171 lines
7.0 KiB
HTML
171 lines
7.0 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!--
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This manual is for Libffi, a portable foreign-function interface
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library.
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Copyright (C) 2008, 2010, 2011 Red Hat, Inc.
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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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-->
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<!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>The Closure API (libffi)</title>
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<meta name="description" content="The Closure API (libffi)">
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<meta name="keywords" content="The Closure API (libffi)">
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<meta name="Generator" content="makeinfo">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index.html#Index" rel="index" title="Index">
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<link href="Using-libffi.html#Using-libffi" rel="up" title="Using libffi">
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<link href="Closure-Example.html#Closure-Example" rel="next" title="Closure Example">
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<link href="Multiple-ABIs.html#Multiple-ABIs" rel="prev" title="Multiple ABIs">
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</style>
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</head>
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<body lang="en">
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<a name="The-Closure-API"></a>
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<div class="header">
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<p>
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Next: <a href="Closure-Example.html#Closure-Example" accesskey="n" rel="next">Closure Example</a>, Previous: <a href="Multiple-ABIs.html#Multiple-ABIs" accesskey="p" rel="prev">Multiple ABIs</a>, Up: <a href="Using-libffi.html#Using-libffi" accesskey="u" rel="up">Using libffi</a> [<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="The-Closure-API-1"></a>
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<h3 class="section">2.5 The Closure API</h3>
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<p><code>libffi</code> 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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</p>
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<p>This facility is called the <em>closure API</em>. Closures are not
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supported on all platforms; you can check the <code>FFI_CLOSURES</code>
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define to determine whether they are supported on the current
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platform.
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<a name="index-closures"></a>
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<a name="index-closure-API"></a>
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<a name="index-FFI_005fCLOSURES"></a>
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</p>
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<p>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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</p>
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<a name="index-ffi_005fclosure_005falloc"></a>
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<dl>
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<dt><a name="index-void-1"></a>Function: <strong>void</strong> <em>*ffi_closure_alloc (size_t <var>size</var>, void **<var>code</var>)</em></dt>
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<dd><p>Allocate a chunk of memory holding <var>size</var> bytes. This returns a
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pointer to the writable address, and sets *<var>code</var> to the
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corresponding executable address.
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</p>
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<p><var>size</var> should be sufficient to hold a <code>ffi_closure</code> object.
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</p></dd></dl>
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<a name="index-ffi_005fclosure_005ffree"></a>
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<dl>
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<dt><a name="index-void-2"></a>Function: <strong>void</strong> <em>ffi_closure_free (void *<var>writable</var>)</em></dt>
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<dd><p>Free memory allocated using <code>ffi_closure_alloc</code>. The argument is
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the writable address that was returned.
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</p></dd></dl>
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<p>Once you have allocated the memory for a closure, you must construct a
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<code>ffi_cif</code> describing the function call. Finally you can prepare
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the closure function:
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</p>
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<a name="index-ffi_005fprep_005fclosure_005floc"></a>
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<dl>
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<dt><a name="index-ffi_005fstatus-2"></a>Function: <strong>ffi_status</strong> <em>ffi_prep_closure_loc (ffi_closure *<var>closure</var>, ffi_cif *<var>cif</var>, void (*<var>fun</var>) (ffi_cif *<var>cif</var>, void *<var>ret</var>, void **<var>args</var>, void *<var>user_data</var>), void *<var>user_data</var>, void *<var>codeloc</var>)</em></dt>
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<dd><p>Prepare a closure function.
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</p>
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<p><var>closure</var> is the address of a <code>ffi_closure</code> object; this is
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the writable address returned by <code>ffi_closure_alloc</code>.
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</p>
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<p><var>cif</var> is the <code>ffi_cif</code> describing the function parameters.
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</p>
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<p><var>user_data</var> is an arbitrary datum that is passed, uninterpreted,
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to your closure function.
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</p>
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<p><var>codeloc</var> is the executable address returned by
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<code>ffi_closure_alloc</code>.
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</p>
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<p><var>fun</var> 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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</p><dl compact="compact">
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<dt><var>cif</var></dt>
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<dd><p>The <code>ffi_cif</code> passed to <code>ffi_prep_closure_loc</code>.
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</p>
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</dd>
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<dt><var>ret</var></dt>
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<dd><p>A pointer to the memory used for the function’s return value.
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<var>fun</var> must fill this, unless the function is declared as returning
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<code>void</code>.
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</p>
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</dd>
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<dt><var>args</var></dt>
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<dd><p>A vector of pointers to memory holding the arguments to the function.
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</p>
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</dd>
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<dt><var>user_data</var></dt>
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<dd><p>The same <var>user_data</var> that was passed to
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<code>ffi_prep_closure_loc</code>.
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</p></dd>
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</dl>
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<p><code>ffi_prep_closure_loc</code> will return <code>FFI_OK</code> if everything
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went ok, and something else on error.
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</p>
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<p>After calling <code>ffi_prep_closure_loc</code>, you can cast <var>codeloc</var>
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to the appropriate pointer-to-function type.
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</p></dd></dl>
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<p>You may see old code referring to <code>ffi_prep_closure</code>. 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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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Closure-Example.html#Closure-Example" accesskey="n" rel="next">Closure Example</a>, Previous: <a href="Multiple-ABIs.html#Multiple-ABIs" accesskey="p" rel="prev">Multiple ABIs</a>, Up: <a href="Using-libffi.html#Using-libffi" accesskey="u" rel="up">Using libffi</a> [<a href="Index.html#Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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