Add closure API example to documentation

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@settitle libffi
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@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