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. If the processor time
used is not available or its value cannot be represented, the function returns
the value (clock_t)-1.
The C standard allows for arbitrary values at the start of the program; subtract
the value returned from a call to clock() at the start of the program
to get maximum portability.
Note that the time can wrap around. On a 32bit system where CLOCKS_PER_SEC
equals 1000000 this function will return the same value approximately every 72
On several other implementations, the value returned by clock() also
includes the times of any children whose status has been collected via
wait() (or another wait-type call). Linux does not include the times of
waited-for children in the value returned by clock(). The
times() function, which explicitly returns (separate) information about
the caller and its children, may be preferable.