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162 lines
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162 lines
8.3 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Pragmas - The C Preprocessor</title>
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<meta name="description" content="The C Preprocessor">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="prev" href="Line-Control.html#Line-Control" title="Line Control">
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<link rel="next" href="Other-Directives.html#Other-Directives" title="Other Directives">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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Copyright (C) 1987-2015 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. A copy of
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the license is included in the
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section entitled ``GNU Free Documentation License''.
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This manual contains no Invariant Sections. The Front-Cover Texts are
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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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</head>
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<body>
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<div class="node">
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<a name="Pragmas"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Other-Directives.html#Other-Directives">Other Directives</a>,
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Previous: <a rel="previous" accesskey="p" href="Line-Control.html#Line-Control">Line Control</a>,
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Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
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<hr>
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</div>
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<h2 class="chapter">7 Pragmas</h2>
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<p>The ‘<samp><span class="samp">#pragma</span></samp>’ directive is the method specified by the C standard
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for providing additional information to the compiler, beyond what is
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conveyed in the language itself. Three forms of this directive
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(commonly known as <dfn>pragmas</dfn>) are specified by the 1999 C standard.
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A C compiler is free to attach any meaning it likes to other pragmas.
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<p>GCC has historically preferred to use extensions to the syntax of the
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language, such as <code>__attribute__</code>, for this purpose. However, GCC
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does define a few pragmas of its own. These mostly have effects on the
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entire translation unit or source file.
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<p>In GCC version 3, all GNU-defined, supported pragmas have been given a
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<code>GCC</code> prefix. This is in line with the <code>STDC</code> prefix on all
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pragmas defined by C99. For backward compatibility, pragmas which were
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recognized by previous versions are still recognized without the
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<code>GCC</code> prefix, but that usage is deprecated. Some older pragmas are
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deprecated in their entirety. They are not recognized with the
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<code>GCC</code> prefix. See <a href="Obsolete-Features.html#Obsolete-Features">Obsolete Features</a>.
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<p><a name="index-g_t_0040code_007b_005fPragma_007d-100"></a>C99 introduces the <code>_Pragma<!-- /@w --></code> operator. This feature addresses a
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major problem with ‘<samp><span class="samp">#pragma</span></samp>’: being a directive, it cannot be
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produced as the result of macro expansion. <code>_Pragma<!-- /@w --></code> is an
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operator, much like <code>sizeof</code> or <code>defined</code>, and can be embedded
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in a macro.
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<p>Its syntax is <code>_Pragma (</code><var>string-literal</var><code>)<!-- /@w --></code>, where
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<var>string-literal</var> can be either a normal or wide-character string
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literal. It is destringized, by replacing all ‘<samp><span class="samp">\\</span></samp>’ with a single
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‘<samp><span class="samp">\</span></samp>’ and all ‘<samp><span class="samp">\"</span></samp>’ with a ‘<samp><span class="samp">"</span></samp>’. The result is then
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processed as if it had appeared as the right hand side of a
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‘<samp><span class="samp">#pragma</span></samp>’ directive. For example,
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<pre class="smallexample"> _Pragma ("GCC dependency \"parse.y\"")
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</pre>
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<p class="noindent">has the same effect as <code>#pragma GCC dependency "parse.y"</code>. The
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same effect could be achieved using macros, for example
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<pre class="smallexample"> #define DO_PRAGMA(x) _Pragma (#x)
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DO_PRAGMA (GCC dependency "parse.y")
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</pre>
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<p>The standard is unclear on where a <code>_Pragma</code> operator can appear.
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The preprocessor does not accept it within a preprocessing conditional
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directive like ‘<samp><span class="samp">#if</span></samp>’. To be safe, you are probably best keeping it
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out of directives other than ‘<samp><span class="samp">#define</span></samp>’, and putting it on a line of
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its own.
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<p>This manual documents the pragmas which are meaningful to the
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preprocessor itself. Other pragmas are meaningful to the C or C++
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compilers. They are documented in the GCC manual.
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<p>GCC plugins may provide their own pragmas.
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<dl>
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<dt><code>#pragma GCC dependency</code><a name="index-g_t_0023pragma-GCC-dependency-101"></a><dd><code>#pragma GCC dependency</code> allows you to check the relative dates of
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the current file and another file. If the other file is more recent than
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the current file, a warning is issued. This is useful if the current
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file is derived from the other file, and should be regenerated. The
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other file is searched for using the normal include search path.
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Optional trailing text can be used to give more information in the
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warning message.
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<pre class="smallexample"> #pragma GCC dependency "parse.y"
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#pragma GCC dependency "/usr/include/time.h" rerun fixincludes
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</pre>
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<br><dt><code>#pragma GCC poison</code><a name="index-g_t_0023pragma-GCC-poison-102"></a><dd>Sometimes, there is an identifier that you want to remove completely
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from your program, and make sure that it never creeps back in. To
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enforce this, you can <dfn>poison</dfn> the identifier with this pragma.
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<code>#pragma GCC poison</code> is followed by a list of identifiers to
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poison. If any of those identifiers appears anywhere in the source
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after the directive, it is a hard error. For example,
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<pre class="smallexample"> #pragma GCC poison printf sprintf fprintf
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sprintf(some_string, "hello");
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</pre>
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<p class="noindent">will produce an error.
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<p>If a poisoned identifier appears as part of the expansion of a macro
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which was defined before the identifier was poisoned, it will <em>not</em>
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cause an error. This lets you poison an identifier without worrying
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about system headers defining macros that use it.
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<p>For example,
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<pre class="smallexample"> #define strrchr rindex
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#pragma GCC poison rindex
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strrchr(some_string, 'h');
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</pre>
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<p class="noindent">will not produce an error.
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<br><dt><code>#pragma GCC system_header</code><a name="index-g_t_0023pragma-GCC-system_005fheader-103"></a><dd>This pragma takes no arguments. It causes the rest of the code in the
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current file to be treated as if it came from a system header.
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See <a href="System-Headers.html#System-Headers">System Headers</a>.
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<br><dt><code>#pragma GCC warning</code><a name="index-g_t_0023pragma-GCC-warning-104"></a><dt><code>#pragma GCC error</code><a name="index-g_t_0023pragma-GCC-error-105"></a><dd><code>#pragma GCC warning "message"</code> causes the preprocessor to issue
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a warning diagnostic with the text ‘<samp><span class="samp">message</span></samp>’. The message
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contained in the pragma must be a single string literal. Similarly,
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<code>#pragma GCC error "message"</code> issues an error message. Unlike
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the ‘<samp><span class="samp">#warning</span></samp>’ and ‘<samp><span class="samp">#error</span></samp>’ directives, these pragmas can be
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embedded in preprocessor macros using ‘<samp><span class="samp">_Pragma</span></samp>’.
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</dl>
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</body></html>
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