NAME
clock — report CPU
time used
SYNOPSIS
#include
<time.h>
clock_t clock(void);
DESCRIPTION
[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.
The clock () function shall return the
implementation's best approximation to the processor time used by the
process since the beginning of an implementation-defined era related only to
the process invocation.
RETURN VALUE
To determine the time in seconds, the value returned by
clock () should be divided by the value of the macro
CLOCKS_PER_SEC.
[XSI]
CLOCKS_PER_SEC is defined to be one million in
<time.h>. If the processor time used is not
available or its value cannot be represented, the function shall return the
value ( clock_t) -1.
ERRORS
The clock () function shall fail if:
- [EOVERFLOW]
- [CX] The processor time used cannot be represented in an object of type clock_t .
EXAMPLES
None.
APPLICATION USAGE
In programming environments where clock_t is a
32-bit integer type and CLOCKS_PER_SEC is one million,
clock () will start failing in less than 36 minutes
of processor time for signed clock_t, or 72 minutes for
unsigned clock_t. Applications intended to be portable to
such environments should use times(3) instead (or
clock_gettime(3) with CLOCK_PROCESS_CPUTIME_ID, if
supported).
In order to measure the time spent in a program,
clock () should be called at the start of the
program and its return value subtracted from the value returned by
subsequent calls. The value returned by clock () is
defined for compatibility across systems that have clocks with different
resolutions. The resolution on any particular system need not be to
microsecond accuracy.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
asctime(3), clock_getres(3) , ctime(3), difftime(3), futimens(3), gmtime(3), localtime(3), mktime(3), strftime(3), strptime(3), time(3), times(3)
XBD <time.h>
CHANGE HISTORY
First released in Issue 1. Derived from Issue 1 of the SVID.
Issue 7
POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0067 [686] is applied.
Issue 8
Austin Group Defect 703 is applied, adding the [EOVERFLOW] error.