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178 lines
9.0 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Standard Predefined Macros - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Predefined-Macros.html#Predefined-Macros" title="Predefined Macros">
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<link rel="next" href="Common-Predefined-Macros.html#Common-Predefined-Macros" title="Common Predefined Macros">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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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) (see below), and the Back-Cover Texts are (b) (see below).
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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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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development.
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<style type="text/css"><!--
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pre.format { font-family:inherit }
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pre.smallformat { font-family:inherit; font-size:smaller }
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span.sansserif { font-family:sans-serif; font-weight:normal; }
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Standard-Predefined-Macros"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Common-Predefined-Macros.html#Common-Predefined-Macros">Common Predefined Macros</a>,
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Up: <a rel="up" accesskey="u" href="Predefined-Macros.html#Predefined-Macros">Predefined Macros</a>
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<hr>
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</div>
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<h4 class="subsection">3.7.1 Standard Predefined Macros</h4>
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<p><a name="index-standard-predefined-macros_002e-60"></a>
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The standard predefined macros are specified by the relevant
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language standards, so they are available with all compilers that
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implement those standards. Older compilers may not provide all of
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them. Their names all start with double underscores.
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<dl>
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<dt><code>__FILE__</code><dd>This macro expands to the name of the current input file, in the form of
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a C string constant. This is the path by which the preprocessor opened
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the file, not the short name specified in ‘<samp><span class="samp">#include</span></samp>’ or as the
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input file name argument. For example,
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<code>"/usr/local/include/myheader.h"</code> is a possible expansion of this
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macro.
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<br><dt><code>__LINE__</code><dd>This macro expands to the current input line number, in the form of a
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decimal integer constant. While we call it a predefined macro, it's
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a pretty strange macro, since its “definition” changes with each
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new line of source code.
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</dl>
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<p><code>__FILE__</code> and <code>__LINE__</code> are useful in generating an error
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message to report an inconsistency detected by the program; the message
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can state the source line at which the inconsistency was detected. For
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example,
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<pre class="smallexample"> fprintf (stderr, "Internal error: "
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"negative string length "
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"%d at %s, line %d.",
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length, __FILE__, __LINE__);
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</pre>
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<p>An ‘<samp><span class="samp">#include</span></samp>’ directive changes the expansions of <code>__FILE__</code>
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and <code>__LINE__</code> to correspond to the included file. At the end of
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that file, when processing resumes on the input file that contained
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the ‘<samp><span class="samp">#include</span></samp>’ directive, the expansions of <code>__FILE__</code> and
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<code>__LINE__</code> revert to the values they had before the
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‘<samp><span class="samp">#include</span></samp>’ (but <code>__LINE__</code> is then incremented by one as
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processing moves to the line after the ‘<samp><span class="samp">#include</span></samp>’).
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<p>A ‘<samp><span class="samp">#line</span></samp>’ directive changes <code>__LINE__</code>, and may change
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<code>__FILE__</code> as well. See <a href="Line-Control.html#Line-Control">Line Control</a>.
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<p>C99 introduces <code>__func__</code>, and GCC has provided <code>__FUNCTION__</code>
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for a long time. Both of these are strings containing the name of the
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current function (there are slight semantic differences; see the GCC
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manual). Neither of them is a macro; the preprocessor does not know the
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name of the current function. They tend to be useful in conjunction
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with <code>__FILE__</code> and <code>__LINE__</code>, though.
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<dl>
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<dt><code>__DATE__</code><dd>This macro expands to a string constant that describes the date on which
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the preprocessor is being run. The string constant contains eleven
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characters and looks like <code>"Feb 12 1996"<!-- /@w --></code>. If the day of the
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month is less than 10, it is padded with a space on the left.
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<p>If GCC cannot determine the current date, it will emit a warning message
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(once per compilation) and <code>__DATE__</code> will expand to
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<code>"??? ?? ????"<!-- /@w --></code>.
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<br><dt><code>__TIME__</code><dd>This macro expands to a string constant that describes the time at
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which the preprocessor is being run. The string constant contains
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eight characters and looks like <code>"23:59:01"</code>.
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<p>If GCC cannot determine the current time, it will emit a warning message
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(once per compilation) and <code>__TIME__</code> will expand to
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<code>"??:??:??"</code>.
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<br><dt><code>__STDC__</code><dd>In normal operation, this macro expands to the constant 1, to signify
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that this compiler conforms to ISO Standard C. If GNU CPP is used with
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a compiler other than GCC, this is not necessarily true; however, the
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preprocessor always conforms to the standard unless the
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<samp><span class="option">-traditional-cpp</span></samp> option is used.
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<p>This macro is not defined if the <samp><span class="option">-traditional-cpp</span></samp> option is used.
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<p>On some hosts, the system compiler uses a different convention, where
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<code>__STDC__</code> is normally 0, but is 1 if the user specifies strict
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conformance to the C Standard. CPP follows the host convention when
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processing system header files, but when processing user files
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<code>__STDC__</code> is always 1. This has been reported to cause problems;
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for instance, some versions of Solaris provide X Windows headers that
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expect <code>__STDC__</code> to be either undefined or 1. See <a href="Invocation.html#Invocation">Invocation</a>.
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<br><dt><code>__STDC_VERSION__</code><dd>This macro expands to the C Standard's version number, a long integer
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constant of the form <var>yyyy</var><var>mm</var><code>L</code> where <var>yyyy</var> and
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<var>mm</var> are the year and month of the Standard version. This signifies
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which version of the C Standard the compiler conforms to. Like
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<code>__STDC__</code>, this is not necessarily accurate for the entire
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implementation, unless GNU CPP is being used with GCC.
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<p>The value <code>199409L</code> signifies the 1989 C standard as amended in
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1994, which is the current default; the value <code>199901L</code> signifies
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the 1999 revision of the C standard. Support for the 1999 revision is
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not yet complete.
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<p>This macro is not defined if the <samp><span class="option">-traditional-cpp</span></samp> option is
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used, nor when compiling C++ or Objective-C.
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<br><dt><code>__STDC_HOSTED__</code><dd>This macro is defined, with value 1, if the compiler's target is a
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<dfn>hosted environment</dfn>. A hosted environment has the complete
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facilities of the standard C library available.
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<br><dt><code>__cplusplus</code><dd>This macro is defined when the C++ compiler is in use. You can use
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<code>__cplusplus</code> to test whether a header is compiled by a C compiler
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or a C++ compiler. This macro is similar to <code>__STDC_VERSION__</code>, in
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that it expands to a version number. Depending on the language standard
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selected, the value of the macro is <code>199711L</code>, as mandated by the
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1998 C++ standard; <code>201103L</code>, per the 2011 C++ standard; an
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unspecified value strictly larger than <code>201103L</code> for the experimental
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languages enabled by <samp><span class="option">-std=c++1y</span></samp> and <samp><span class="option">-std=gnu++1y</span></samp>.
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<br><dt><code>__OBJC__</code><dd>This macro is defined, with value 1, when the Objective-C compiler is in
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use. You can use <code>__OBJC__</code> to test whether a header is compiled
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by a C compiler or an Objective-C compiler.
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<br><dt><code>__ASSEMBLER__</code><dd>This macro is defined with value 1 when preprocessing assembly
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language.
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</dl>
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</body></html>
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