clock - determine processor time
() function returns an approximation of processor time used by
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
For an explanation of the terms used in this section, see attributes
POSIX.1-2001, POSIX.1-2008, C89, C99. XSI 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
equals 1000000 this function will return the same value
approximately every 72 minutes.
On several other implementations, the value returned by clock
includes the times of any children whose status has been collected via
(2) (or another wait-type call). Linux does not include the times
of waited-for children in the value returned by clock
(2) function, which explicitly returns (separate) information
about the caller and its children, may be preferable.
In glibc 2.17 and earlier, clock
() was implemented on top of
(2). For improved accuracy, since glibc 2.18, it is implemented on
top of clock_gettime
(2) (using the CLOCK_PROCESS_CPUTIME_ID