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<h2 class="chapter">7 Pragmas</h2>

<p>The &lsquo;<samp><span class="samp">#pragma</span></samp>&rsquo; directive is the method specified by the C standard
for providing additional information to the compiler, beyond what is
conveyed in the language itself.  Three forms of this directive
(commonly known as <dfn>pragmas</dfn>) are specified by the 1999 C standard. 
A C compiler is free to attach any meaning it likes to other pragmas.

   <p>GCC has historically preferred to use extensions to the syntax of the
language, such as <code>__attribute__</code>, for this purpose.  However, GCC
does define a few pragmas of its own.  These mostly have effects on the
entire translation unit or source file.

   <p>In GCC version 3, all GNU-defined, supported pragmas have been given a
<code>GCC</code> prefix.  This is in line with the <code>STDC</code> prefix on all
pragmas defined by C99.  For backward compatibility, pragmas which were
recognized by previous versions are still recognized without the
<code>GCC</code> prefix, but that usage is deprecated.  Some older pragmas are
deprecated in their entirety.  They are not recognized with the
<code>GCC</code> prefix.  See <a href="Obsolete-Features.html#Obsolete-Features">Obsolete Features</a>.

   <p><a name="index-g_t_0040code_007b_005fPragma_007d-100"></a>C99 introduces the <code>_Pragma<!-- /@w --></code> operator.  This feature addresses a
major problem with &lsquo;<samp><span class="samp">#pragma</span></samp>&rsquo;: being a directive, it cannot be
produced as the result of macro expansion.  <code>_Pragma<!-- /@w --></code> is an
operator, much like <code>sizeof</code> or <code>defined</code>, and can be embedded
in a macro.

   <p>Its syntax is <code>_Pragma&nbsp;(</code><var>string-literal</var><code>)<!-- /@w --></code>, where
<var>string-literal</var> can be either a normal or wide-character string
literal.  It is destringized, by replacing all &lsquo;<samp><span class="samp">\\</span></samp>&rsquo; with a single
&lsquo;<samp><span class="samp">\</span></samp>&rsquo; and all &lsquo;<samp><span class="samp">\"</span></samp>&rsquo; with a &lsquo;<samp><span class="samp">"</span></samp>&rsquo;.  The result is then
processed as if it had appeared as the right hand side of a
&lsquo;<samp><span class="samp">#pragma</span></samp>&rsquo; directive.  For example,

<pre class="smallexample">     _Pragma ("GCC dependency \"parse.y\"")
</pre>
   <p class="noindent">has the same effect as <code>#pragma GCC dependency "parse.y"</code>.  The
same effect could be achieved using macros, for example

<pre class="smallexample">     #define DO_PRAGMA(x) _Pragma (#x)
     DO_PRAGMA (GCC dependency "parse.y")
</pre>
   <p>The standard is unclear on where a <code>_Pragma</code> operator can appear. 
The preprocessor does not accept it within a preprocessing conditional
directive like &lsquo;<samp><span class="samp">#if</span></samp>&rsquo;.  To be safe, you are probably best keeping it
out of directives other than &lsquo;<samp><span class="samp">#define</span></samp>&rsquo;, and putting it on a line of
its own.

   <p>This manual documents the pragmas which are meaningful to the
preprocessor itself.  Other pragmas are meaningful to the C or C++
compilers.  They are documented in the GCC manual.

   <p>GCC plugins may provide their own pragmas.

     <dl>
<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
the current file and another file.  If the other file is more recent than
the current file, a warning is issued.  This is useful if the current
file is derived from the other file, and should be regenerated.  The
other file is searched for using the normal include search path. 
Optional trailing text can be used to give more information in the
warning message.

     <pre class="smallexample">          #pragma GCC dependency "parse.y"
          #pragma GCC dependency "/usr/include/time.h" rerun fixincludes
</pre>
     <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
from your program, and make sure that it never creeps back in.  To
enforce this, you can <dfn>poison</dfn> the identifier with this pragma. 
<code>#pragma GCC poison</code> is followed by a list of identifiers to
poison.  If any of those identifiers appears anywhere in the source
after the directive, it is a hard error.  For example,

     <pre class="smallexample">          #pragma GCC poison printf sprintf fprintf
          sprintf(some_string, "hello");
</pre>
     <p class="noindent">will produce an error.

     <p>If a poisoned identifier appears as part of the expansion of a macro
which was defined before the identifier was poisoned, it will <em>not</em>
cause an error.  This lets you poison an identifier without worrying
about system headers defining macros that use it.

     <p>For example,

     <pre class="smallexample">          #define strrchr rindex
          #pragma GCC poison rindex
          strrchr(some_string, 'h');
</pre>
     <p class="noindent">will not produce an error.

     <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
current file to be treated as if it came from a system header. 
See <a href="System-Headers.html#System-Headers">System Headers</a>.

   </dl>

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