142 responsibility to ensure this. If CIF declares that the function
143 returns `void' (using `ffi_type_void'), then RVALUE is ignored.
144 If RVALUE is `NULL', then the return value is discarded.
146 AVALUES is a vector of `void *' pointers that point to the memory
147 locations holding the argument values for a call. If CIF declares
148 that the function has no arguments (i.e., NARGS was 0), then
152 File: libffi.info, Node: Simple Example, Next: Types, Prev: The Basics, Up: Using libffi
157 Here is a trivial example that calls `puts' a few times.
170 /* Initialize the argument info vectors */
171 args[0] = &ffi_type_pointer;
174 /* Initialize the cif */
175 if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
176 &ffi_type_uint, args) == FFI_OK)
179 ffi_call(&cif, puts, &rc, values);
180 /* rc now holds the result of the call to puts */
182 /* values holds a pointer to the function's arg, so to
183 call puts() again all we need to do is change the
186 ffi_call(&cif, puts, &rc, values);
193 File: libffi.info, Node: Types, Next: Multiple ABIs, Prev: Simple Example, Up: Using libffi
200 * Primitive Types:: Built-in types.
201 * Structures:: Structure types.
202 * Type Example:: Structure type example.
205 File: libffi.info, Node: Primitive Types, Next: Structures, Up: Types
207 2.3.1 Primitive Types
208 ---------------------
210 `Libffi' provides a number of built-in type descriptors that can be
211 used to describe argument and return types:
214 The type `void'. This cannot be used for argument types, only for
218 An unsigned, 8-bit integer type.
221 A signed, 8-bit integer type.
224 An unsigned, 16-bit integer type.
227 A signed, 16-bit integer type.
230 An unsigned, 32-bit integer type.
233 A signed, 32-bit integer type.
236 An unsigned, 64-bit integer type.
239 A signed, 64-bit integer type.
248 The C `unsigned char' type.
251 The C `signed char' type. (Note that there is not an exact
252 equivalent to the C `char' type in `libffi'; ordinarily you should
253 either use `ffi_type_schar' or `ffi_type_uchar' depending on
254 whether `char' is signed.)
257 The C `unsigned short' type.
263 The C `unsigned int' type.
269 The C `unsigned long' type.
274 `ffi_type_longdouble'
275 On platforms that have a C `long double' type, this is defined.
276 On other platforms, it is not.
279 A generic `void *' pointer. You should use this for all pointers,
280 regardless of their real type.
282 Each of these is of type `ffi_type', so you must take the address
283 when passing to `ffi_prep_cif'.
286 File: libffi.info, Node: Structures, Next: Type Example, Prev: Primitive Types, Up: Types
291 Although `libffi' has no special support for unions or bit-fields, it
292 is perfectly happy passing structures back and forth. You must first
293 describe the structure to `libffi' by creating a new `ffi_type' object
297 The `ffi_type' has the following members:
299 This is set by `libffi'; you should initialize it to zero.
301 `unsigned short alignment'
302 This is set by `libffi'; you should initialize it to zero.
304 `unsigned short type'
305 For a structure, this should be set to `FFI_TYPE_STRUCT'.
307 `ffi_type **elements'
308 This is a `NULL'-terminated array of pointers to `ffi_type'
309 objects. There is one element per field of the struct.
312 File: libffi.info, Node: Type Example, Prev: Structures, Up: Types
317 The following example initializes a `ffi_type' object representing the
318 `tm' struct from Linux's `time.h'.
320 Here is how the struct is defined:
332 /* Those are for future use. */
333 long int __tm_gmtoff__;
334 __const char *__tm_zone__;
337 Here is the corresponding code to describe this struct to `libffi':
341 ffi_type *tm_type_elements[12];
344 tm_type.size = tm_type.alignment = 0;
345 tm_type.elements = &tm_type_elements;
347 for (i = 0; i < 9; i++)
348 tm_type_elements[i] = &ffi_type_sint;
350 tm_type_elements[9] = &ffi_type_slong;
351 tm_type_elements[10] = &ffi_type_pointer;
352 tm_type_elements[11] = NULL;
354 /* tm_type can now be used to represent tm argument types and
355 return types for ffi_prep_cif() */
359 File: libffi.info, Node: Multiple ABIs, Next: The Closure API, Prev: Types, Up: Using libffi
364 A given platform may provide multiple different ABIs at once. For
365 instance, the x86 platform has both `stdcall' and `fastcall' functions.
367 `libffi' provides some support for this. However, this is
368 necessarily platform-specific.
371 File: libffi.info, Node: The Closure API, Prev: Multiple ABIs, Up: Using libffi
376 `libffi' also provides a way to write a generic function - a function
377 that can accept and decode any combination of arguments. This can be
378 useful when writing an interpreter, or to provide wrappers for
381 This facility is called the "closure API". Closures are not
382 supported on all platforms; you can check the `FFI_CLOSURES' define to
383 determine whether they are supported on the current platform.
385 Because closures work by assembling a tiny function at runtime, they
386 require special allocation on platforms that have a non-executable
387 heap. Memory management for closures is handled by a pair of functions:
389 -- Function: void *ffi_closure_alloc (size_t SIZE, void **CODE)
390 Allocate a chunk of memory holding SIZE bytes. This returns a
391 pointer to the writable address, and sets *CODE to the
392 corresponding executable address.
394 SIZE should be sufficient to hold a `ffi_closure' object.
396 -- Function: void ffi_closure_free (void *WRITABLE)
397 Free memory allocated using `ffi_closure_alloc'. The argument is
398 the writable address that was returned.
400 Once you have allocated the memory for a closure, you must construct
401 a `ffi_cif' describing the function call. Finally you can prepare the
404 -- Function: ffi_status ffi_prep_closure_loc (ffi_closure *CLOSURE,
405 ffi_cif *CIF, void (*FUN) (ffi_cif *CIF, void *RET, void
406 **ARGS, void *USER_DATA), void *USER_DATA, void *CODELOC)
407 Prepare a closure function.
409 CLOSURE is the address of a `ffi_closure' object; this is the
410 writable address returned by `ffi_closure_alloc'.
412 CIF is the `ffi_cif' describing the function parameters.
414 USER_DATA is an arbitrary datum that is passed, uninterpreted, to
415 your closure function.
417 CODELOC is the executable address returned by `ffi_closure_alloc'.
419 FUN is the function which will be called when the closure is
420 invoked. It is called with the arguments:
422 The `ffi_cif' passed to `ffi_prep_closure_loc'.
425 A pointer to the memory used for the function's return value.
426 FUN must fill this, unless the function is declared as
430 A vector of pointers to memory holding the arguments to the
434 The same USER_DATA that was passed to `ffi_prep_closure_loc'.
436 `ffi_prep_closure_loc' will return `FFI_OK' if everything went ok,
437 and something else on error.
439 After calling `ffi_prep_closure_loc', you can cast CODELOC to the
440 appropriate pointer-to-function type.
442 You may see old code referring to `ffi_prep_closure'. This function
443 is deprecated, as it cannot handle the need for separate writable and
444 executable addresses.
447 File: libffi.info, Node: Missing Features, Next: Index, Prev: Using libffi, Up: Top
452 `libffi' is missing a few features. We welcome patches to add support
455 * There is no support for calling varargs functions. This may work
456 on some platforms, depending on how the ABI is defined, but it is
459 * There is no support for bit fields in structures.
463 * The "raw" API is undocumented.
466 File: libffi.info, Node: Index, Prev: Missing Features, Up: Top
474 * : Structures. (line 12)
475 * ABI: Introduction. (line 13)
476 * Application Binary Interface: Introduction. (line 13)
477 * calling convention: Introduction. (line 13)
478 * cif: The Basics. (line 14)
479 * closure API: The Closure API. (line 13)
480 * closures: The Closure API. (line 13)
481 * FFI: Introduction. (line 31)
482 * ffi_call: The Basics. (line 41)
483 * ffi_closure_alloca: The Closure API. (line 19)
484 * ffi_closure_free: The Closure API. (line 26)
485 * FFI_CLOSURES: The Closure API. (line 13)
486 * ffi_prep_cif: The Basics. (line 16)
487 * ffi_prep_closure_loc: The Closure API. (line 34)
488 * ffi_status <1>: The Closure API. (line 37)
489 * ffi_status: The Basics. (line 18)
490 * ffi_type: Structures. (line 11)
491 * ffi_type_double: Primitive Types. (line 41)
492 * ffi_type_float: Primitive Types. (line 38)
493 * ffi_type_longdouble: Primitive Types. (line 71)
494 * ffi_type_pointer: Primitive Types. (line 75)
495 * ffi_type_schar: Primitive Types. (line 47)
496 * ffi_type_sint: Primitive Types. (line 62)
497 * ffi_type_sint16: Primitive Types. (line 23)
498 * ffi_type_sint32: Primitive Types. (line 29)
499 * ffi_type_sint64: Primitive Types. (line 35)
500 * ffi_type_sint8: Primitive Types. (line 17)
501 * ffi_type_slong: Primitive Types. (line 68)
502 * ffi_type_sshort: Primitive Types. (line 56)
503 * ffi_type_uchar: Primitive Types. (line 44)
504 * ffi_type_uint: Primitive Types. (line 59)
505 * ffi_type_uint16: Primitive Types. (line 20)
506 * ffi_type_uint32: Primitive Types. (line 26)
507 * ffi_type_uint64: Primitive Types. (line 32)
508 * ffi_type_uint8: Primitive Types. (line 14)
509 * ffi_type_ulong: Primitive Types. (line 65)
510 * ffi_type_ushort: Primitive Types. (line 53)
511 * ffi_type_void: Primitive Types. (line 10)
512 * Foreign Function Interface: Introduction. (line 31)
513 * void <1>: The Closure API. (line 20)
514 * void: The Basics. (line 43)
520 Node: Introduction
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521 Node: Using libffi
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522 Node: The Basics
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523 Node: Simple Example
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525 Node: Primitive Types
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526 Node: Structures
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527 Node: Type Example
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528 Node: Multiple ABIs
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529 Node: The Closure API
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530 Node: Missing Features
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