623 lines
17 KiB
Plaintext
623 lines
17 KiB
Plaintext
\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 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
|