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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 ‘<samp><span class="samp">#include</span></samp>’ 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 “definition” 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 ‘<samp><span class="samp">#include</span></samp>’ 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 ‘<samp><span class="samp">#include</span></samp>’ directive, the expansions of <code>__FILE__</code> and |
| <code>__LINE__</code> revert to the values they had before the |
| ‘<samp><span class="samp">#include</span></samp>’ (but <code>__LINE__</code> is then incremented by one as |
| processing moves to the line after the ‘<samp><span class="samp">#include</span></samp>’). |
| |
| <p>A ‘<samp><span class="samp">#line</span></samp>’ 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 12 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>"??? ?? ????"<!-- /@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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