Subversion Repositories openrisc

\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 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.

@samp{libffi} does not yet handle varargs functions; see @ref{Missing

Features} for more information.

@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

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.

@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 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

There is no support for calling varargs functions.  This may work on

some platforms, depending on how the ABI is defined, but it is not

reliable.

@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

@node Index

@unnumbered Index

@printindex cp

@bye