542 lines
15 KiB
Plaintext
542 lines
15 KiB
Plaintext
\input texinfo @c -*-texinfo-*-
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@c %**start of header
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@setfilename libffi.info
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@settitle libffi
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@setchapternewpage off
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@c %**end of header
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@c Merge the standard indexes into a single one.
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@syncodeindex fn cp
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@syncodeindex vr cp
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@syncodeindex ky cp
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@syncodeindex pg cp
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@syncodeindex tp cp
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@include version.texi
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@copying
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This manual is for Libffi, a portable foreign-function interface
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library.
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Copyright @copyright{} 2008 Red Hat, Inc.
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@quotation
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU General Public License as published by the
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Free Software Foundation; either version 2, or (at your option) any
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later version. A copy of the license is included in the
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section entitled ``GNU General Public License''.
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@end quotation
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@end copying
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@dircategory Development
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@direntry
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* libffi: (libffi). Portable foreign-function interface library.
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@end direntry
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@titlepage
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@title Libffi
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@page
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@vskip 0pt plus 1filll
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@insertcopying
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@end titlepage
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@ifnottex
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@node Top
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@top libffi
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@insertcopying
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@menu
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* Introduction:: What is libffi?
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* Using libffi:: How to use libffi.
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* Missing Features:: Things libffi can't do.
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* Index:: Index.
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@end menu
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@end ifnottex
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@node Introduction
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@chapter What is libffi?
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Compilers for high level languages generate code that follow certain
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conventions. These conventions are necessary, in part, for separate
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compilation to work. One such convention is the @dfn{calling
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convention}. The calling convention is a set of assumptions made by
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the compiler about where function arguments will be found on entry to
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a function. A calling convention also specifies where the return
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value for a function is found. The calling convention is also
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sometimes called the @dfn{ABI} or @dfn{Application Binary Interface}.
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@cindex calling convention
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@cindex ABI
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@cindex Application Binary Interface
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Some programs may not know at the time of compilation what arguments
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are to be passed to a function. For instance, an interpreter may be
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told at run-time about the number and types of arguments used to call
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a given function. @samp{Libffi} can be used in such programs to
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provide a bridge from the interpreter program to compiled code.
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The @samp{libffi} library provides a portable, high level programming
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interface to various calling conventions. This allows a programmer to
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call any function specified by a call interface description at run
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time.
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@acronym{FFI} stands for Foreign Function Interface. A foreign
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function interface is the popular name for the interface that allows
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code written in one language to call code written in another language.
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The @samp{libffi} library really only provides the lowest, machine
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dependent layer of a fully featured foreign function interface. A
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layer must exist above @samp{libffi} that handles type conversions for
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values passed between the two languages.
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@cindex FFI
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@cindex Foreign Function Interface
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@node Using libffi
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@chapter Using libffi
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@menu
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* The Basics:: The basic libffi API.
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* Simple Example:: A simple example.
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* Types:: libffi type descriptions.
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* Multiple ABIs:: Different passing styles on one platform.
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* The Closure API:: Writing a generic function.
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@end menu
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@node The Basics
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@section The Basics
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@samp{Libffi} assumes that you have a pointer to the function you wish
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to call and that you know the number and types of arguments to pass
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it, as well as the return type of the function.
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The first thing you must do is create an @code{ffi_cif} object that
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matches the signature of the function you wish to call. This is a
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separate step because it is common to make multiple calls using a
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single @code{ffi_cif}. The @dfn{cif} in @code{ffi_cif} stands for
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Call InterFace. To prepare a call interface object, use the function
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@code{ffi_prep_cif}.
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@cindex cif
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@findex ffi_prep_cif
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@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})
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This initializes @var{cif} according to the given parameters.
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@var{abi} is the ABI to use; normally @code{FFI_DEFAULT_ABI} is what
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you want. @ref{Multiple ABIs} for more information.
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@var{nargs} is the number of arguments that this function accepts.
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@samp{libffi} does not yet handle varargs functions; see @ref{Missing
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Features} for more information.
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@var{rtype} is a pointer to an @code{ffi_type} structure that
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describes the return type of the function. @xref{Types}.
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@var{argtypes} is a vector of @code{ffi_type} pointers.
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@var{argtypes} must have @var{nargs} elements. If @var{nargs} is 0,
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this argument is ignored.
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@code{ffi_prep_cif} returns a @code{libffi} status code, of type
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@code{ffi_status}. This will be either @code{FFI_OK} if everything
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worked properly; @code{FFI_BAD_TYPEDEF} if one of the @code{ffi_type}
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objects is incorrect; or @code{FFI_BAD_ABI} if the @var{abi} parameter
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is invalid.
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@end defun
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To call a function using an initialized @code{ffi_cif}, use the
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@code{ffi_call} function:
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@findex ffi_call
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@defun void ffi_call (ffi_cif *@var{cif}, void *@var{fn}, void *@var{rvalue}, void **@var{avalues})
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This calls the function @var{fn} according to the description given in
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@var{cif}. @var{cif} must have already been prepared using
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@code{ffi_prep_cif}.
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@var{rvalue} is a pointer to a chunk of memory that will hold the
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result of the function call. This must be large enough to hold the
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result and must be suitably aligned; it is the caller's responsibility
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to ensure this. If @var{cif} declares that the function returns
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@code{void} (using @code{ffi_type_void}), then @var{rvalue} is
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ignored. If @var{rvalue} is @samp{NULL}, then the return value is
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discarded.
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@var{avalues} is a vector of @code{void *} pointers that point to the
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memory locations holding the argument values for a call. If @var{cif}
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declares that the function has no arguments (i.e., @var{nargs} was 0),
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then @var{avalues} is ignored.
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@end defun
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@node Simple Example
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@section Simple Example
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Here is a trivial example that calls @code{puts} a few times.
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@example
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#include <stdio.h>
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#include <ffi.h>
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int main()
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@{
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ffi_cif cif;
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ffi_type *args[1];
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void *values[1];
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char *s;
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int rc;
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/* Initialize the argument info vectors */
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args[0] = &ffi_type_pointer;
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values[0] = &s;
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/* Initialize the cif */
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if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 1,
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&ffi_type_uint, args) == FFI_OK)
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@{
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s = "Hello World!";
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ffi_call(&cif, puts, &rc, values);
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/* rc now holds the result of the call to puts */
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/* values holds a pointer to the function's arg, so to
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call puts() again all we need to do is change the
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value of s */
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s = "This is cool!";
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ffi_call(&cif, puts, &rc, values);
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@}
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return 0;
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@}
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@end example
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@node Types
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@section Types
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@menu
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* Primitive Types:: Built-in types.
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* Structures:: Structure types.
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* Type Example:: Structure type example.
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@end menu
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@node Primitive Types
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@subsection Primitive Types
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@code{Libffi} provides a number of built-in type descriptors that can
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be used to describe argument and return types:
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@table @code
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@item ffi_type_void
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@tindex ffi_type_void
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The type @code{void}. This cannot be used for argument types, only
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for return values.
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@item ffi_type_uint8
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@tindex ffi_type_uint8
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An unsigned, 8-bit integer type.
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@item ffi_type_sint8
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@tindex ffi_type_sint8
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A signed, 8-bit integer type.
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@item ffi_type_uint16
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@tindex ffi_type_uint16
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An unsigned, 16-bit integer type.
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@item ffi_type_sint16
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@tindex ffi_type_sint16
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A signed, 16-bit integer type.
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@item ffi_type_uint32
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@tindex ffi_type_uint32
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An unsigned, 32-bit integer type.
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@item ffi_type_sint32
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@tindex ffi_type_sint32
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A signed, 32-bit integer type.
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@item ffi_type_uint64
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@tindex ffi_type_uint64
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An unsigned, 64-bit integer type.
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@item ffi_type_sint64
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@tindex ffi_type_sint64
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A signed, 64-bit integer type.
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@item ffi_type_float
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@tindex ffi_type_float
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The C @code{float} type.
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@item ffi_type_double
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@tindex ffi_type_double
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The C @code{double} type.
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@item ffi_type_uchar
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@tindex ffi_type_uchar
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The C @code{unsigned char} type.
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@item ffi_type_schar
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@tindex ffi_type_schar
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The C @code{signed char} type. (Note that there is not an exact
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equivalent to the C @code{char} type in @code{libffi}; ordinarily you
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should either use @code{ffi_type_schar} or @code{ffi_type_uchar}
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depending on whether @code{char} is signed.)
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@item ffi_type_ushort
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@tindex ffi_type_ushort
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The C @code{unsigned short} type.
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@item ffi_type_sshort
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@tindex ffi_type_sshort
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The C @code{short} type.
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@item ffi_type_uint
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@tindex ffi_type_uint
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The C @code{unsigned int} type.
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@item ffi_type_sint
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@tindex ffi_type_sint
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The C @code{int} type.
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@item ffi_type_ulong
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@tindex ffi_type_ulong
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The C @code{unsigned long} type.
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@item ffi_type_slong
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@tindex ffi_type_slong
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The C @code{long} type.
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@item ffi_type_longdouble
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@tindex ffi_type_longdouble
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On platforms that have a C @code{long double} type, this is defined.
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On other platforms, it is not.
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@item ffi_type_pointer
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@tindex ffi_type_pointer
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A generic @code{void *} pointer. You should use this for all
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pointers, regardless of their real type.
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@end table
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Each of these is of type @code{ffi_type}, so you must take the address
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when passing to @code{ffi_prep_cif}.
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@node Structures
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@subsection Structures
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Although @samp{libffi} has no special support for unions or
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bit-fields, it is perfectly happy passing structures back and forth.
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You must first describe the structure to @samp{libffi} by creating a
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new @code{ffi_type} object for it.
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@tindex ffi_type
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@deftp ffi_type
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The @code{ffi_type} has the following members:
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@table @code
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@item size_t size
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This is set by @code{libffi}; you should initialize it to zero.
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@item unsigned short alignment
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This is set by @code{libffi}; you should initialize it to zero.
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@item unsigned short type
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For a structure, this should be set to @code{FFI_TYPE_STRUCT}.
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@item ffi_type **elements
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This is a @samp{NULL}-terminated array of pointers to @code{ffi_type}
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objects. There is one element per field of the struct.
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@end table
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@end deftp
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@node Type Example
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@subsection Type Example
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The following example initializes a @code{ffi_type} object
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representing the @code{tm} struct from Linux's @file{time.h}.
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Here is how the struct is defined:
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@example
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struct tm @{
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int tm_sec;
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int tm_min;
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int tm_hour;
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int tm_mday;
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int tm_mon;
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int tm_year;
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int tm_wday;
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int tm_yday;
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int tm_isdst;
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/* Those are for future use. */
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long int __tm_gmtoff__;
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__const char *__tm_zone__;
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@};
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@end example
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Here is the corresponding code to describe this struct to
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@code{libffi}:
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@example
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@{
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ffi_type tm_type;
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ffi_type *tm_type_elements[12];
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int i;
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tm_type.size = tm_type.alignment = 0;
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tm_type.elements = &tm_type_elements;
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for (i = 0; i < 9; i++)
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tm_type_elements[i] = &ffi_type_sint;
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tm_type_elements[9] = &ffi_type_slong;
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tm_type_elements[10] = &ffi_type_pointer;
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tm_type_elements[11] = NULL;
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/* tm_type can now be used to represent tm argument types and
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return types for ffi_prep_cif() */
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@}
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@end example
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@node Multiple ABIs
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@section Multiple ABIs
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A given platform may provide multiple different ABIs at once. For
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instance, the x86 platform has both @samp{stdcall} and @samp{fastcall}
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functions.
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@code{libffi} provides some support for this. However, this is
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necessarily platform-specific.
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@c FIXME: document the platforms
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@node The Closure API
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@section The Closure API
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@code{libffi} also provides a way to write a generic function -- a
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function that can accept and decode any combination of arguments.
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This can be useful when writing an interpreter, or to provide wrappers
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for arbitrary functions.
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This facility is called the @dfn{closure API}. Closures are not
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supported on all platforms; you can check the @code{FFI_CLOSURES}
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define to determine whether they are supported on the current
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platform.
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@cindex closures
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@cindex closure API
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@findex FFI_CLOSURES
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Because closures work by assembling a tiny function at runtime, they
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require special allocation on platforms that have a non-executable
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heap. Memory management for closures is handled by a pair of
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functions:
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@findex ffi_closure_alloc
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@defun void *ffi_closure_alloc (size_t @var{size}, void **@var{code})
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Allocate a chunk of memory holding @var{size} bytes. This returns a
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pointer to the writable address, and sets *@var{code} to the
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corresponding executable address.
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@var{size} should be sufficient to hold a @code{ffi_closure} object.
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@end defun
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@findex ffi_closure_free
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@defun void ffi_closure_free (void *@var{writable})
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Free memory allocated using @code{ffi_closure_alloc}. The argument is
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the writable address that was returned.
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@end defun
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Once you have allocated the memory for a closure, you must construct a
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@code{ffi_cif} describing the function call. Finally you can prepare
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the closure function:
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@findex ffi_prep_closure_loc
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@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})
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Prepare a closure function.
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@var{closure} is the address of a @code{ffi_closure} object; this is
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the writable address returned by @code{ffi_closure_alloc}.
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@var{cif} is the @code{ffi_cif} describing the function parameters.
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@var{user_data} is an arbitrary datum that is passed, uninterpreted,
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to your closure function.
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@var{codeloc} is the executable address returned by
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@code{ffi_closure_alloc}.
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@var{fun} is the function which will be called when the closure is
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invoked. It is called with the arguments:
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@table @var
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@item cif
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The @code{ffi_cif} passed to @code{ffi_prep_closure_loc}.
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@item ret
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A pointer to the memory used for the function's return value.
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@var{fun} must fill this, unless the function is declared as returning
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@code{void}.
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@c FIXME: is this NULL for void-returning functions?
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@item args
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A vector of pointers to memory holding the arguments to the function.
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@item user_data
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The same @var{user_data} that was passed to
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@code{ffi_prep_closure_loc}.
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@end table
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@code{ffi_prep_closure_loc} will return @code{FFI_OK} if everything
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went ok, and something else on error.
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@c FIXME: what?
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After calling @code{ffi_prep_closure_loc}, you can cast @var{codeloc}
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to the appropriate pointer-to-function type.
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@end defun
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@c FIXME: example
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You may see old code referring to @code{ffi_prep_closure}. This
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function is deprecated, as it cannot handle the need for separate
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writable and executable addresses.
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@node Missing Features
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@chapter Missing Features
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@code{libffi} is missing a few features. We welcome patches to add
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support for these.
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@itemize @bullet
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@item
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There is no support for calling varargs functions. This may work on
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some platforms, depending on how the ABI is defined, but it is not
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reliable.
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@item
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There is no support for bit fields in structures.
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@item
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The closure API is
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@item
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The ``raw'' API is undocumented.
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@c argument promotion?
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@c unions?
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@c anything else?
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@end itemize
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@node Index
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@unnumbered Index
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@printindex cp
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@bye
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