Which Cross Platform Preprocessor Defines? (__WIN32__ or __WIN32 or WIN32 )?

I often see __WIN32, WIN32 or __WIN32__. I assume that this depends on the used preprocessor (either one from visual studio, or gcc etc).

Do I now have to check first for os and then for the used compiler? We are using here G++ 4.4.x, Visual Studio 2008 and Xcode (which I assume is a gcc again) and ATM we are using just __WIN32__, __APPLE__ and __LINUX__.

This article answers your question:

• C/C++ tip: How to detect the operating system type using compiler predefined macros (plus archive.org link in case it vanishes).

The article is quite long, and includes tables that are hard to reproduce, but here's the essence:

You can detect Unix-style OS with:

#if !defined(_WIN32) && (defined(__unix__) || defined(__unix) || (defined(__APPLE__) && defined(__MACH__)))
    /* UNIX-style OS. ------------------------------------------- */

#endif

Once you know it's Unix, you can find if it's POSIX and the POSIX version with:

#include <unistd.h>
#if defined(_POSIX_VERSION)
	/* POSIX compliant */
#endif

You can check for BSD-derived systems with:

#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
#include <sys/param.h>
#if defined(BSD)
	/* BSD (DragonFly BSD, FreeBSD, OpenBSD, NetBSD). ----------- */

#endif
#endif

and Linux with:

#if defined(__linux__)
	/* Linux  */
#endif

and Apple's operating systems with

#if defined(__APPLE__) && defined(__MACH__)
	/* Apple OSX and iOS (Darwin) */
#include <TargetConditionals.h>
#if TARGET_IPHONE_SIMULATOR == 1
	/* iOS in Xcode simulator */
#elif TARGET_OS_IPHONE == 1
	/* iOS on iPhone, iPad, etc. */    
#elif TARGET_OS_MAC == 1
	/* OS X */
#endif
#endif

Windows with Cygwin

#if defined(__CYGWIN__) && !defined(_WIN32)
	/* Cygwin POSIX under Microsoft Windows. */
#endif

And non-POSIX Windows with:

#if defined(_WIN64)
	/* Microsoft Windows (64-bit) */
#elif defined(_WIN32)
	/* Microsoft Windows (32-bit) */
#endif

The full article lists the following symbols, and shows which systems define them and when: _AIX, __APPLE__, __CYGWIN32__, __CYGWIN__, __DragonFly__, __FreeBSD__, __gnu_linux, hpux, __hpux, linux, __linux, __linux__, __MACH__, __MINGW32__, __MINGW64__, __NetBSD__, __OpenBSD__, _POSIX_IPV6, _POSIX_MAPPED_FILES, _POSIX_SEMAPHORES, _POSIX_THREADS, _POSIX_VERSION, sun, __sun, __SunOS, __sun__, __SVR4, __svr4__, TARGET_IPHONE_SIMULATOR, TARGET_OS_EMBEDDED, TARGET_OS_IPHONE, TARGET_OS_MAC, UNIX, unix, __unix, __unix__, WIN32, _WIN32, __WIN32, __WIN32__, WIN64, _WIN64, __WIN64, __WIN64__, WINNT, __WINNT, __WINNT__.

A related article (archive.org link) covers detecting compilers and compiler versions. It lists the following symbols: __clang__, __GNUC__, __GNUG__, __HP_aCC, __HP_cc, __IBMCPP__, __IBMC__, __ICC, __INTEL_COMPILER, _MSC_VER, __PGI, __SUNPRO_C, __SUNPRO_CC for detecting compilers, and __clang_major__, __clang_minor__, __clang_patchlevel__, __clang_version__, __GNUC_MINOR__, __GNUC_PATCHLEVEL__, __GNUC__, __GNUG__, __HP_aCC, __HP_cc, __IBMCPP__, __IBMC__, __ICC, __INTEL_COMPILER, __INTEL_COMPILER_BUILD_DATE, _MSC_BUILD, _MSC_FULL_VER, _MSC_VER, __PGIC_MINOR__, __PGIC_PATCHLEVEL__, __PGIC__, __SUNPRO_C, __SUNPRO_CC, __VERSION__, __xlC_ver__, __xlC__, __xlc__ for detecting compiler versions.

It depends what you are trying to do. You can check the compiler if your program wants to make use of some specific functions (from the gcc toolchain for example). You can check for operating system ( _WINDOWS, __unix__ ) if you want to use some OS specific functions (regardless of compiler - for example CreateProcess on Windows and fork on unix).

Macros for Visual C

Macros for gcc

You must check the documentation of each compiler in order to be able to detect the differences when compiling. I remember that the gnu toolchain(gcc) has some functions in the C library (libc) that are not on other toolchains (like Visual C for example). This way if you want to use those functions out of commodity then you must detect that you are using GCC, so the code you must use would be the following:

#ifdef __GNUC__
// do my gcc specific stuff
#else
// ... handle this for other compilers
#endif

Don't see why you have to. You might have to remember to specify the definition manually on your compiler's commandline, but that's all. For the record, Visual Studio's definition is _WIN32 (with one underscore) rather than __WIN32. If it's not defined then it's not defined, and it won't matter.

I've rebuild my answer... Damn, editing berserk :P:

You don't need to use partical one. And probably for MacOSX, Linux and other Unix-likes you don't need to use any at all.

Most popular one is (as far as Google tells the truth) is _WIN32.

You never define it "by hand" in your source code. It is defined in one of these ways:
as a commandline preprocessor/compiler flag (like g++ -D _WIN32)
or it is predefined by compiler itself (most of Windows compilers predefine _WIN32, and sometimes other like WIN32 or _WIN32_ too. -- Then you don't need to worry about defining it at all, compiler does the whole work.

---

And my old answer:

You don't 'have to' anything. It's just for multi-platform compatibility. Often version of code for all Unix-likes (including Linux, MacOSX, BSD, Solaris...) and other POSIX platform will be completely the same and there must be some changes for Windows. So people write their code generally for Unix-likes and put some Windows-only (eg. DirectX instructions, Windows-like file paths...) parts between #ifdef _WIN32 and #endif.

If you have some parts eg. X-Window-system only, or MacOS-only, you do similar with something like #ifdef X_WINDOW or #ifdef MACOS. Then, you need set a proper preprocessor definition while compiling (with gcc using -D flag, like eg. gcc -D _WIN32).

If you don't write any platform-dependent code, then you don't need to care for such a #ifdef, #else, #endif blocks. And most of Windows compilers/preprocessors AFAIK have predefined some symbols like _WIN32 (most popular, as far as google tells the truth), WIN32, _WIN32_, etc. So compiling it on Windows most probably you don't need to make anything else than just compiling.

Sigh - don't rely on compiler anything - specify which platform you are building for in your Makefile. Simply put, anything beginning with _ is implementation dependent and not portable.

I tried your method once upon a time, on a very large project, and in between bouncing around between Sun-C++ and GCC we just decided to go with Makefile control rather than trying to deduce what the compilers were going to do.