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This is ../libffi/doc/libffi.info, produced by makeinfo version 4.13
from ../libffi/doc/libffi.texi.
This manual is for Libffi, a portable foreign-function interface
library.
Copyright (C) 2008, 2010, 2011 Red Hat, Inc.
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".
INFO-DIR-SECTION Development
START-INFO-DIR-ENTRY
* libffi: (libffi). Portable foreign-function interface library.
END-INFO-DIR-ENTRY

File: libffi.info, Node: Top, Next: Introduction, Up: (dir)
libffi
******
This manual is for Libffi, a portable foreign-function interface
library.
Copyright (C) 2008, 2010, 2011 Red Hat, Inc.
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".
* Menu:
* Introduction:: What is libffi?
* Using libffi:: How to use libffi.
* Missing Features:: Things libffi can't do.
* Index:: Index.

File: libffi.info, Node: Introduction, Next: Using libffi, Prev: Top, Up: Top
1 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 "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 "ABI" or
"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. `Libffi' can be used in such programs to provide a
bridge from the interpreter program to compiled code.
The `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.
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
`libffi' library really only provides the lowest, machine dependent
layer of a fully featured foreign function interface. A layer must
exist above `libffi' that handles type conversions for values passed
between the two languages.

File: libffi.info, Node: Using libffi, Next: Missing Features, Prev: Introduction, Up: Top
2 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.

File: libffi.info, Node: The Basics, Next: Simple Example, Up: Using libffi
2.1 The Basics
==============
`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 `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 `ffi_cif'. The "cif" in `ffi_cif' stands for Call InterFace.
To prepare a call interface object, use the function `ffi_prep_cif'.
-- Function: ffi_status ffi_prep_cif (ffi_cif *CIF, ffi_abi ABI,
unsigned int NARGS, ffi_type *RTYPE, ffi_type **ARGTYPES)
This initializes CIF according to the given parameters.
ABI is the ABI to use; normally `FFI_DEFAULT_ABI' is what you
want. *note Multiple ABIs:: for more information.
NARGS is the number of arguments that this function accepts.
RTYPE is a pointer to an `ffi_type' structure that describes the
return type of the function. *Note Types::.
ARGTYPES is a vector of `ffi_type' pointers. ARGTYPES must have
NARGS elements. If NARGS is 0, this argument is ignored.
`ffi_prep_cif' returns a `libffi' status code, of type
`ffi_status'. This will be either `FFI_OK' if everything worked
properly; `FFI_BAD_TYPEDEF' if one of the `ffi_type' objects is
incorrect; or `FFI_BAD_ABI' if the ABI parameter is invalid.
If the function being called is variadic (varargs) then
`ffi_prep_cif_var' must be used instead of `ffi_prep_cif'.
-- Function: ffi_status ffi_prep_cif_var (ffi_cif *CIF, ffi_abi
varabi, unsigned int NFIXEDARGS, unsigned int varntotalargs,
ffi_type *RTYPE, ffi_type **ARGTYPES)
This initializes CIF according to the given parameters for a call
to a variadic function. In general it's operation is the same as
for `ffi_prep_cif' except that:
NFIXEDARGS is the number of fixed arguments, prior to any variadic
arguments. It must be greater than zero.
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 `ffi_prep_cif_var' with
NFIXEDARGS=NOTOTALARGS is NOT equivalent to a call to
`ffi_prep_cif'.
To call a function using an initialized `ffi_cif', use the
`ffi_call' function:
-- Function: void ffi_call (ffi_cif *CIF, void *FN, void *RVALUE, void
**AVALUES)
This calls the function FN according to the description given in
CIF. CIF must have already been prepared using `ffi_prep_cif'.
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 CIF declares that the function
returns `void' (using `ffi_type_void'), then RVALUE is ignored.
If RVALUE is `NULL', then the return value is discarded.
AVALUES is a vector of `void *' pointers that point to the memory
locations holding the argument values for a call. If CIF declares
that the function has no arguments (i.e., NARGS was 0), then
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.

File: libffi.info, Node: Simple Example, Next: Types, Prev: The Basics, Up: Using libffi
2.2 Simple Example
==================
Here is a trivial example that calls `puts' a few times.
#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;
}

File: libffi.info, Node: Types, Next: Multiple ABIs, Prev: Simple Example, Up: Using libffi
2.3 Types
=========
* Menu:
* Primitive Types:: Built-in types.
* Structures:: Structure types.
* Type Example:: Structure type example.

File: libffi.info, Node: Primitive Types, Next: Structures, Up: Types
2.3.1 Primitive Types
---------------------
`Libffi' provides a number of built-in type descriptors that can be
used to describe argument and return types:
`ffi_type_void'
The type `void'. This cannot be used for argument types, only for
return values.
`ffi_type_uint8'
An unsigned, 8-bit integer type.
`ffi_type_sint8'
A signed, 8-bit integer type.
`ffi_type_uint16'
An unsigned, 16-bit integer type.
`ffi_type_sint16'
A signed, 16-bit integer type.
`ffi_type_uint32'
An unsigned, 32-bit integer type.
`ffi_type_sint32'
A signed, 32-bit integer type.
`ffi_type_uint64'
An unsigned, 64-bit integer type.
`ffi_type_sint64'
A signed, 64-bit integer type.
`ffi_type_float'
The C `float' type.
`ffi_type_double'
The C `double' type.
`ffi_type_uchar'
The C `unsigned char' type.
`ffi_type_schar'
The C `signed char' type. (Note that there is not an exact
equivalent to the C `char' type in `libffi'; ordinarily you should
either use `ffi_type_schar' or `ffi_type_uchar' depending on
whether `char' is signed.)
`ffi_type_ushort'
The C `unsigned short' type.
`ffi_type_sshort'
The C `short' type.
`ffi_type_uint'
The C `unsigned int' type.
`ffi_type_sint'
The C `int' type.
`ffi_type_ulong'
The C `unsigned long' type.
`ffi_type_slong'
The C `long' type.
`ffi_type_longdouble'
On platforms that have a C `long double' type, this is defined.
On other platforms, it is not.
`ffi_type_pointer'
A generic `void *' pointer. You should use this for all pointers,
regardless of their real type.
Each of these is of type `ffi_type', so you must take the address
when passing to `ffi_prep_cif'.

File: libffi.info, Node: Structures, Next: Type Example, Prev: Primitive Types, Up: Types
2.3.2 Structures
----------------
Although `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 `libffi' by creating a new `ffi_type' object
for it.
-- Data type: ffi_type
The `ffi_type' has the following members:
`size_t size'
This is set by `libffi'; you should initialize it to zero.
`unsigned short alignment'
This is set by `libffi'; you should initialize it to zero.
`unsigned short type'
For a structure, this should be set to `FFI_TYPE_STRUCT'.
`ffi_type **elements'
This is a `NULL'-terminated array of pointers to `ffi_type'
objects. There is one element per field of the struct.

File: libffi.info, Node: Type Example, Prev: Structures, Up: Types
2.3.3 Type Example
------------------
The following example initializes a `ffi_type' object representing the
`tm' struct from Linux's `time.h'.
Here is how the struct is defined:
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__;
};
Here is the corresponding code to describe this struct to `libffi':
{
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() */
}

File: libffi.info, Node: Multiple ABIs, Next: The Closure API, Prev: Types, Up: Using libffi
2.4 Multiple ABIs
=================
A given platform may provide multiple different ABIs at once. For
instance, the x86 platform has both `stdcall' and `fastcall' functions.
`libffi' provides some support for this. However, this is
necessarily platform-specific.

File: libffi.info, Node: The Closure API, Next: Closure Example, Prev: Multiple ABIs, Up: Using libffi
2.5 The Closure API
===================
`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 "closure API". Closures are not
supported on all platforms; you can check the `FFI_CLOSURES' define to
determine whether they are supported on the current platform.
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:
-- Function: void *ffi_closure_alloc (size_t SIZE, void **CODE)
Allocate a chunk of memory holding SIZE bytes. This returns a
pointer to the writable address, and sets *CODE to the
corresponding executable address.
SIZE should be sufficient to hold a `ffi_closure' object.
-- Function: void ffi_closure_free (void *WRITABLE)
Free memory allocated using `ffi_closure_alloc'. The argument is
the writable address that was returned.
Once you have allocated the memory for a closure, you must construct
a `ffi_cif' describing the function call. Finally you can prepare the
closure function:
-- Function: ffi_status ffi_prep_closure_loc (ffi_closure *CLOSURE,
ffi_cif *CIF, void (*FUN) (ffi_cif *CIF, void *RET, void
**ARGS, void *USER_DATA), void *USER_DATA, void *CODELOC)
Prepare a closure function.
CLOSURE is the address of a `ffi_closure' object; this is the
writable address returned by `ffi_closure_alloc'.
CIF is the `ffi_cif' describing the function parameters.
USER_DATA is an arbitrary datum that is passed, uninterpreted, to
your closure function.
CODELOC is the executable address returned by `ffi_closure_alloc'.
FUN is the function which will be called when the closure is
invoked. It is called with the arguments:
CIF
The `ffi_cif' passed to `ffi_prep_closure_loc'.
RET
A pointer to the memory used for the function's return value.
FUN must fill this, unless the function is declared as
returning `void'.
ARGS
A vector of pointers to memory holding the arguments to the
function.
USER_DATA
The same USER_DATA that was passed to `ffi_prep_closure_loc'.
`ffi_prep_closure_loc' will return `FFI_OK' if everything went ok,
and something else on error.
After calling `ffi_prep_closure_loc', you can cast CODELOC to the
appropriate pointer-to-function type.
You may see old code referring to `ffi_prep_closure'. This function
is deprecated, as it cannot handle the need for separate writable and
executable addresses.

File: libffi.info, Node: Closure Example, Prev: The Closure API, Up: Using libffi
2.6 Closure Example
===================
A trivial example that creates a new `puts' by binding `fputs' with
`stdin'.
#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;
}

File: libffi.info, Node: Missing Features, Next: Index, Prev: Using libffi, Up: Top
3 Missing Features
******************
`libffi' is missing a few features. We welcome patches to add support
for these.
* Variadic closures.
* There is no support for bit fields in structures.
* The closure API is
* The "raw" API is undocumented.
Note that variadic support is very new and tested on a relatively
small number of platforms.

File: libffi.info, Node: Index, Prev: Missing Features, Up: Top
Index
*****
[index]
* Menu:
* ABI: Introduction. (line 13)
* Application Binary Interface: Introduction. (line 13)
* calling convention: Introduction. (line 13)
* cif: The Basics. (line 14)
* closure API: The Closure API. (line 13)
* closures: The Closure API. (line 13)
* FFI: Introduction. (line 31)
* ffi_call: The Basics. (line 63)
* ffi_closure_alloc: The Closure API. (line 19)
* ffi_closure_free: The Closure API. (line 26)
* FFI_CLOSURES: The Closure API. (line 13)
* ffi_prep_cif: The Basics. (line 16)
* ffi_prep_cif_var: The Basics. (line 39)
* ffi_prep_closure_loc: The Closure API. (line 34)
* ffi_status <1>: The Closure API. (line 37)
* ffi_status: The Basics. (line 18)
* ffi_type: Structures. (line 11)
* ffi_type_double: Primitive Types. (line 41)
* ffi_type_float: Primitive Types. (line 38)
* ffi_type_longdouble: Primitive Types. (line 71)
* ffi_type_pointer: Primitive Types. (line 75)
* ffi_type_schar: Primitive Types. (line 47)
* ffi_type_sint: Primitive Types. (line 62)
* ffi_type_sint16: Primitive Types. (line 23)
* ffi_type_sint32: Primitive Types. (line 29)
* ffi_type_sint64: Primitive Types. (line 35)
* ffi_type_sint8: Primitive Types. (line 17)
* ffi_type_slong: Primitive Types. (line 68)
* ffi_type_sshort: Primitive Types. (line 56)
* ffi_type_uchar: Primitive Types. (line 44)
* ffi_type_uint: Primitive Types. (line 59)
* ffi_type_uint16: Primitive Types. (line 20)
* ffi_type_uint32: Primitive Types. (line 26)
* ffi_type_uint64: Primitive Types. (line 32)
* ffi_type_uint8: Primitive Types. (line 14)
* ffi_type_ulong: Primitive Types. (line 65)
* ffi_type_ushort: Primitive Types. (line 53)
* ffi_type_void: Primitive Types. (line 10)
* Foreign Function Interface: Introduction. (line 31)
* void <1>: The Closure API. (line 20)
* void: The Basics. (line 65)

Tag Table:
Node: Top712
Node: Introduction1460
Node: Using libffi3096
Node: The Basics3582
Node: Simple Example7224
Node: Types8251
Node: Primitive Types8534
Node: Structures10354
Node: Type Example11224
Node: Multiple ABIs12447
Node: The Closure API12818
Node: Closure Example15762
Node: Missing Features17321
Node: Index17774

End Tag Table

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\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

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