Identifying the target platform

A common problem is identifying the target platform at compile-time so that specific code can be used, perhaps to work around a quirk, or to access a non-standard feature of the target platform.

Standard compiler platform macros

The following table is a partial extract of [Pre-defined C/C++ Compiler Macros], part of a useful resource to compiler macro definitions.

Please update the table carefully with your platform or corrections:

Platform macros

Platform	Defined macro	Other notes
Darwin	DARWIN	Darwin is not necessarily OS X, see below
FreeBSD	__FreeBSD__	Consider using the BSD macro
Linux	linux or __linux	I also found LINUX
MacOS X	__MACOSX__ or __APPLE__
NetBSD	__NetBSD__	Consider using the BSD macro
OpenBSD	__OpenBSD__	Consider using the BSD macro
Solaris	sun or __sun	SunOS versions < 5 will not have __SVR4 or __svr4__ defined
Windows	_WIN32 or __WIN32__

You can use #if wrappers like:

#if defined (__FreeBSD__)
...
#elif defined (__linux) || defined (linux)
...
#else
...
#endif

Using GNUStep-Make's support

Chris Vetter shared a tip on the use of your GNUmakefile.preamble with the following line:

ADDITIONAL_CPPFLAGS += -D$(GNUSTEP_HOST_OS)

and then the #if wrappers will look like:

#if defined( freebsd )
...
#elif defined( netbsdelf )
...
#else
...
#endif

To be on the safe side, you may then want to mix the compiler defaults and GNUstep's definitions. It's also good style to add an error or at least a warning in case a particular platform is not (yet) supported.

#if defined( freebsd ) || defined( __FreeBSD__ )
...
#elif defined( darwin ) || defined( DARWIN )
...
#else
# error Do not know how to handle this feature on your platform.
#endif

Always keep in mind that not everyone is using your preferred platform and that implementations may vary. This is especially true with respect to the implementation and usage of /proc on Linux as compared to other systems.

It's much better generating an error (as mentioned above) than just assuming your code will work on a different platform. The former will most likely get someone involved in your pet project and have them send in patches for their particular system. The latter will probably just annoy people, especially if your code screws up (on) their system due to differences...