clock (3)
Leading comments
Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de) %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical t...
NAME
clock - determine processor timeSYNOPSIS
#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 attributes(7).| Interface | Attribute | Value |
| clock() | Thread safety | MT-Safe |
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, C89, C99. XSI requires that CLOCKS_PER_SEC equals 1000000 independent of the actual resolution.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.
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.
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).