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 the constant 1, 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.