clock(3) Library Functions Manual clock(3)
NAME
clock - determine processor time
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <time.h>
clock_t clock(void);
DESCRIPTION
The clock() function returns an approximation of processor time used by
the program.
RETURN VALUE
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.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌───────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├───────────────────────────────────────────┼───────────────┼─────────┤
│ clock() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────┴───────────────┴─────────┘
VERSIONS
XSI requires that CLOCKS_PER_SEC equals 1000000 independent of the ac-
tual resolution.
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.
STANDARDS
C11, POSIX.1-2008.
HISTORY
POSIX.1-2001, C89.
In glibc 2.17 and earlier, clock() was implemented on top of times(2).
For improved accuracy, since glibc 2.18, it is implemented on top of
clock_gettime(2) (using the CLOCK_PROCESS_CPUTIME_ID clock).
NOTES
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.
SEE ALSO
clock_gettime(2), getrusage(2), times(2)
Linux man-pages 6.7 2023-10-31 clock(3)
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