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| author | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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| committer | alk3pInjection <webmaster@raspii.tech> | 2024-02-04 16:16:35 +0800 |
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diff --git a/share/doc/cpp/Pragmas.html b/share/doc/cpp/Pragmas.html new file mode 100644 index 0000000..3aa7f4b --- /dev/null +++ b/share/doc/cpp/Pragmas.html @@ -0,0 +1,231 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1987-2023 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. A copy of +the license is included in the +section entitled "GNU Free Documentation License". + +This manual contains no Invariant Sections. The Front-Cover Texts are +(a) (see below), and the Back-Cover Texts are (b) (see below). + +(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 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The C Preprocessor: Pragmas</title> + +<meta name="description" content="The C Preprocessor: Pragmas"> +<meta name="keywords" content="The C Preprocessor: Pragmas"> +<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="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="index.html#Top" rel="up" title="Top"> +<link href="Other-Directives.html#Other-Directives" rel="next" title="Other Directives"> +<link href="Line-Control.html#Line-Control" rel="previous" title="Line Control"> +<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="Pragmas"></a> +<div class="header"> +<p> +Next: <a href="Other-Directives.html#Other-Directives" accesskey="n" rel="next">Other Directives</a>, Previous: <a href="Line-Control.html#Line-Control" accesskey="p" rel="previous">Line Control</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Pragmas-1"></a> +<h2 class="chapter">7 Pragmas</h2> + +<a name="index-pragma-directive"></a> + +<p>The ‘<samp>#pragma</samp>’ directive is the method specified by the C standard +for providing additional information to the compiler, beyond what is +conveyed in the language itself. The forms of this directive +(commonly known as <em>pragmas</em>) specified by C standard are prefixed with +<code>STDC</code>. A C compiler is free to attach any meaning it likes to other +pragmas. Most GNU-defined, supported pragmas have been given a +<code>GCC</code> prefix. +</p> +<a name="index-_005fPragma"></a> +<p>C99 introduced the <code><span class="nolinebreak">_Pragma</span><!-- /@w --></code> operator. This feature addresses a +major problem with ‘<samp>#pragma</samp>’: being a directive, it cannot be +produced as the result of macro expansion. <code><span class="nolinebreak">_Pragma</span><!-- /@w --></code> is an +operator, much like <code>sizeof</code> or <code>defined</code>, and can be embedded +in a macro. +</p> +<p>Its syntax is <code><span class="nolinebreak">_Pragma</span> (<var><span class="nolinebreak">string-literal</span></var>)<!-- /@w --></code>, where +<var>string-literal</var> can be either a normal or wide-character string +literal. It is destringized, by replacing all ‘<samp>\\</samp>’ with a single +‘<samp>\</samp>’ and all ‘<samp>\"</samp>’ with a ‘<samp>"</samp>’. The result is then +processed as if it had appeared as the right hand side of a +‘<samp>#pragma</samp>’ directive. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">_Pragma ("GCC dependency \"parse.y\"") +</pre></div> + +<p>has the same effect as <code>#pragma GCC dependency "parse.y"</code>. The +same effect could be achieved using macros, for example +</p> +<div class="smallexample"> +<pre class="smallexample">#define DO_PRAGMA(x) _Pragma (#x) +DO_PRAGMA (GCC dependency "parse.y") +</pre></div> + +<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 ‘<samp>#if</samp>’. To be safe, you are probably best keeping it +out of directives other than ‘<samp>#define</samp>’, and putting it on a line of +its own. +</p> +<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> +<p>GCC plugins may provide their own pragmas. +</p> +<dl compact="compact"> +<dt><code>#pragma GCC dependency</code> +<a name="index-_0023pragma-GCC-dependency"></a> +</dt> +<dd><p><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. +</p> +<div class="smallexample"> +<pre class="smallexample">#pragma GCC dependency "parse.y" +#pragma GCC dependency "/usr/include/time.h" rerun fixincludes +</pre></div> + +</dd> +<dt><code>#pragma GCC poison</code> +<a name="index-_0023pragma-GCC-poison"></a> +</dt> +<dd><p>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 <em>poison</em> 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, +</p> +<div class="smallexample"> +<pre class="smallexample">#pragma GCC poison printf sprintf fprintf +sprintf(some_string, "hello"); +</pre></div> + +<p>will produce an error. +</p> +<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> +<p>For example, +</p> +<div class="smallexample"> +<pre class="smallexample">#define strrchr rindex +#pragma GCC poison rindex +strrchr(some_string, 'h'); +</pre></div> + +<p>will not produce an error. +</p> +</dd> +<dt><code>#pragma GCC system_header</code> +<a name="index-_0023pragma-GCC-system_005fheader-1"></a> +</dt> +<dd><p>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>. +</p> +</dd> +<dt><code>#pragma GCC warning</code> +<a name="index-_0023pragma-GCC-warning"></a> +</dt> +<dt><code>#pragma GCC error</code> +<a name="index-_0023pragma-GCC-error"></a> +</dt> +<dd><p><code>#pragma GCC warning "message"</code> causes the preprocessor to issue +a warning diagnostic with the text ‘<samp>message</samp>’. The message +contained in the pragma must be a single string literal. Similarly, +<code>#pragma GCC error "message"</code> issues an error message. Unlike +the ‘<samp>#warning</samp>’ and ‘<samp>#error</samp>’ directives, these pragmas can be +embedded in preprocessor macros using ‘<samp>_Pragma</samp>’. +</p> +</dd> +<dt><code>#pragma once</code> +<a name="index-_0023pragma-once"></a> +</dt> +<dd><p>If <code>#pragma once</code> is seen when scanning a header file, that +file will never be read again, no matter what. It is a less-portable +alternative to using ‘<samp>#ifndef</samp>’ to guard the contents of header files +against multiple inclusions. +</p> +</dd> +<dt><code>#pragma region {tokens}...</code> +<a name="index-_0023pragma-region-_007btokens_007d_002e_002e_002e"></a> +</dt> +<dt><code>#pragma endregion {tokens}...</code> +<a name="index-_0023pragma-endregion-_007btokens_007d_002e_002e_002e"></a> +</dt> +<dd><p>These pragmas are accepted, but have no effect. +</p> +</dd> +</dl> + +<hr> +<div class="header"> +<p> +Next: <a href="Other-Directives.html#Other-Directives" accesskey="n" rel="next">Other Directives</a>, Previous: <a href="Line-Control.html#Line-Control" accesskey="p" rel="previous">Line Control</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |