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| 22 | <title>Pragma Basics (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Pragmas.html" rel="up" title="Pragmas">
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| 33 | <link href="Severity-Pragmas.html" rel="next" title="Severity Pragmas">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Pragma-Basics"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Severity-Pragmas.html" accesskey="n" rel="next">Severity Pragmas</a>, Up: <a href="Pragmas.html" accesskey="u" rel="up">Pragmas</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Pragma-Basics-1"></span><h4 class="subsection">30.1.1 Pragma Basics</h4>
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| 64 |
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| 65 | <p>C defines two syntactical forms for pragmas, the line form and the
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| 66 | token form. You can write any pragma in either form, with the same
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| 67 | meaning.
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| 68 | </p>
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| 69 | <p>The line form is a line in the source code, like this:
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">#pragma <var>line</var>
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| 73 | </pre></div>
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| 74 |
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| 75 | <p>The line pragma has no effect on the parsing of the lines around it.
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| 76 | This form has the drawback that it can’t be generated by a macro expansion.
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| 77 | </p>
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| 78 | <p>The token form is a series of tokens; it can appear anywhere in the
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| 79 | program between the other tokens.
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| 80 | </p>
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| 81 | <div class="example">
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| 82 | <pre class="example">_Pragma (<var>stringconstant</var>)
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| 83 | </pre></div>
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| 84 |
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| 85 | <p>The pragma has no effect on the syntax of the tokens that surround it;
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| 86 | thus, here’s a pragma in the middle of an <code>if</code> statement:
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| 87 | </p>
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| 88 | <div class="example">
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| 89 | <pre class="example">if _Pragma ("hello") (x > 1)
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| 90 | </pre></div>
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| 91 |
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| 92 | <p>However, that’s an unclear thing to do; for the sake of
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| 93 | understandability, it is better to put a pragma on a line by itself
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| 94 | and not embedded in the middle of another construct.
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| 95 | </p>
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| 96 | <p>Both forms of pragma have a textual argument. In a line pragma, the
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| 97 | text is the rest of the line. The textual argument to <code>_Pragma</code>
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| 98 | uses the same syntax as a C string constant: surround the text with
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| 99 | two ‘<samp>"</samp>’ characters, and add a backslash before each ‘<samp>"</samp>’ or
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| 100 | ‘<samp>\</samp>’ character in it.
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| 101 | </p>
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| 102 | <p>With either syntax, the textual argument specifies what to do.
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| 103 | It begins with one or several words that specify the operation.
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| 104 | If the compiler does not recognize them, it ignores the pragma.
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| 105 | </p>
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| 106 | <p>Here are the pragma operations supported in GNU C.
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| 107 | </p>
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| 108 | <dl compact="compact">
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| 109 | <dt><code>#pragma GCC dependency "<var>file</var>" [<var>message</var>]</code></dt>
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| 110 | <dt><code>_Pragma ("GCC dependency \"<var>file</var>\" [<var>message</var>]")</code></dt>
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| 111 | <dd><p>Declares that the current source file depends on <var>file</var>, so GNU C
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| 112 | compares the file times and gives a warning if <var>file</var> is newer
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| 113 | than the current source file.
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| 114 | </p>
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| 115 | <p>This directive searches for <var>file</var> the way <code>#include</code>
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| 116 | searches for a non-system header file.
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| 117 | </p>
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| 118 | <p>If <var>message</var> is given, the warning message includes that text.
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| 119 | </p>
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| 120 | <p>Examples:
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| 121 | </p>
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| 122 | <div class="example">
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| 123 | <pre class="example">#pragma GCC dependency "parse.y"
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| 124 | _pragma ("GCC dependency \"/usr/include/time.h\" \
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| 125 | rerun fixincludes")
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| 126 | </pre></div>
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| 127 |
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| 128 | </dd>
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| 129 | <dt><code>#pragma GCC poison <var>identifiers</var></code></dt>
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| 130 | <dt><code>_Pragma ("GCC poison <var>identifiers</var>")</code></dt>
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| 131 | <dd><p>Poisons the identifiers listed in <var>identifiers</var>.
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| 132 | </p>
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| 133 | <p>This is useful to make sure all mention of <var>identifiers</var> has been
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| 134 | deleted from the program and that no reference to them creeps back in.
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| 135 | If any of those identifiers appears anywhere in the source after the
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| 136 | directive, it causes a compilation error. For example,
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| 137 | </p>
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| 138 | <div class="example">
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| 139 | <pre class="example">#pragma GCC poison printf sprintf fprintf
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| 140 | sprintf(some_string, "hello");
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| 141 | </pre></div>
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| 142 |
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| 143 | <p>generates an error.
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| 144 | </p>
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| 145 | <p>If a poisoned identifier appears as part of the expansion of a macro
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| 146 | that was defined before the identifier was poisoned, it will <em>not</em>
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| 147 | cause an error. Thus, system headers that define macros that use
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| 148 | the identifier will not cause errors.
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| 149 | </p>
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| 150 | <p>For example,
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| 151 | </p>
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| 152 | <div class="example">
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| 153 | <pre class="example">#define strrchr rindex
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| 154 | _Pragma ("GCC poison rindex")
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| 155 | strrchr(some_string, 'h');
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| 156 | </pre></div>
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| 157 |
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| 158 | <p>does not cause a compilation error.
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| 159 | </p>
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| 160 | </dd>
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| 161 | <dt><code>#pragma GCC system_header</code></dt>
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| 162 | <dt><code>_Pragma ("GCC system_header")</code></dt>
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| 163 | <dd><p>Specify treating the rest of the current source file as if it came
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| 164 | from a system header file. See <a href="https://gcc.gnu.org/onlinedocs/gcc/System-Headers.html#System-Headers">System Headers</a> in <cite>Using the GNU Compiler Collection</cite>.
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| 165 | </p>
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| 166 | </dd>
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| 167 | <dt><code>#pragma GCC warning <var>message</var></code></dt>
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| 168 | <dt><code>_Pragma ("GCC warning <var>message</var>")</code></dt>
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| 169 | <dd><p>Equivalent to <code>#warning</code>. Its advantage is that the
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| 170 | <code>_Pragma</code> form can be included in a macro definition.
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| 171 | </p>
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| 172 | </dd>
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| 173 | <dt><code>#pragma GCC error <var>message</var></code></dt>
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| 174 | <dt><code>_Pragma ("GCC error <var>message</var>")</code></dt>
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| 175 | <dd><p>Equivalent to <code>#error</code>. Its advantage is that the
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| 176 | <code>_Pragma</code> form can be included in a macro definition.
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| 177 | </p>
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| 178 | </dd>
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| 179 | <dt><code>#pragma GCC message <var>message</var></code></dt>
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| 180 | <dt><code>_Pragma ("GCC message <var>message</var>")</code></dt>
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| 181 | <dd><p>Similar to ‘<samp>GCC warning</samp>’ and ‘<samp>GCC error</samp>’, this simply prints an
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| 182 | informational message, and could be used to include additional warning
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| 183 | or error text without triggering more warnings or errors. (Note that
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| 184 | unlike ‘<samp>warning</samp>’ and ‘<samp>error</samp>’, ‘<samp>message</samp>’ does not include
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| 185 | ‘<samp>GCC</samp>’ as part of the pragma.)
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| 186 | </p></dd>
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| 187 | </dl>
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| 188 |
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| 189 | <hr>
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| 190 | <div class="header">
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| 191 | <p>
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| 192 | Next: <a href="Severity-Pragmas.html" accesskey="n" rel="next">Severity Pragmas</a>, Up: <a href="Pragmas.html" accesskey="u" rel="up">Pragmas</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 193 | </div>
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| 194 |
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| 195 |
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| 196 |
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