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<title>Using the GNU Compiler Collection (GCC): Environment Variables</title>

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<a name="Environment-Variables"></a>
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Next: <a href="Precompiled-Headers.html#Precompiled-Headers" accesskey="n" rel="next">Precompiled Headers</a>, Previous: <a href="Spec-Files.html#Spec-Files" accesskey="p" rel="prev">Spec Files</a>, Up: <a href="Invoking-GCC.html#Invoking-GCC" accesskey="u" rel="up">Invoking GCC</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Environment-Variables-Affecting-GCC"></a>
<h3 class="section">3.20 Environment Variables Affecting GCC</h3>
<a name="index-environment-variables"></a>

<p>This section describes several environment variables that affect how GCC
operates.  Some of them work by specifying directories or prefixes to use
when searching for various kinds of files.  Some are used to specify other
aspects of the compilation environment.
</p>
<p>Note that you can also specify places to search using options such as
<samp>-B</samp>, <samp>-I</samp> and <samp>-L</samp> (see <a href="Directory-Options.html#Directory-Options">Directory Options</a>).  These
take precedence over places specified using environment variables, which
in turn take precedence over those specified by the configuration of GCC.
See <a href="http://gcc.gnu.org/onlinedocs/gccint/Driver.html#Driver">Controlling the Compilation Driver <samp>gcc</samp></a> in <cite>GNU Compiler Collection (GCC) Internals</cite>.
</p>
<dl compact="compact">
<dt><code>LANG</code></dt>
<dt><code>LC_CTYPE</code></dt>
<dt><code>LC_MESSAGES</code></dt>
<dt><code>LC_ALL</code></dt>
<dd><a name="index-LANG"></a>
<a name="index-LC_005fCTYPE"></a>
<a name="index-LC_005fMESSAGES"></a>
<a name="index-LC_005fALL"></a>
<a name="index-locale"></a>
<p>These environment variables control the way that GCC uses
localization information which allows GCC to work with different
national conventions.  GCC inspects the locale categories
<code>LC_CTYPE</code> and <code>LC_MESSAGES</code> if it has been configured to do
so.  These locale categories can be set to any value supported by your
installation.  A typical value is &lsquo;<samp>en_GB.UTF-8</samp>&rsquo; for English in the United
Kingdom encoded in UTF-8.
</p>
<p>The <code>LC_CTYPE</code> environment variable specifies character
classification.  GCC uses it to determine the character boundaries in
a string; this is needed for some multibyte encodings that contain quote
and escape characters that are otherwise interpreted as a string
end or escape.
</p>
<p>The <code>LC_MESSAGES</code> environment variable specifies the language to
use in diagnostic messages.
</p>
<p>If the <code>LC_ALL</code> environment variable is set, it overrides the value
of <code>LC_CTYPE</code> and <code>LC_MESSAGES</code>; otherwise, <code>LC_CTYPE</code>
and <code>LC_MESSAGES</code> default to the value of the <code>LANG</code>
environment variable.  If none of these variables are set, GCC
defaults to traditional C English behavior.
</p>
</dd>
<dt><code>TMPDIR</code></dt>
<dd><a name="index-TMPDIR"></a>
<p>If <code>TMPDIR</code> is set, it specifies the directory to use for temporary
files.  GCC uses temporary files to hold the output of one stage of
compilation which is to be used as input to the next stage: for example,
the output of the preprocessor, which is the input to the compiler
proper.
</p>
</dd>
<dt><code>GCC_COMPARE_DEBUG</code></dt>
<dd><a name="index-GCC_005fCOMPARE_005fDEBUG"></a>
<p>Setting <code>GCC_COMPARE_DEBUG</code> is nearly equivalent to passing
<samp>-fcompare-debug</samp> to the compiler driver.  See the documentation
of this option for more details.
</p>
</dd>
<dt><code>GCC_EXEC_PREFIX</code></dt>
<dd><a name="index-GCC_005fEXEC_005fPREFIX"></a>
<p>If <code>GCC_EXEC_PREFIX</code> is set, it specifies a prefix to use in the
names of the subprograms executed by the compiler.  No slash is added
when this prefix is combined with the name of a subprogram, but you can
specify a prefix that ends with a slash if you wish.
</p>
<p>If <code>GCC_EXEC_PREFIX</code> is not set, GCC attempts to figure out
an appropriate prefix to use based on the pathname it is invoked with.
</p>
<p>If GCC cannot find the subprogram using the specified prefix, it
tries looking in the usual places for the subprogram.
</p>
<p>The default value of <code>GCC_EXEC_PREFIX</code> is
<samp><var>prefix</var>/lib/gcc/</samp> where <var>prefix</var> is the prefix to
the installed compiler. In many cases <var>prefix</var> is the value
of <code>prefix</code> when you ran the <samp>configure</samp> script.
</p>
<p>Other prefixes specified with <samp>-B</samp> take precedence over this prefix.
</p>
<p>This prefix is also used for finding files such as <samp>crt0.o</samp> that are
used for linking.
</p>
<p>In addition, the prefix is used in an unusual way in finding the
directories to search for header files.  For each of the standard
directories whose name normally begins with &lsquo;<samp>/usr/local/lib/gcc</samp>&rsquo;
(more precisely, with the value of <code>GCC_INCLUDE_DIR</code>), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name.  Thus, with <samp>-Bfoo/</samp>, GCC searches
<samp>foo/bar</samp> just before it searches the standard directory 
<samp>/usr/local/lib/bar</samp>.
If a standard directory begins with the configured
<var>prefix</var> then the value of <var>prefix</var> is replaced by
<code>GCC_EXEC_PREFIX</code> when looking for header files.
</p>
</dd>
<dt><code>COMPILER_PATH</code></dt>
<dd><a name="index-COMPILER_005fPATH"></a>
<p>The value of <code>COMPILER_PATH</code> is a colon-separated list of
directories, much like <code>PATH</code>.  GCC tries the directories thus
specified when searching for subprograms, if it cannot find the
subprograms using <code>GCC_EXEC_PREFIX</code>.
</p>
</dd>
<dt><code>LIBRARY_PATH</code></dt>
<dd><a name="index-LIBRARY_005fPATH"></a>
<p>The value of <code>LIBRARY_PATH</code> is a colon-separated list of
directories, much like <code>PATH</code>.  When configured as a native compiler,
GCC tries the directories thus specified when searching for special
linker files, if it cannot find them using <code>GCC_EXEC_PREFIX</code>.  Linking
using GCC also uses these directories when searching for ordinary
libraries for the <samp>-l</samp> option (but directories specified with
<samp>-L</samp> come first).
</p>
</dd>
<dt><code>LANG</code></dt>
<dd><a name="index-LANG-1"></a>
<a name="index-locale-definition"></a>
<p>This variable is used to pass locale information to the compiler.  One way in
which this information is used is to determine the character set to be used
when character literals, string literals and comments are parsed in C and C++.
When the compiler is configured to allow multibyte characters,
the following values for <code>LANG</code> are recognized:
</p>
<dl compact="compact">
<dt>&lsquo;<samp>C-JIS</samp>&rsquo;</dt>
<dd><p>Recognize JIS characters.
</p></dd>
<dt>&lsquo;<samp>C-SJIS</samp>&rsquo;</dt>
<dd><p>Recognize SJIS characters.
</p></dd>
<dt>&lsquo;<samp>C-EUCJP</samp>&rsquo;</dt>
<dd><p>Recognize EUCJP characters.
</p></dd>
</dl>

<p>If <code>LANG</code> is not defined, or if it has some other value, then the
compiler uses <code>mblen</code> and <code>mbtowc</code> as defined by the default locale to
recognize and translate multibyte characters.
</p></dd>
</dl>

<p>Some additional environment variables affect the behavior of the
preprocessor.
</p>


<dl compact="compact">
<dt><code>CPATH</code>
<a name="index-CPATH"></a>
</dt>
<dt><code>C_INCLUDE_PATH</code>
<a name="index-C_005fINCLUDE_005fPATH"></a>
</dt>
<dt><code>CPLUS_INCLUDE_PATH</code>
<a name="index-CPLUS_005fINCLUDE_005fPATH"></a>
</dt>
<dt><code>OBJC_INCLUDE_PATH</code>
<a name="index-OBJC_005fINCLUDE_005fPATH"></a>
</dt>
<dd><p>Each variable&rsquo;s value is a list of directories separated by a special
character, much like <code>PATH</code>, in which to look for header files.
The special character, <code>PATH_SEPARATOR</code>, is target-dependent and
determined at GCC build time.  For Microsoft Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
</p>
<p><code>CPATH</code> specifies a list of directories to be searched as if
specified with <samp>-I</samp>, but after any paths given with <samp>-I</samp>
options on the command line.  This environment variable is used
regardless of which language is being preprocessed.
</p>
<p>The remaining environment variables apply only when preprocessing the
particular language indicated.  Each specifies a list of directories
to be searched as if specified with <samp>-isystem</samp>, but after any
paths given with <samp>-isystem</samp> options on the command line.
</p>
<p>In all these variables, an empty element instructs the compiler to
search its current working directory.  Empty elements can appear at the
beginning or end of a path.  For instance, if the value of
<code>CPATH</code> is <code>:/special/include</code>, that has the same
effect as &lsquo;<samp><span class="nolinebreak">-I.</span>&nbsp;<span class="nolinebreak">-I/special/include</span><!-- /@w --></samp>&rsquo;.
</p>

</dd>
<dt><code>DEPENDENCIES_OUTPUT</code>
<a name="index-DEPENDENCIES_005fOUTPUT"></a>
</dt>
<dd><a name="index-dependencies-for-make-as-output"></a>
<p>If this variable is set, its value specifies how to output
dependencies for Make based on the non-system header files processed
by the compiler.  System header files are ignored in the dependency
output.
</p>
<p>The value of <code>DEPENDENCIES_OUTPUT</code> can be just a file name, in
which case the Make rules are written to that file, guessing the target
name from the source file name.  Or the value can have the form
&lsquo;<samp><var>file</var> <var>target</var></samp>&rsquo;, in which case the rules are written to
file <var>file</var> using <var>target</var> as the target name.
</p>
<p>In other words, this environment variable is equivalent to combining
the options <samp>-MM</samp> and <samp>-MF</samp>
(see <a href="Preprocessor-Options.html#Preprocessor-Options">Preprocessor Options</a>),
with an optional <samp>-MT</samp> switch too.
</p>
</dd>
<dt><code>SUNPRO_DEPENDENCIES</code>
<a name="index-SUNPRO_005fDEPENDENCIES"></a>
</dt>
<dd><a name="index-dependencies-for-make-as-output-1"></a>
<p>This variable is the same as <code>DEPENDENCIES_OUTPUT</code> (see above),
except that system header files are not ignored, so it implies
<samp>-M</samp> rather than <samp>-MM</samp>.  However, the dependence on the
main input file is omitted.
See <a href="Preprocessor-Options.html#Preprocessor-Options">Preprocessor Options</a>.
</p>
</dd>
<dt><code>SOURCE_DATE_EPOCH</code>
<a name="index-SOURCE_005fDATE_005fEPOCH"></a>
</dt>
<dd><p>If this variable is set, its value specifies a UNIX timestamp to be
used in replacement of the current date and time in the <code>__DATE__</code>
and <code>__TIME__</code> macros, so that the embedded timestamps become
reproducible.
</p>
<p>The value of <code>SOURCE_DATE_EPOCH</code> must be a UNIX timestamp,
defined as the number of seconds (excluding leap seconds) since
01 Jan 1970 00:00:00 represented in ASCII; identical to the output of
&lsquo;<samp><code>date +%s</code></samp>&rsquo; on GNU/Linux and other systems that support the
<code>%s</code> extension in the <code>date</code> command.
</p>
<p>The value should be a known timestamp such as the last modification
time of the source or package and it should be set by the build
process.
</p>
</dd>
</dl>


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