Windows
_WIN32 Both 32 bit and 64 bit
_WIN64 64 bit only
Unix (Linux, *BSD, Mac OS X)
unix
__unix
__unix__
Mac OS X
__APPLE__
__MACH__
Linux
__linux__
linux Obsolete (not POSIX compliant)
__linux Obsolete (not POSIX compliant)
FreeBSD
__FreeBSD__
Android
__ANDROID__