clock() returns a clock_t containing the processor time since a specific point in time (implementation defined). This return value should be divided by CLOCKS_PER_SEC to get the time in seconds. CLOCKS_PER_SEC == 1,000,000 in time.h clock() returns (clock_t) - 1 if the processor time used cannot be represented or is somehow unavailable clock() is best used to get a delta between two times in a program. It is acceptable for clock() to wrap on some implementations. (No specifics on when/where.)