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| <h4 class="subsection">3.7.3 System-specific Predefined Macros</h4> |
| |
| <p><a name="index-system_002dspecific-predefined-macros-62"></a><a name="index-predefined-macros_002c-system_002dspecific-63"></a><a name="index-reserved-namespace-64"></a> |
| The C preprocessor normally predefines several macros that indicate what |
| type of system and machine is in use. They are obviously different on |
| each target supported by GCC. This manual, being for all systems and |
| machines, cannot tell you what their names are, but you can use |
| <samp><span class="command">cpp -dM</span></samp> to see them all. See <a href="Invocation.html#Invocation">Invocation</a>. All system-specific |
| predefined macros expand to the constant 1, so you can test them with |
| either ‘<samp><span class="samp">#ifdef</span></samp>’ or ‘<samp><span class="samp">#if</span></samp>’. |
| |
| <p>The C standard requires that all system-specific macros be part of the |
| <dfn>reserved namespace</dfn>. All names which begin with two underscores, |
| or an underscore and a capital letter, are reserved for the compiler and |
| library to use as they wish. However, historically system-specific |
| macros have had names with no special prefix; for instance, it is common |
| to find <code>unix</code> defined on Unix systems. For all such macros, GCC |
| provides a parallel macro with two underscores added at the beginning |
| and the end. If <code>unix</code> is defined, <code>__unix__</code> will be defined |
| too. There will never be more than two underscores; the parallel of |
| <code>_mips</code> is <code>__mips__</code>. |
| |
| <p>When the <samp><span class="option">-ansi</span></samp> option, or any <samp><span class="option">-std</span></samp> option that |
| requests strict conformance, is given to the compiler, all the |
| system-specific predefined macros outside the reserved namespace are |
| suppressed. The parallel macros, inside the reserved namespace, remain |
| defined. |
| |
| <p>We are slowly phasing out all predefined macros which are outside the |
| reserved namespace. You should never use them in new programs, and we |
| encourage you to correct older code to use the parallel macros whenever |
| you find it. We don't recommend you use the system-specific macros that |
| are in the reserved namespace, either. It is better in the long run to |
| check specifically for features you need, using a tool such as |
| <samp><span class="command">autoconf</span></samp>. |
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