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<h4 class="subsection">3.7.1 Standard Predefined Macros</h4>

<p><a name="index-standard-predefined-macros_002e-60"></a>
The standard predefined macros are specified by the relevant
language standards, so they are available with all compilers that
implement those standards.  Older compilers may not provide all of
them.  Their names all start with double underscores.

     <dl>
<dt><code>__FILE__</code><dd>This macro expands to the name of the current input file, in the form of
a C string constant.  This is the path by which the preprocessor opened
the file, not the short name specified in &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; or as the
input file name argument.  For example,
<code>"/usr/local/include/myheader.h"</code> is a possible expansion of this
macro.

     <br><dt><code>__LINE__</code><dd>This macro expands to the current input line number, in the form of a
decimal integer constant.  While we call it a predefined macro, it's
a pretty strange macro, since its &ldquo;definition&rdquo; changes with each
new line of source code. 
</dl>

   <p><code>__FILE__</code> and <code>__LINE__</code> are useful in generating an error
message to report an inconsistency detected by the program; the message
can state the source line at which the inconsistency was detected.  For
example,

<pre class="smallexample">     fprintf (stderr, "Internal error: "
                      "negative string length "
                      "%d at %s, line %d.",
              length, __FILE__, __LINE__);
</pre>
   <p>An &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; directive changes the expansions of <code>__FILE__</code>
and <code>__LINE__</code> to correspond to the included file.  At the end of
that file, when processing resumes on the input file that contained
the &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; directive, the expansions of <code>__FILE__</code> and
<code>__LINE__</code> revert to the values they had before the
&lsquo;<samp><span class="samp">#include</span></samp>&rsquo; (but <code>__LINE__</code> is then incremented by one as
processing moves to the line after the &lsquo;<samp><span class="samp">#include</span></samp>&rsquo;).

   <p>A &lsquo;<samp><span class="samp">#line</span></samp>&rsquo; directive changes <code>__LINE__</code>, and may change
<code>__FILE__</code> as well.  See <a href="Line-Control.html#Line-Control">Line Control</a>.

   <p>C99 introduces <code>__func__</code>, and GCC has provided <code>__FUNCTION__</code>
for a long time.  Both of these are strings containing the name of the
current function (there are slight semantic differences; see the GCC
manual).  Neither of them is a macro; the preprocessor does not know the
name of the current function.  They tend to be useful in conjunction
with <code>__FILE__</code> and <code>__LINE__</code>, though.

     <dl>
<dt><code>__DATE__</code><dd>This macro expands to a string constant that describes the date on which
the preprocessor is being run.  The string constant contains eleven
characters and looks like <code>"Feb&nbsp;12&nbsp;1996"<!-- /@w --></code>.  If the day of the
month is less than 10, it is padded with a space on the left.

     <p>If GCC cannot determine the current date, it will emit a warning message
(once per compilation) and <code>__DATE__</code> will expand to
<code>"???&nbsp;??&nbsp;????"<!-- /@w --></code>.

     <br><dt><code>__TIME__</code><dd>This macro expands to a string constant that describes the time at
which the preprocessor is being run.  The string constant contains
eight characters and looks like <code>"23:59:01"</code>.

     <p>If GCC cannot determine the current time, it will emit a warning message
(once per compilation) and <code>__TIME__</code> will expand to
<code>"??:??:??"</code>.

     <br><dt><code>__STDC__</code><dd>In normal operation, this macro expands to the constant 1, to signify
that this compiler conforms to ISO Standard C.  If GNU CPP is used with
a compiler other than GCC, this is not necessarily true; however, the
preprocessor always conforms to the standard unless the
<samp><span class="option">-traditional-cpp</span></samp> option is used.

     <p>This macro is not defined if the <samp><span class="option">-traditional-cpp</span></samp> option is used.

     <p>On some hosts, the system compiler uses a different convention, where
<code>__STDC__</code> is normally 0, but is 1 if the user specifies strict
conformance to the C Standard.  CPP follows the host convention when
processing system header files, but when processing user files
<code>__STDC__</code> is always 1.  This has been reported to cause problems;
for instance, some versions of Solaris provide X Windows headers that
expect <code>__STDC__</code> to be either undefined or 1.  See <a href="Invocation.html#Invocation">Invocation</a>.

     <br><dt><code>__STDC_VERSION__</code><dd>This macro expands to the C Standard's version number, a long integer
constant of the form <var>yyyy</var><var>mm</var><code>L</code> where <var>yyyy</var> and
<var>mm</var> are the year and month of the Standard version.  This signifies
which version of the C Standard the compiler conforms to.  Like
<code>__STDC__</code>, this is not necessarily accurate for the entire
implementation, unless GNU CPP is being used with GCC.

     <p>The value <code>199409L</code> signifies the 1989 C standard as amended in
1994, which is the current default; the value <code>199901L</code> signifies
the 1999 revision of the C standard.  Support for the 1999 revision is
not yet complete.

     <p>This macro is not defined if the <samp><span class="option">-traditional-cpp</span></samp> option is
used, nor when compiling C++ or Objective-C.

     <br><dt><code>__STDC_HOSTED__</code><dd>This macro is defined, with value 1, if the compiler's target is a
<dfn>hosted environment</dfn>.  A hosted environment has the complete
facilities of the standard C library available.

     <br><dt><code>__cplusplus</code><dd>This macro is defined when the C++ compiler is in use.  You can use
<code>__cplusplus</code> to test whether a header is compiled by a C compiler
or a C++ compiler.  This macro is similar to <code>__STDC_VERSION__</code>, in
that it expands to a version number.  A fully conforming implementation
of the 1998 C++ standard will define this macro to <code>199711L</code>.  The
GNU C++ compiler is not yet fully conforming, so it uses <code>1</code>
instead.  It is hoped to complete the implementation of standard C++
in the near future.

     <br><dt><code>__OBJC__</code><dd>This macro is defined, with value 1, when the Objective-C compiler is in
use.  You can use <code>__OBJC__</code> to test whether a header is compiled
by a C compiler or an Objective-C compiler.

     <br><dt><code>__ASSEMBLER__</code><dd>This macro is defined with value 1 when preprocessing assembly
language.

   </dl>

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