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\input texinfo   @c -*-texinfo-*-
2
@c %**start of header
3
@setfilename libffi.info
4
@settitle libffi
5
@setchapternewpage off
6
@c %**end of header
7
 
8
@c Merge the standard indexes into a single one.
9
@syncodeindex fn cp
10
@syncodeindex vr cp
11
@syncodeindex ky cp
12
@syncodeindex pg cp
13
@syncodeindex tp cp
14
 
15
@include version.texi
16
 
17
@copying
18
 
19
This manual is for Libffi, a portable foreign-function interface
20
library.
21
 
22
Copyright @copyright{} 2008, 2010 Red Hat, Inc.
23
 
24
@quotation
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''.
30
 
31
@end quotation
32
@end copying
33
 
34
@dircategory Development
35
@direntry
36
* libffi: (libffi).             Portable foreign-function interface library.
37
@end direntry
38
 
39
@titlepage
40
@title Libffi
41
@page
42
@vskip 0pt plus 1filll
43
@insertcopying
44
@end titlepage
45
 
46
 
47
@ifnottex
48
@node Top
49
@top libffi
50
 
51
@insertcopying
52
 
53
@menu
54
* Introduction::                What is libffi?
55
* Using libffi::                How to use libffi.
56
* Missing Features::            Things libffi can't do.
57
* Index::                       Index.
58
@end menu
59
 
60
@end ifnottex
61
 
62
 
63
@node Introduction
64
@chapter What is libffi?
65
 
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
75
@cindex ABI
76
@cindex Application Binary Interface
77
 
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.
83
 
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
87
time.
88
 
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.
96
@cindex FFI
97
@cindex Foreign Function Interface
98
 
99
 
100
@node Using libffi
101
@chapter Using libffi
102
 
103
@menu
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.
110
@end menu
111
 
112
 
113
@node The Basics
114
@section The Basics
115
 
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.
119
 
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
125
@code{ffi_prep_cif}.
126
@cindex cif
127
 
128
@findex ffi_prep_cif
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.
131
 
132
@var{abi} is the ABI to use; normally @code{FFI_DEFAULT_ABI} is what
133
you want.  @ref{Multiple ABIs} for more information.
134
 
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.
138
 
139
@var{rtype} is a pointer to an @code{ffi_type} structure that
140
describes the return type of the function.  @xref{Types}.
141
 
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.
145
 
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
150
is invalid.
151
@end defun
152
 
153
 
154
To call a function using an initialized @code{ffi_cif}, use the
155
@code{ffi_call} function:
156
 
157
@findex ffi_call
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
161
@code{ffi_prep_cif}.
162
 
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
169
discarded.
170
 
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.
175
@end defun
176
 
177
 
178
@node Simple Example
179
@section Simple Example
180
 
181
Here is a trivial example that calls @code{puts} a few times.
182
 
183
@example
184
#include <stdio.h>
185
#include <ffi.h>
186
 
187
int main()
188
@{
189
  ffi_cif cif;
190
  ffi_type *args[1];
191
  void *values[1];
192
  char *s;
193
  int rc;
194
 
195
  /* Initialize the argument info vectors */
196
  args[0] = &ffi_type_pointer;
197
  values[0] = &s;
198
 
199
  /* Initialize the cif */
200
  if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
201
                       &ffi_type_uint, args) == FFI_OK)
202
    @{
203
      s = "Hello World!";
204
      ffi_call(&cif, puts, &rc, values);
205
      /* rc now holds the result of the call to puts */
206
 
207
      /* values holds a pointer to the function's arg, so to
208
         call puts() again all we need to do is change the
209
         value of s */
210
      s = "This is cool!";
211
      ffi_call(&cif, puts, &rc, values);
212
    @}
213
 
214
  return 0;
215
@}
216
@end example
217
 
218
 
219
@node Types
220
@section Types
221
 
222
@menu
223
* Primitive Types::             Built-in types.
224
* Structures::                  Structure types.
225
* Type Example::                Structure type example.
226
@end menu
227
 
228
@node Primitive Types
229
@subsection Primitive Types
230
 
231
@code{Libffi} provides a number of built-in type descriptors that can
232
be used to describe argument and return types:
233
 
234
@table @code
235
@item ffi_type_void
236
@tindex ffi_type_void
237
The type @code{void}.  This cannot be used for argument types, only
238
for return values.
239
 
240
@item ffi_type_uint8
241
@tindex ffi_type_uint8
242
An unsigned, 8-bit integer type.
243
 
244
@item ffi_type_sint8
245
@tindex ffi_type_sint8
246
A signed, 8-bit integer type.
247
 
248
@item ffi_type_uint16
249
@tindex ffi_type_uint16
250
An unsigned, 16-bit integer type.
251
 
252
@item ffi_type_sint16
253
@tindex ffi_type_sint16
254
A signed, 16-bit integer type.
255
 
256
@item ffi_type_uint32
257
@tindex ffi_type_uint32
258
An unsigned, 32-bit integer type.
259
 
260
@item ffi_type_sint32
261
@tindex ffi_type_sint32
262
A signed, 32-bit integer type.
263
 
264
@item ffi_type_uint64
265
@tindex ffi_type_uint64
266
An unsigned, 64-bit integer type.
267
 
268
@item ffi_type_sint64
269
@tindex ffi_type_sint64
270
A signed, 64-bit integer type.
271
 
272
@item ffi_type_float
273
@tindex ffi_type_float
274
The C @code{float} type.
275
 
276
@item ffi_type_double
277
@tindex ffi_type_double
278
The C @code{double} type.
279
 
280
@item ffi_type_uchar
281
@tindex ffi_type_uchar
282
The C @code{unsigned char} type.
283
 
284
@item ffi_type_schar
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.)
290
 
291
@item ffi_type_ushort
292
@tindex ffi_type_ushort
293
The C @code{unsigned short} type.
294
 
295
@item ffi_type_sshort
296
@tindex ffi_type_sshort
297
The C @code{short} type.
298
 
299
@item ffi_type_uint
300
@tindex ffi_type_uint
301
The C @code{unsigned int} type.
302
 
303
@item ffi_type_sint
304
@tindex ffi_type_sint
305
The C @code{int} type.
306
 
307
@item ffi_type_ulong
308
@tindex ffi_type_ulong
309
The C @code{unsigned long} type.
310
 
311
@item ffi_type_slong
312
@tindex ffi_type_slong
313
The C @code{long} type.
314
 
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.
319
 
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.
324
@end table
325
 
326
Each of these is of type @code{ffi_type}, so you must take the address
327
when passing to @code{ffi_prep_cif}.
328
 
329
 
330
@node Structures
331
@subsection Structures
332
 
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.
337
 
338
@tindex ffi_type
339
@deftp ffi_type
340
The @code{ffi_type} has the following members:
341
@table @code
342
@item size_t size
343
This is set by @code{libffi}; you should initialize it to zero.
344
 
345
@item unsigned short alignment
346
This is set by @code{libffi}; you should initialize it to zero.
347
 
348
@item unsigned short type
349
For a structure, this should be set to @code{FFI_TYPE_STRUCT}.
350
 
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.
354
@end table
355
@end deftp
356
 
357
 
358
@node Type Example
359
@subsection Type Example
360
 
361
The following example initializes a @code{ffi_type} object
362
representing the @code{tm} struct from Linux's @file{time.h}.
363
 
364
Here is how the struct is defined:
365
 
366
@example
367
struct tm @{
368
    int tm_sec;
369
    int tm_min;
370
    int tm_hour;
371
    int tm_mday;
372
    int tm_mon;
373
    int tm_year;
374
    int tm_wday;
375
    int tm_yday;
376
    int tm_isdst;
377
    /* Those are for future use. */
378
    long int __tm_gmtoff__;
379
    __const char *__tm_zone__;
380
@};
381
@end example
382
 
383
Here is the corresponding code to describe this struct to
384
@code{libffi}:
385
 
386
@example
387
    @{
388
      ffi_type tm_type;
389
      ffi_type *tm_type_elements[12];
390
      int i;
391
 
392
      tm_type.size = tm_type.alignment = 0;
393
      tm_type.elements = &tm_type_elements;
394
 
395
      for (i = 0; i < 9; i++)
396
          tm_type_elements[i] = &ffi_type_sint;
397
 
398
      tm_type_elements[9] = &ffi_type_slong;
399
      tm_type_elements[10] = &ffi_type_pointer;
400
      tm_type_elements[11] = NULL;
401
 
402
      /* tm_type can now be used to represent tm argument types and
403
         return types for ffi_prep_cif() */
404
    @}
405
@end example
406
 
407
 
408
@node Multiple ABIs
409
@section Multiple ABIs
410
 
411
A given platform may provide multiple different ABIs at once.  For
412
instance, the x86 platform has both @samp{stdcall} and @samp{fastcall}
413
functions.
414
 
415
@code{libffi} provides some support for this.  However, this is
416
necessarily platform-specific.
417
 
418
@c FIXME: document the platforms
419
 
420
@node The Closure API
421
@section The Closure API
422
 
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.
427
 
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
431
platform.
432
@cindex closures
433
@cindex closure API
434
@findex FFI_CLOSURES
435
 
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
439
functions:
440
 
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.
446
 
447
@var{size} should be sufficient to hold a @code{ffi_closure} object.
448
@end defun
449
 
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.
454
@end defun
455
 
456
 
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:
460
 
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.
464
 
465
@var{closure} is the address of a @code{ffi_closure} object; this is
466
the writable address returned by @code{ffi_closure_alloc}.
467
 
468
@var{cif} is the @code{ffi_cif} describing the function parameters.
469
 
470
@var{user_data} is an arbitrary datum that is passed, uninterpreted,
471
to your closure function.
472
 
473
@var{codeloc} is the executable address returned by
474
@code{ffi_closure_alloc}.
475
 
476
@var{fun} is the function which will be called when the closure is
477
invoked.  It is called with the arguments:
478
@table @var
479
@item cif
480
The @code{ffi_cif} passed to @code{ffi_prep_closure_loc}.
481
 
482
@item ret
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
485
@code{void}.
486
@c FIXME: is this NULL for void-returning functions?
487
 
488
@item args
489
A vector of pointers to memory holding the arguments to the function.
490
 
491
@item user_data
492
The same @var{user_data} that was passed to
493
@code{ffi_prep_closure_loc}.
494
@end table
495
 
496
@code{ffi_prep_closure_loc} will return @code{FFI_OK} if everything
497
went ok, and something else on error.
498
@c FIXME: what?
499
 
500
After calling @code{ffi_prep_closure_loc}, you can cast @var{codeloc}
501
to the appropriate pointer-to-function type.
502
@end defun
503
 
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.
507
 
508
@node Closure Example
509
@section Closure Example
510
 
511
A trivial example that creates a new @code{puts} by binding
512
@code{fputs} with @code{stdin}.
513
 
514
@example
515
#include <stdio.h>
516
#include <ffi.h>
517
 
518
/* Acts like puts with the file given at time of enclosure. */
519
void puts_binding(ffi_cif *cif, unsigned int *ret, void* args[],
520
                  FILE *stream)
521
@{
522
  *ret = fputs(*(char **)args[0], stream);
523
@}
524
 
525
int main()
526
@{
527
  ffi_cif cif;
528
  ffi_type *args[1];
529
  ffi_closure *closure;
530
 
531
  int (*bound_puts)(char *);
532
  int rc;
533
 
534
  /* Allocate closure and bound_puts */
535
  closure = ffi_closure_alloc(sizeof(ffi_closure), &bound_puts);
536
 
537
  if (closure)
538
    @{
539
      /* Initialize the argument info vectors */
540
      args[0] = &ffi_type_pointer;
541
 
542
      /* Initialize the cif */
543
      if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
544
                       &ffi_type_uint, args) == FFI_OK)
545
        @{
546
          /* Initialize the closure, setting stream to stdout */
547
          if (ffi_prep_closure_loc(closure, &cif, puts_binding,
548
                                   stdout, bound_puts) == FFI_OK)
549
            @{
550
              rc = bound_puts("Hello World!");
551
              /* rc now holds the result of the call to fputs */
552
            @}
553
        @}
554
    @}
555
 
556
  /* Deallocate both closure, and bound_puts */
557
  ffi_closure_free(closure);
558
 
559
  return 0;
560
@}
561
 
562
@end example
563
 
564
 
565
@node Missing Features
566
@chapter Missing Features
567
 
568
@code{libffi} is missing a few features.  We welcome patches to add
569
support for these.
570
 
571
@itemize @bullet
572
@item
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
575
reliable.
576
 
577
@item
578
There is no support for bit fields in structures.
579
 
580
@item
581
The closure API is
582
 
583
@c FIXME: ...
584
 
585
@item
586
The ``raw'' API is undocumented.
587
@c argument promotion?
588
@c unions?
589
@c anything else?
590
@end itemize
591
 
592
 
593
@node Index
594
@unnumbered Index
595
 
596
@printindex cp
597
 
598
@bye

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