They do it by having something like

Code:
#ifdef WIN32
int myTime ( void ) {
  int result = ftime();  // or whatever
  return result;
}
#elif POSIX
int myTime ( void ) {
  int result = gettimeofday();  // or whatever
  return result;
}
#else
#error bad platform for myTime
#endif
Except there are better ways of doing it, but that's the idea.

You wrap the implementation specific parts of the code up in a consistent internal API call (in this case, myTime() ), which you can call from the rest of your ANSI-C89 code.

> even though the source is OPEN i am not able to make out how they have done it
Dig deeper, and look for the platform specific API calls. They will be there somewhere for sure.