The Unix clock function calculates the CPU time, not the wall clock time.

[gpaulissen]

Depending on the environment either clock(), ftime() or getrusage() were used for calculating a double time (in seconds and after the digit the fractional seconds). The clock() function does its job on Windows however not on Unix.

See also 8 Ways to Measure Execution Time in C/C++.

[gpaulissen]

Now first clock_gettime() and gettimeofday() are used before the others.