CentOS 7.0 - man page for clock (centos section 3)
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CLOCK(3) Linux Programmer's Manual CLOCK(3)
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. If the processor time used is not available or its value
cannot be represented, the function returns the value (clock_t) -1.
C89, C99, POSIX.1-2001. POSIX requires that CLOCKS_PER_SEC equals 1000000 independent of
the actual resolution.
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 32-bit system where CLOCKS_PER_SEC equals
1000000 this function will return the same value approximately every 72 minutes.
On several other implementations, the value returned by clock() also includes the times of
any children whose status has been collected via wait(2) (or another wait-type call).
Linux does not include the times of waited-for children in the value returned by clock().
The times(2) function, which explicitly returns (separate) information about the caller
and its children, may be preferable.
clock_gettime(2), getrusage(2), times(2)
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
GNU 2008-08-28 CLOCK(3)
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