209 lines
9.0 KiB
HTML
209 lines
9.0 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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<!-- Copyright (C) 1988-2016 Free Software Foundation, 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 Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Using the GNU Compiler Collection (GCC): Constructing Calls</title>
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<meta name="description" content="Using the GNU Compiler Collection (GCC): Constructing Calls">
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<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Constructing Calls">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions">
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<link href="Typeof.html#Typeof" rel="next" title="Typeof">
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<link href="Nested-Functions.html#Nested-Functions" rel="prev" title="Nested Functions">
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<style type="text/css">
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<!--
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-->
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Constructing-Calls"></a>
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<div class="header">
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<p>
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Next: <a href="Typeof.html#Typeof" accesskey="n" rel="next">Typeof</a>, Previous: <a href="Nested-Functions.html#Nested-Functions" accesskey="p" rel="prev">Nested Functions</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Constructing-Function-Calls"></a>
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<h3 class="section">6.5 Constructing Function Calls</h3>
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<a name="index-constructing-calls"></a>
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<a name="index-forwarding-calls"></a>
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<p>Using the built-in functions described below, you can record
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the arguments a function received, and call another function
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with the same arguments, without knowing the number or types
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of the arguments.
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</p>
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<p>You can also record the return value of that function call,
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and later return that value, without knowing what data type
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the function tried to return (as long as your caller expects
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that data type).
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</p>
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<p>However, these built-in functions may interact badly with some
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sophisticated features or other extensions of the language. It
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is, therefore, not recommended to use them outside very simple
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functions acting as mere forwarders for their arguments.
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</p>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fapply_005fargs"></a>Built-in Function: <em>void *</em> <strong>__builtin_apply_args</strong> <em>()</em></dt>
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<dd><p>This built-in function returns a pointer to data
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describing how to perform a call with the same arguments as are passed
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to the current function.
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</p>
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<p>The function saves the arg pointer register, structure value address,
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and all registers that might be used to pass arguments to a function
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into a block of memory allocated on the stack. Then it returns the
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address of that block.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fapply"></a>Built-in Function: <em>void *</em> <strong>__builtin_apply</strong> <em>(void (*<var>function</var>)(), void *<var>arguments</var>, size_t <var>size</var>)</em></dt>
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<dd><p>This built-in function invokes <var>function</var>
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with a copy of the parameters described by <var>arguments</var>
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and <var>size</var>.
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</p>
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<p>The value of <var>arguments</var> should be the value returned by
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<code>__builtin_apply_args</code>. The argument <var>size</var> specifies the size
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of the stack argument data, in bytes.
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</p>
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<p>This function returns a pointer to data describing
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how to return whatever value is returned by <var>function</var>. The data
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is saved in a block of memory allocated on the stack.
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</p>
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<p>It is not always simple to compute the proper value for <var>size</var>. The
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value is used by <code>__builtin_apply</code> to compute the amount of data
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that should be pushed on the stack and copied from the incoming argument
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area.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005freturn"></a>Built-in Function: <em>void</em> <strong>__builtin_return</strong> <em>(void *<var>result</var>)</em></dt>
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<dd><p>This built-in function returns the value described by <var>result</var> from
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the containing function. You should specify, for <var>result</var>, a value
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returned by <code>__builtin_apply</code>.
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</p></dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fva_005farg_005fpack"></a>Built-in Function: <em></em> <strong>__builtin_va_arg_pack</strong> <em>()</em></dt>
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<dd><p>This built-in function represents all anonymous arguments of an inline
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function. It can be used only in inline functions that are always
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inlined, never compiled as a separate function, such as those using
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<code>__attribute__ ((__always_inline__))</code> or
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<code>__attribute__ ((__gnu_inline__))</code> extern inline functions.
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It must be only passed as last argument to some other function
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with variable arguments. This is useful for writing small wrapper
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inlines for variable argument functions, when using preprocessor
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macros is undesirable. For example:
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</p><div class="smallexample">
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<pre class="smallexample">extern int myprintf (FILE *f, const char *format, ...);
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extern inline __attribute__ ((__gnu_inline__)) int
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myprintf (FILE *f, const char *format, ...)
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{
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int r = fprintf (f, "myprintf: ");
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if (r < 0)
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return r;
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int s = fprintf (f, format, __builtin_va_arg_pack ());
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if (s < 0)
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return s;
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return r + s;
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}
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</pre></div>
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</dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fva_005farg_005fpack_005flen"></a>Built-in Function: <em>size_t</em> <strong>__builtin_va_arg_pack_len</strong> <em>()</em></dt>
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<dd><p>This built-in function returns the number of anonymous arguments of
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an inline function. It can be used only in inline functions that
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are always inlined, never compiled as a separate function, such
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as those using <code>__attribute__ ((__always_inline__))</code> or
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<code>__attribute__ ((__gnu_inline__))</code> extern inline functions.
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For example following does link- or run-time checking of open
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arguments for optimized code:
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</p><div class="smallexample">
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<pre class="smallexample">#ifdef __OPTIMIZE__
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extern inline __attribute__((__gnu_inline__)) int
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myopen (const char *path, int oflag, ...)
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{
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if (__builtin_va_arg_pack_len () > 1)
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warn_open_too_many_arguments ();
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if (__builtin_constant_p (oflag))
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{
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if ((oflag & O_CREAT) != 0 && __builtin_va_arg_pack_len () < 1)
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{
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warn_open_missing_mode ();
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return __open_2 (path, oflag);
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}
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return open (path, oflag, __builtin_va_arg_pack ());
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}
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if (__builtin_va_arg_pack_len () < 1)
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return __open_2 (path, oflag);
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return open (path, oflag, __builtin_va_arg_pack ());
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}
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#endif
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</pre></div>
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</dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Typeof.html#Typeof" accesskey="n" rel="next">Typeof</a>, Previous: <a href="Nested-Functions.html#Nested-Functions" accesskey="p" rel="prev">Nested Functions</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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