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() instead (or clock_gettime() 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 , clock_getres , ctime , difftime , futimens , gmtime , localtime , mktime , strftime , strptime , time , times

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.