void MyMultiplatformFunction(...)
{

common code

#ifdef PLATFORM_LINUX

code specific to linux only

#elif defined(PLATFORM_POSIX)

code specific to posix systems (linux, bsd, solaris, osx?)

#elif defined(PLATFORM_WIN64) 

code specific to windows 64-bit only

#elif defined(PLATFORM_WIN32) 

code specific to windows 32/64

#endif

common code

}