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175 lines
7.8 KiB
HTML
175 lines
7.8 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Constructing Calls - Using the GNU Compiler Collection (GCC)</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="Using the GNU Compiler Collection (GCC)">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions">
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<link rel="prev" href="Nested-Functions.html#Nested-Functions" title="Nested Functions">
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<link rel="next" href="Typeof.html#Typeof" title="Typeof">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 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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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Constructing-Calls"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Typeof.html#Typeof">Typeof</a>,
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Previous: <a rel="previous" accesskey="p" href="Nested-Functions.html#Nested-Functions">Nested Functions</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.5 Constructing Function Calls</h3>
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<p><a name="index-constructing-calls-2882"></a><a name="index-forwarding-calls-2883"></a>
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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>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>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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<div class="defun">
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— Built-in Function: void * <b>__builtin_apply_args</b> ()<var><a name="index-g_t_005f_005fbuiltin_005fapply_005fargs-2884"></a></var><br>
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<blockquote><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>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></blockquote></div>
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<div class="defun">
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— Built-in Function: void * <b>__builtin_apply</b> (<var>void </var>(<var>*function</var>)()<var>, void *arguments, size_t size</var>)<var><a name="index-g_t_005f_005fbuiltin_005fapply-2885"></a></var><br>
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<blockquote><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>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>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>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></blockquote></div>
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<div class="defun">
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— Built-in Function: void <b>__builtin_return</b> (<var>void *result</var>)<var><a name="index-g_t_005f_005fbuiltin_005freturn-2886"></a></var><br>
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<blockquote><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></blockquote></div>
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<div class="defun">
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— Built-in Function: <b>__builtin_va_arg_pack</b> ()<var><a name="index-g_t_005f_005fbuiltin_005fva_005farg_005fpack-2887"></a></var><br>
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<blockquote><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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<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>
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</blockquote></div>
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<div class="defun">
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— Built-in Function: size_t <b>__builtin_va_arg_pack_len</b> ()<var><a name="index-g_t_005f_005fbuiltin_005fva_005farg_005fpack_005flen-2888"></a></var><br>
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<blockquote><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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<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>
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</blockquote></div>
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</body></html>
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