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766 lines
22 KiB
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766 lines
22 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, 2010, 2011 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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* Closure Example:: A closure example.
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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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@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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If the function being called is variadic (varargs) then
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@code{ffi_prep_cif_var} must be used instead of @code{ffi_prep_cif}.
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@findex ffi_prep_cif_var
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@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})
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This initializes @var{cif} according to the given parameters for
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a call to a variadic function. In general it's operation is the
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same as for @code{ffi_prep_cif} except that:
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@var{nfixedargs} is the number of fixed arguments, prior to any
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variadic arguments. It must be greater than zero.
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@var{ntotalargs} the total number of arguments, including variadic
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and fixed arguments.
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Note that, different cif's must be prepped for calls to the same
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function when different numbers of arguments are passed.
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Also note that a call to @code{ffi_prep_cif_var} with
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@var{nfixedargs}=@var{nototalargs} is NOT equivalent to a call to
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@code{ffi_prep_cif}.
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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, no smaller than the system register size (generally 32 or 64
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bits), 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.
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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. Note that argument values may be
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modified by the callee (for instance, structs passed by value); the
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burden of copying pass-by-value arguments is placed on the caller.
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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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ffi_arg 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_sint, 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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* Complex:: Complex types.
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* Complex Type Example:: Complex 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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@item ffi_type_complex_float
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@tindex ffi_type_complex_float
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The C @code{_Complex float} type.
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@item ffi_type_complex_double
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@tindex ffi_type_complex_double
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The C @code{_Complex double} type.
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@item ffi_type_complex_longdouble
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@tindex ffi_type_complex_longdouble
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The C @code{_Complex long double} type.
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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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@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 {Data type} 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.type = FFI_TYPE_STRUCT;
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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 Complex
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@subsection Complex Types
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@samp{libffi} supports the complex types defined by the C99
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standard (@code{_Complex float}, @code{_Complex double} and
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@code{_Complex long double} with the built-in type descriptors
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@code{ffi_type_complex_float}, @code{ffi_type_complex_double} and
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@code{ffi_type_complex_longdouble}.
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Custom complex types like @code{_Complex int} can also be used.
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An @code{ffi_type} object has to be defined to describe the
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complex type to @samp{libffi}.
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@tindex ffi_type
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@deftp {Data type} ffi_type
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@table @code
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@item size_t size
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This must be manually set to the size of the complex type.
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@item unsigned short alignment
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This must be manually set to the alignment of the complex type.
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@item unsigned short type
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For a complex type, this must be set to @code{FFI_TYPE_COMPLEX}.
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@item ffi_type **elements
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This is a @samp{NULL}-terminated array of pointers to
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@code{ffi_type} objects. The first element is set to the
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@code{ffi_type} of the complex's base type. The second element
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must be set to @code{NULL}.
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@end table
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@end deftp
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The section @ref{Complex Type Example} shows a way to determine
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the @code{size} and @code{alignment} members in a platform
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independent way.
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For platforms that have no complex support in @code{libffi} yet,
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the functions @code{ffi_prep_cif} and @code{ffi_prep_args} abort
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the program if they encounter a complex type.
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@node Complex Type Example
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|
@subsection Complex Type Example
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This example demonstrates how to use complex types:
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@example
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#include <stdio.h>
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#include <ffi.h>
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#include <complex.h>
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void complex_fn(_Complex float cf,
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_Complex double cd,
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_Complex long double cld)
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@{
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printf("cf=%f+%fi\ncd=%f+%fi\ncld=%f+%fi\n",
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(float)creal (cf), (float)cimag (cf),
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(float)creal (cd), (float)cimag (cd),
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(float)creal (cld), (float)cimag (cld));
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@}
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int main()
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@{
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ffi_cif cif;
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ffi_type *args[3];
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void *values[3];
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_Complex float cf;
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_Complex double cd;
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_Complex long double cld;
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|
|
/* Initialize the argument info vectors */
|
|
args[0] = &ffi_type_complex_float;
|
|
args[1] = &ffi_type_complex_double;
|
|
args[2] = &ffi_type_complex_longdouble;
|
|
values[0] = &cf;
|
|
values[1] = &cd;
|
|
values[2] = &cld;
|
|
|
|
/* Initialize the cif */
|
|
if (ffi_prep_cif(&cif, FFI_DEFAULT_ABI, 3,
|
|
&ffi_type_void, args) == FFI_OK)
|
|
@{
|
|
cf = 1.0 + 20.0 * I;
|
|
cd = 300.0 + 4000.0 * I;
|
|
cld = 50000.0 + 600000.0 * I;
|
|
/* Call the function */
|
|
ffi_call(&cif, (void (*)(void))complex_fn, 0, values);
|
|
@}
|
|
|
|
return 0;
|
|
@}
|
|
@end example
|
|
|
|
This is an example for defining a custom complex type descriptor
|
|
for compilers that support them:
|
|
|
|
@example
|
|
/*
|
|
* This macro can be used to define new complex type descriptors
|
|
* in a platform independent way.
|
|
*
|
|
* name: Name of the new descriptor is ffi_type_complex_<name>.
|
|
* type: The C base type of the complex type.
|
|
*/
|
|
#define FFI_COMPLEX_TYPEDEF(name, type, ffitype) \
|
|
static ffi_type *ffi_elements_complex_##name [2] = @{ \
|
|
(ffi_type *)(&ffitype), NULL \
|
|
@}; \
|
|
struct struct_align_complex_##name @{ \
|
|
char c; \
|
|
_Complex type x; \
|
|
@}; \
|
|
ffi_type ffi_type_complex_##name = @{ \
|
|
sizeof(_Complex type), \
|
|
offsetof(struct struct_align_complex_##name, x), \
|
|
FFI_TYPE_COMPLEX, \
|
|
(ffi_type **)ffi_elements_complex_##name \
|
|
@}
|
|
|
|
/* Define new complex type descriptors using the macro: */
|
|
/* ffi_type_complex_sint */
|
|
FFI_COMPLEX_TYPEDEF(sint, int, ffi_type_sint);
|
|
/* ffi_type_complex_uchar */
|
|
FFI_COMPLEX_TYPEDEF(uchar, unsigned char, ffi_type_uint8);
|
|
@end example
|
|
|
|
The new type descriptors can then be used like one of the built-in
|
|
type descriptors in the previous 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{stdout}.
|
|
|
|
@example
|
|
#include <stdio.h>
|
|
#include <ffi.h>
|
|
|
|
/* Acts like puts with the file given at time of enclosure. */
|
|
void puts_binding(ffi_cif *cif, void *ret, void* args[],
|
|
void *stream)
|
|
@{
|
|
*(ffi_arg *)ret = fputs(*(char **)args[0], (FILE *)stream);
|
|
@}
|
|
|
|
typedef int (*puts_t)(char *);
|
|
|
|
int main()
|
|
@{
|
|
ffi_cif cif;
|
|
ffi_type *args[1];
|
|
ffi_closure *closure;
|
|
|
|
void *bound_puts;
|
|
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_sint, 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 = ((puts_t)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 ``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
|