When compiling a C/C++ program under Windows using Visual Studio (or a compiler that tries to be compatible) there is a predefined macro _WIN32 (Source: http://msdn.microsoft.com/en-us/library/b0084kay.aspx) that you can use for platform-specific #ifdef-s. What I am looking for is an analogon under Linux: a macro which tells me that I am compiling for Linux/an OS that claims to be (more or less) POSIX-compatible. So I looked into gcc documentation and found this: http://gcc.gnu.org/onlinedocs/cpp/System_002dspecific-Predefined-Macros.html Applied to my program, the following macros (gcc 4.4.5 - Ubuntu 10.10) looked promising (I hope that I didn't drop an important macro): #define __USE_BSD 1 #define __unix__ 1 #define __linux 1 #define __unix 1 #define __linux__ 1 #define _POSIX_SOURCE 1 #define __STDC_HOSTED__ 1 #define __STDC_IEC_559__ 1 #define __gnu_linux__ 1 #define __USE_SVID 1 #define __USE_XOPEN2K 1 #define __USE_POSIX199506 1 #define _G_USING_THUNKS 1 #define __USE_XOPEN2K8 1 #define _BSD_SOURCE 1 #define unix 1 #define linux 1 #define __USE_POSIX 1 #define __USE_POSIX199309 1 #define __SSP__ 1 #define _SVID_SOURCE 1 #define _G_HAVE_SYS_CDEFS 1 #define __USE_POSIX_IMPLICITLY 1 Where do I find a detailed documentation of them - as to the mentioned Windows-specific macros above? Additionally I'd be interested in macros normally defined for other POSIX-compliant operating systems as *BSD etc.