DESCRIPTION clock
calculates the best available approximation of the cumulative amount of
time used by your program since it started. To convert the result into
seconds, divide by the macro, CLOCKS_PER_SEC.
RETURNS The amount of processor time used so far by your program, in units
defined by the machine-dependent macro, CLOCKS_PER_SEC.
If no measurement is available, the result is -1.
COMPLIANCE ANSI C requires clock
and CLOCKS_PER_SEC.