System-specific Predefined Macros - The C Preprocessor
Description: The C Preprocessor

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#### 3.7.3 System-specific Predefined Macros

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 `cpp -dM` to see them all. See Invocation. All system-specific predefined macros expand to a constant value, so you can test them with either ‘`#ifdef`’ or ‘`#if`’.

The C standard requires that all system-specific macros be part of the reserved namespace. 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 `unix` defined on Unix systems. For all such macros, GCC provides a parallel macro with two underscores added at the beginning and the end. If `unix` is defined, `__unix__` will be defined too. There will never be more than two underscores; the parallel of `_mips` is `__mips__`.

When the `-ansi` option, or any `-std` 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.

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 `autoconf`.