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238 lines
11 KiB
HTML
238 lines
11 KiB
HTML
4 years ago
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1987-2018 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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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Standard Predefined Macros (The C Preprocessor)</title>
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<meta name="description" content="Standard Predefined Macros (The C Preprocessor)">
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<meta name="keywords" content="Standard Predefined Macros (The C Preprocessor)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Predefined-Macros.html#Predefined-Macros" rel="up" title="Predefined Macros">
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<link href="Common-Predefined-Macros.html#Common-Predefined-Macros" rel="next" title="Common Predefined Macros">
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<link href="Predefined-Macros.html#Predefined-Macros" rel="prev" title="Predefined Macros">
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<style type="text/css">
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a.summary-letter {text-decoration: none}
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blockquote.smallquotation {font-size: smaller}
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div.display {margin-left: 3.2em}
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div.example {margin-left: 3.2em}
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kbd {font-style: oblique}
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pre.display {font-family: inherit}
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pre.format {font-family: inherit}
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pre.menu-comment {font-family: serif}
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span.nolinebreak {white-space: nowrap}
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span.roman {font-family: initial; font-weight: normal}
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span.sansserif {font-family: sans-serif; font-weight: normal}
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ul.no-bullet {list-style: none}
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Standard-Predefined-Macros"></a>
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<div class="header">
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<p>
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Next: <a href="Common-Predefined-Macros.html#Common-Predefined-Macros" accesskey="n" rel="next">Common Predefined Macros</a>, Up: <a href="Predefined-Macros.html#Predefined-Macros" accesskey="u" rel="up">Predefined Macros</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>
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</div>
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<hr>
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<a name="Standard-Predefined-Macros-1"></a>
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<h4 class="subsection">3.7.1 Standard Predefined Macros</h4>
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<a name="index-standard-predefined-macros_002e"></a>
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<p>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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</p>
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<dl compact="compact">
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<dt><code>__FILE__</code></dt>
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<dd><p>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>#include</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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</p>
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</dd>
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<dt><code>__LINE__</code></dt>
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<dd><p>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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</p></dd>
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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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</p>
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<div class="smallexample">
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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></div>
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<p>An ‘<samp>#include</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>#include</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>#include</samp>’ (but <code>__LINE__</code> is then incremented by one as
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processing moves to the line after the ‘<samp>#include</samp>’).
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</p>
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<p>A ‘<samp>#line</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>
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<p>C99 introduced <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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</p>
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<dl compact="compact">
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<dt><code>__DATE__</code></dt>
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<dd><p>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>
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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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</p>
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</dd>
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<dt><code>__TIME__</code></dt>
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<dd><p>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>
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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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</p>
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</dd>
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<dt><code>__STDC__</code></dt>
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<dd><p>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>-traditional-cpp</samp> option is used.
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</p>
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<p>This macro is not defined if the <samp>-traditional-cpp</samp> option is used.
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</p>
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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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</p>
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</dd>
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<dt><code>__STDC_VERSION__</code></dt>
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<dd><p>This macro expands to the C Standard’s version number, a long integer
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constant of the form <code><var>yyyy</var><var>mm</var>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>
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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; the value <code>201112L</code>
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signifies the 2011 revision of the C standard; the value
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<code>201710L</code> signifies the 2017 revision of the C standard (which is
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otherwise identical to the 2011 version apart from correction of
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defects).
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</p>
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<p>This macro is not defined if the <samp>-traditional-cpp</samp> option is
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used, nor when compiling C++ or Objective-C.
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</p>
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</dd>
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<dt><code>__STDC_HOSTED__</code></dt>
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<dd><p>This macro is defined, with value 1, if the compiler’s target is a
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<em>hosted environment</em>. A hosted environment has the complete
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facilities of the standard C library available.
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</p>
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</dd>
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<dt><code>__cplusplus</code></dt>
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<dd><p>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
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<code>199711L</code> for the 1998 C++ standard,
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<code>201103L</code> for the 2011 C++ standard,
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<code>201402L</code> for the 2014 C++ standard,
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<code>201703L</code> for the 2017 C++ standard,
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or an unspecified value strictly larger than <code>201703L</code> for the
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experimental languages enabled by <samp>-std=c++2a</samp> and
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<samp>-std=gnu++2a</samp>.
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</p>
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</dd>
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<dt><code>__OBJC__</code></dt>
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<dd><p>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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</p>
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</dd>
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<dt><code>__ASSEMBLER__</code></dt>
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<dd><p>This macro is defined with value 1 when preprocessing assembly
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language.
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</p>
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</dd>
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</dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Common-Predefined-Macros.html#Common-Predefined-Macros" accesskey="n" rel="next">Common Predefined Macros</a>, Up: <a href="Predefined-Macros.html#Predefined-Macros" accesskey="u" rel="up">Predefined Macros</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>
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</div>
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</body>
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</html>
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