| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being "Funding Free Software", the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| "GNU Free Documentation License". |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>Using the GNU Compiler Collection (GCC): Constructing Calls</title> |
| |
| <meta name="description" content="Using the GNU Compiler Collection (GCC): Constructing Calls"> |
| <meta name="keywords" content="Using the GNU Compiler Collection (GCC): Constructing Calls"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions"> |
| <link href="Typeof.html#Typeof" rel="next" title="Typeof"> |
| <link href="Nested-Functions.html#Nested-Functions" rel="prev" title="Nested Functions"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Constructing-Calls"></a> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| <hr> |
| <a name="Constructing-Function-Calls"></a> |
| <h3 class="section">6.5 Constructing Function Calls</h3> |
| <a name="index-constructing-calls"></a> |
| <a name="index-forwarding-calls"></a> |
| |
| <p>Using the built-in functions described below, you can record |
| the arguments a function received, and call another function |
| with the same arguments, without knowing the number or types |
| of the arguments. |
| </p> |
| <p>You can also record the return value of that function call, |
| and later return that value, without knowing what data type |
| the function tried to return (as long as your caller expects |
| that data type). |
| </p> |
| <p>However, these built-in functions may interact badly with some |
| sophisticated features or other extensions of the language. It |
| is, therefore, not recommended to use them outside very simple |
| functions acting as mere forwarders for their arguments. |
| </p> |
| <dl> |
| <dt><a name="index-_005f_005fbuiltin_005fapply_005fargs"></a>Built-in Function: <em>void *</em> <strong>__builtin_apply_args</strong> <em>()</em></dt> |
| <dd><p>This built-in function returns a pointer to data |
| describing how to perform a call with the same arguments as are passed |
| to the current function. |
| </p> |
| <p>The function saves the arg pointer register, structure value address, |
| and all registers that might be used to pass arguments to a function |
| into a block of memory allocated on the stack. Then it returns the |
| address of that block. |
| </p></dd></dl> |
| |
| <dl> |
| <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> |
| <dd><p>This built-in function invokes <var>function</var> |
| with a copy of the parameters described by <var>arguments</var> |
| and <var>size</var>. |
| </p> |
| <p>The value of <var>arguments</var> should be the value returned by |
| <code>__builtin_apply_args</code>. The argument <var>size</var> specifies the size |
| of the stack argument data, in bytes. |
| </p> |
| <p>This function returns a pointer to data describing |
| how to return whatever value is returned by <var>function</var>. The data |
| is saved in a block of memory allocated on the stack. |
| </p> |
| <p>It is not always simple to compute the proper value for <var>size</var>. The |
| value is used by <code>__builtin_apply</code> to compute the amount of data |
| that should be pushed on the stack and copied from the incoming argument |
| area. |
| </p></dd></dl> |
| |
| <dl> |
| <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> |
| <dd><p>This built-in function returns the value described by <var>result</var> from |
| the containing function. You should specify, for <var>result</var>, a value |
| returned by <code>__builtin_apply</code>. |
| </p></dd></dl> |
| |
| <dl> |
| <dt><a name="index-_005f_005fbuiltin_005fva_005farg_005fpack"></a>Built-in Function: <em></em> <strong>__builtin_va_arg_pack</strong> <em>()</em></dt> |
| <dd><p>This built-in function represents all anonymous arguments of an inline |
| function. It can be used only in inline functions that are always |
| inlined, never compiled as a separate function, such as those using |
| <code>__attribute__ ((__always_inline__))</code> or |
| <code>__attribute__ ((__gnu_inline__))</code> extern inline functions. |
| It must be only passed as last argument to some other function |
| with variable arguments. This is useful for writing small wrapper |
| inlines for variable argument functions, when using preprocessor |
| macros is undesirable. For example: |
| </p><div class="smallexample"> |
| <pre class="smallexample">extern int myprintf (FILE *f, const char *format, ...); |
| extern inline __attribute__ ((__gnu_inline__)) int |
| myprintf (FILE *f, const char *format, ...) |
| { |
| int r = fprintf (f, "myprintf: "); |
| if (r < 0) |
| return r; |
| int s = fprintf (f, format, __builtin_va_arg_pack ()); |
| if (s < 0) |
| return s; |
| return r + s; |
| } |
| </pre></div> |
| </dd></dl> |
| |
| <dl> |
| <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> |
| <dd><p>This built-in function returns the number of anonymous arguments of |
| an inline function. It can be used only in inline functions that |
| are always inlined, never compiled as a separate function, such |
| as those using <code>__attribute__ ((__always_inline__))</code> or |
| <code>__attribute__ ((__gnu_inline__))</code> extern inline functions. |
| For example following does link- or run-time checking of open |
| arguments for optimized code: |
| </p><div class="smallexample"> |
| <pre class="smallexample">#ifdef __OPTIMIZE__ |
| extern inline __attribute__((__gnu_inline__)) int |
| myopen (const char *path, int oflag, ...) |
| { |
| if (__builtin_va_arg_pack_len () > 1) |
| warn_open_too_many_arguments (); |
| |
| if (__builtin_constant_p (oflag)) |
| { |
| if ((oflag & O_CREAT) != 0 && __builtin_va_arg_pack_len () < 1) |
| { |
| warn_open_missing_mode (); |
| return __open_2 (path, oflag); |
| } |
| return open (path, oflag, __builtin_va_arg_pack ()); |
| } |
| |
| if (__builtin_va_arg_pack_len () < 1) |
| return __open_2 (path, oflag); |
| |
| return open (path, oflag, __builtin_va_arg_pack ()); |
| } |
| #endif |
| </pre></div> |
| </dd></dl> |
| |
| <hr> |
| <div class="header"> |
| <p> |
| 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> |
| </div> |
| |
| |
| |
| </body> |
| </html> |