clock - Determine processor time
The clock() function returns an approximation of processor
time used by the program.
The value returned is the CPU time used so far as a clock_t;
to get the number of seconds used, divide by CLOCKS_PER_SEC.
The C standard allows for arbitrary values at the start of
the program; take the difference between the value returned
from a call to clock() at the start of the program and the
end to get maximum portability.
The times() function call returns more information.