Unix, C, and C++
Function Reference
System Dependencies and Predefined Constants

The C and C++ standards leave a lot of things undefined or incompletely defined, and different operating systems and compilers will will make different decisions. Sometimes it is essential to know what system a program is running under.

Detecting the System Type

Every commonly used C or C++ compiler pre-defines some special constants that the program can check. These are pre-processor definitions (#define), not proper C++ constants, and they should only be used in conjunction with the preprocessor directives #ifdef and #ifndef.

I recommend putting the following sequence of definitions at the top of any program that needs to know what kind of system it is running on. No #includes are needed to make this work: 
      const int sys_unix=1, sys_windows=2, sys_unknown=0;
      
      #if defined(unix)
        const int system = sys_unix;
      #elif defined (_WIN32)
        const int system = sys_windows;
      #else
        const int system = sys_unknown;
      #endif
With those definitions in place, proper C and C++ statements can make use of the information: 
      void main(void)
      { if (system==sys_unix)
          printf("Unix\n");
        if (system==sys_windows)
          printf("windows32\n");
        if (system==sys_unknown)
          printf("unknown\n"); }

Very Long (64 bit) Integers

Under the gcc compilers for unix, 64 bit integers are called "long long int"; under the microsoft compilers they are called "__int64" (that's two underlines at the beginning).

This sequence provides uniform system-independent type names of "int64", and "unit64" for the unsigned version, that can be used in all declarations. 
      #if defined(_WIN32)
      #define int64 __int64
      #define uint64 __int64
      #elif defined(unix)
      #define int64 long long int
      #define uint64 unsigned long long int
      #endif