File: m4.info,  Node: Platform macros,  Next: Syscmd,  Up: Shell commands

13.1 Determining the platform
=============================

Sometimes it is desirable for an input file to know which platform ‘m4’
is running on.  GNU ‘m4’ provides several macros that are predefined to
expand to the empty string; checking for their existence will confirm
platform details.

 -- Optional builtin: __gnu__
 -- Optional builtin: __os2__
 -- Optional builtin: os2
 -- Optional builtin: __unix__
 -- Optional builtin: unix
 -- Optional builtin: __windows__
 -- Optional builtin: windows
     Each of these macros is conditionally defined as needed to describe
     the environment of ‘m4’.  If defined, each macro expands to the
     empty string.  For now, these macros silently ignore all arguments,
     but in a future release of M4, they might warn if arguments are
     present.

   When GNU extensions are in effect (that is, when you did not use the
‘-G’ option, *note Invoking m4: Limits control.), GNU ‘m4’ will define
the macro ‘__gnu__’ to expand to the empty string.

     $ m4
     __gnu__
     ⇒
     __gnu__(`ignored')
     ⇒
     Extensions are ifdef(`__gnu__', `active', `inactive')
     ⇒Extensions are active

     $ m4 -G
     __gnu__
     ⇒__gnu__
     __gnu__(`ignored')
     ⇒__gnu__(ignored)
     Extensions are ifdef(`__gnu__', `active', `inactive')
     ⇒Extensions are inactive

   On UNIX systems, GNU ‘m4’ will define ‘__unix__’ by default, or
‘unix’ when the ‘-G’ option is specified.

   On native Windows systems, GNU ‘m4’ will define ‘__windows__’ by
default, or ‘windows’ when the ‘-G’ option is specified.

   On OS/2 systems, GNU ‘m4’ will define ‘__os2__’ by default, or ‘os2’
when the ‘-G’ option is specified.

   If GNU ‘m4’ does not provide a platform macro for your system, please
report that as a bug.

     define(`provided', `0')
     ⇒
     ifdef(`__unix__', `define(`provided', incr(provided))')
     ⇒
     ifdef(`__windows__', `define(`provided', incr(provided))')
     ⇒
     ifdef(`__os2__', `define(`provided', incr(provided))')
     ⇒
     provided
     ⇒1