Source

cpython_sandbox / Modules / _ctypes / libffi / doc / libffi.texi

Full commit
  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
\input texinfo   @c -*-texinfo-*-
@c %**start of header
@setfilename libffi.info
@settitle libffi
@setchapternewpage off
@c %**end of header

@c Merge the standard indexes into a single one.
@syncodeindex fn cp
@syncodeindex vr cp
@syncodeindex ky cp
@syncodeindex pg cp
@syncodeindex tp cp

@include version.texi

@copying

This manual is for Libffi, a portable foreign-function interface
library.

Copyright @copyright{} 2008, 2010, 2011 Red Hat, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2, or (at your option) any
later version.  A copy of the license is included in the
section entitled ``GNU General Public License''.

@end quotation
@end copying

@dircategory Development
@direntry
* libffi: (libffi).             Portable foreign-function interface library.
@end direntry

@titlepage
@title Libffi
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage


@ifnottex
@node Top
@top libffi

@insertcopying

@menu
* Introduction::                What is libffi?
* Using libffi::                How to use libffi.
* Missing Features::            Things libffi can't do.
* Index::                       Index.
@end menu

@end ifnottex


@node Introduction
@chapter What is libffi?

Compilers for high level languages generate code that follow certain
conventions.  These conventions are necessary, in part, for separate
compilation to work.  One such convention is the @dfn{calling
convention}.  The calling convention is a set of assumptions made by
the compiler about where function arguments will be found on entry to
a function.  A calling convention also specifies where the return
value for a function is found.  The calling convention is also
sometimes called the @dfn{ABI} or @dfn{Application Binary Interface}.
@cindex calling convention
@cindex ABI
@cindex Application Binary Interface

Some programs may not know at the time of compilation what arguments
are to be passed to a function.  For instance, an interpreter may be
told at run-time about the number and types of arguments used to call
a given function.  @samp{Libffi} can be used in such programs to
provide a bridge from the interpreter program to compiled code.

The @samp{libffi} library provides a portable, high level programming
interface to various calling conventions.  This allows a programmer to
call any function specified by a call interface description at run
time.

@acronym{FFI} stands for Foreign Function Interface.  A foreign
function interface is the popular name for the interface that allows
code written in one language to call code written in another language.
The @samp{libffi} library really only provides the lowest, machine
dependent layer of a fully featured foreign function interface.  A
layer must exist above @samp{libffi} that handles type conversions for
values passed between the two languages.
@cindex FFI
@cindex Foreign Function Interface


@node Using libffi
@chapter Using libffi

@menu
* The Basics::                  The basic libffi API.
* Simple Example::              A simple example.
* Types::                       libffi type descriptions.
* Multiple ABIs::               Different passing styles on one platform.
* The Closure API::             Writing a generic function.
* Closure Example::             A closure example.
@end menu


@node The Basics
@section The Basics

@samp{Libffi} assumes that you have a pointer to the function you wish
to call and that you know the number and types of arguments to pass
it, as well as the return type of the function.

The first thing you must do is create an @code{ffi_cif} object that
matches the signature of the function you wish to call.  This is a
separate step because it is common to make multiple calls using a
single @code{ffi_cif}.  The @dfn{cif} in @code{ffi_cif} stands for
Call InterFace.  To prepare a call interface object, use the function
@code{ffi_prep_cif}.
@cindex cif

@findex ffi_prep_cif
@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})
This initializes @var{cif} according to the given parameters.

@var{abi} is the ABI to use; normally @code{FFI_DEFAULT_ABI} is what
you want.  @ref{Multiple ABIs} for more information.

@var{nargs} is the number of arguments that this function accepts.

@var{rtype} is a pointer to an @code{ffi_type} structure that
describes the return type of the function.  @xref{Types}.

@var{argtypes} is a vector of @code{ffi_type} pointers.
@var{argtypes} must have @var{nargs} elements.  If @var{nargs} is 0,
this argument is ignored.

@code{ffi_prep_cif} returns a @code{libffi} status code, of type
@code{ffi_status}.  This will be either @code{FFI_OK} if everything
worked properly; @code{FFI_BAD_TYPEDEF} if one of the @code{ffi_type}
objects is incorrect; or @code{FFI_BAD_ABI} if the @var{abi} parameter
is invalid.
@end defun

If the function being called is variadic (varargs) then
@code{ffi_prep_cif_var} must be used instead of @code{ffi_prep_cif}.

@findex ffi_prep_cif_var
@defun ffi_status ffi_prep_cif_var (ffi_cif *@var{cif}, ffi_abi var{abi}, unsigned int @var{nfixedargs}, unsigned int var{ntotalargs}, ffi_type *@var{rtype}, ffi_type **@var{argtypes})
This initializes @var{cif} according to the given parameters for
a call to a variadic function.  In general it's operation is the
same as for @code{ffi_prep_cif} except that:

@var{nfixedargs} is the number of fixed arguments, prior to any
variadic arguments.  It must be greater than zero.

@var{ntotalargs} the total number of arguments, including variadic
and fixed arguments.

Note that, different cif's must be prepped for calls to the same
function when different numbers of arguments are passed.

Also note that a call to @code{ffi_prep_cif_var} with
@var{nfixedargs}=@var{nototalargs} is NOT equivalent to a call to
@code{ffi_prep_cif}.

@end defun


To call a function using an initialized @code{ffi_cif}, use the
@code{ffi_call} function:

@findex ffi_call
@defun void ffi_call (ffi_cif *@var{cif}, void *@var{fn}, void *@var{rvalue}, void **@var{avalues})
This calls the function @var{fn} according to the description given in
@var{cif}.  @var{cif} must have already been prepared using
@code{ffi_prep_cif}.

@var{rvalue} is a pointer to a chunk of memory that will hold the
result of the function call.  This must be large enough to hold the
result and must be suitably aligned; it is the caller's responsibility
to ensure this.  If @var{cif} declares that the function returns
@code{void} (using @code{ffi_type_void}), then @var{rvalue} is
ignored.  If @var{rvalue} is @samp{NULL}, then the return value is
discarded.

@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.
@end defun


@node Simple Example
@section Simple Example

Here is a trivial example that calls @code{puts} a few times.

@example
#include <stdio.h>
#include <ffi.h>

int main()
@{
  ffi_cif cif;
  ffi_type *args[1];
  void *values[1];
  char *s;
  int rc;
  
  /* Initialize the argument info vectors */    
  args[0] = &ffi_type_pointer;
  values[0] = &s;
  
  /* Initialize the cif */
  if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1, 
		       &ffi_type_uint, args) == FFI_OK)
    @{
      s = "Hello World!";
      ffi_call(&cif, puts, &rc, values);
      /* rc now holds the result of the call to puts */
      
      /* values holds a pointer to the function's arg, so to 
         call puts() again all we need to do is change the 
         value of s */
      s = "This is cool!";
      ffi_call(&cif, puts, &rc, values);
    @}
  
  return 0;
@}
@end example


@node Types
@section Types

@menu
* Primitive Types::             Built-in types.
* Structures::                  Structure types.
* Type Example::                Structure type example.
@end menu

@node Primitive Types
@subsection Primitive Types

@code{Libffi} provides a number of built-in type descriptors that can
be used to describe argument and return types:

@table @code
@item ffi_type_void
@tindex ffi_type_void
The type @code{void}.  This cannot be used for argument types, only
for return values.

@item ffi_type_uint8
@tindex ffi_type_uint8
An unsigned, 8-bit integer type.

@item ffi_type_sint8
@tindex ffi_type_sint8
A signed, 8-bit integer type.

@item ffi_type_uint16
@tindex ffi_type_uint16
An unsigned, 16-bit integer type.

@item ffi_type_sint16
@tindex ffi_type_sint16
A signed, 16-bit integer type.

@item ffi_type_uint32
@tindex ffi_type_uint32
An unsigned, 32-bit integer type.

@item ffi_type_sint32
@tindex ffi_type_sint32
A signed, 32-bit integer type.

@item ffi_type_uint64
@tindex ffi_type_uint64
An unsigned, 64-bit integer type.

@item ffi_type_sint64
@tindex ffi_type_sint64
A signed, 64-bit integer type.

@item ffi_type_float
@tindex ffi_type_float
The C @code{float} type.

@item ffi_type_double
@tindex ffi_type_double
The C @code{double} type.

@item ffi_type_uchar
@tindex ffi_type_uchar
The C @code{unsigned char} type.

@item ffi_type_schar
@tindex ffi_type_schar
The C @code{signed char} type.  (Note that there is not an exact
equivalent to the C @code{char} type in @code{libffi}; ordinarily you
should either use @code{ffi_type_schar} or @code{ffi_type_uchar}
depending on whether @code{char} is signed.)

@item ffi_type_ushort
@tindex ffi_type_ushort
The C @code{unsigned short} type.

@item ffi_type_sshort
@tindex ffi_type_sshort
The C @code{short} type.

@item ffi_type_uint
@tindex ffi_type_uint
The C @code{unsigned int} type.

@item ffi_type_sint
@tindex ffi_type_sint
The C @code{int} type.

@item ffi_type_ulong
@tindex ffi_type_ulong
The C @code{unsigned long} type.

@item ffi_type_slong
@tindex ffi_type_slong
The C @code{long} type.

@item ffi_type_longdouble
@tindex ffi_type_longdouble
On platforms that have a C @code{long double} type, this is defined.
On other platforms, it is not.

@item ffi_type_pointer
@tindex ffi_type_pointer
A generic @code{void *} pointer.  You should use this for all
pointers, regardless of their real type.
@end table

Each of these is of type @code{ffi_type}, so you must take the address
when passing to @code{ffi_prep_cif}.


@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.
You must first describe the structure to @samp{libffi} by creating a
new @code{ffi_type} object for it.

@tindex ffi_type
@deftp {Data type} ffi_type
The @code{ffi_type} has the following members:
@table @code
@item size_t size
This is set by @code{libffi}; you should initialize it to zero.

@item unsigned short alignment
This is set by @code{libffi}; you should initialize it to zero.

@item unsigned short type
For a structure, this should be set to @code{FFI_TYPE_STRUCT}.

@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.
@end table
@end deftp


@node Type Example
@subsection Type Example

The following example initializes a @code{ffi_type} object
representing the @code{tm} struct from Linux's @file{time.h}.

Here is how the struct is defined:

@example
struct tm @{
    int tm_sec;
    int tm_min;
    int tm_hour;
    int tm_mday;
    int tm_mon;
    int tm_year;
    int tm_wday;
    int tm_yday;
    int tm_isdst;
    /* Those are for future use. */
    long int __tm_gmtoff__;
    __const char *__tm_zone__;
@};
@end example

Here is the corresponding code to describe this struct to
@code{libffi}:

@example
    @{
      ffi_type tm_type;
      ffi_type *tm_type_elements[12];
      int i;

      tm_type.size = tm_type.alignment = 0;
      tm_type.elements = &tm_type_elements;
    
      for (i = 0; i < 9; i++)
          tm_type_elements[i] = &ffi_type_sint;

      tm_type_elements[9] = &ffi_type_slong;
      tm_type_elements[10] = &ffi_type_pointer;
      tm_type_elements[11] = NULL;

      /* tm_type can now be used to represent tm argument types and
	 return types for ffi_prep_cif() */
    @}
@end example


@node Multiple ABIs
@section Multiple ABIs

A given platform may provide multiple different ABIs at once.  For
instance, the x86 platform has both @samp{stdcall} and @samp{fastcall}
functions.

@code{libffi} provides some support for this.  However, this is
necessarily platform-specific.

@c FIXME: document the platforms

@node The Closure API
@section The Closure API

@code{libffi} also provides a way to write a generic function -- a
function that can accept and decode any combination of arguments.
This can be useful when writing an interpreter, or to provide wrappers
for arbitrary functions.

This facility is called the @dfn{closure API}.  Closures are not
supported on all platforms; you can check the @code{FFI_CLOSURES}
define to determine whether they are supported on the current
platform.
@cindex closures
@cindex closure API
@findex FFI_CLOSURES

Because closures work by assembling a tiny function at runtime, they
require special allocation on platforms that have a non-executable
heap.  Memory management for closures is handled by a pair of
functions:

@findex ffi_closure_alloc
@defun void *ffi_closure_alloc (size_t @var{size}, void **@var{code})
Allocate a chunk of memory holding @var{size} bytes.  This returns a
pointer to the writable address, and sets *@var{code} to the
corresponding executable address.

@var{size} should be sufficient to hold a @code{ffi_closure} object.
@end defun

@findex ffi_closure_free
@defun void ffi_closure_free (void *@var{writable})
Free memory allocated using @code{ffi_closure_alloc}.  The argument is
the writable address that was returned.
@end defun


Once you have allocated the memory for a closure, you must construct a
@code{ffi_cif} describing the function call.  Finally you can prepare
the closure function:

@findex ffi_prep_closure_loc
@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})
Prepare a closure function.

@var{closure} is the address of a @code{ffi_closure} object; this is
the writable address returned by @code{ffi_closure_alloc}.

@var{cif} is the @code{ffi_cif} describing the function parameters.

@var{user_data} is an arbitrary datum that is passed, uninterpreted,
to your closure function.

@var{codeloc} is the executable address returned by
@code{ffi_closure_alloc}.

@var{fun} is the function which will be called when the closure is
invoked.  It is called with the arguments:
@table @var
@item cif
The @code{ffi_cif} passed to @code{ffi_prep_closure_loc}.

@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}.
@c FIXME: is this NULL for void-returning functions?

@item args
A vector of pointers to memory holding the arguments to the function.

@item user_data
The same @var{user_data} that was passed to
@code{ffi_prep_closure_loc}.
@end table

@code{ffi_prep_closure_loc} will return @code{FFI_OK} if everything
went ok, and something else on error.
@c FIXME: what?

After calling @code{ffi_prep_closure_loc}, you can cast @var{codeloc}
to the appropriate pointer-to-function type.
@end defun

You may see old code referring to @code{ffi_prep_closure}.  This
function is deprecated, as it cannot handle the need for separate
writable and executable addresses.

@node Closure Example
@section Closure Example

A trivial example that creates a new @code{puts} by binding 
@code{fputs} with @code{stdin}.

@example
#include <stdio.h>
#include <ffi.h>

/* Acts like puts with the file given at time of enclosure. */
void puts_binding(ffi_cif *cif, unsigned int *ret, void* args[], 
                  FILE *stream)
@{
  *ret = fputs(*(char **)args[0], stream);
@}

int main()
@{
  ffi_cif cif;
  ffi_type *args[1];
  ffi_closure *closure;

  int (*bound_puts)(char *);
  int rc;
  
  /* Allocate closure and bound_puts */
  closure = ffi_closure_alloc(sizeof(ffi_closure), &bound_puts);

  if (closure)
    @{
      /* Initialize the argument info vectors */
      args[0] = &ffi_type_pointer;

      /* Initialize the cif */
      if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
                       &ffi_type_uint, args) == FFI_OK)
        @{
          /* Initialize the closure, setting stream to stdout */
          if (ffi_prep_closure_loc(closure, &cif, puts_binding, 
                                   stdout, bound_puts) == FFI_OK)
            @{
              rc = bound_puts("Hello World!");
              /* rc now holds the result of the call to fputs */
            @}
        @}
    @}

  /* Deallocate both closure, and bound_puts */
  ffi_closure_free(closure);

  return 0;
@}

@end example


@node Missing Features
@chapter Missing Features

@code{libffi} is missing a few features.  We welcome patches to add
support for these.

@itemize @bullet
@item
Variadic closures.

@item
There is no support for bit fields in structures.

@item
The closure API is

@c FIXME: ...

@item
The ``raw'' API is undocumented.
@c argument promotion?
@c unions?
@c anything else?
@end itemize

Note that variadic support is very new and tested on a relatively
small number of platforms.

@node Index
@unnumbered Index

@printindex cp

@bye