The Open Group Base Specifications Issue 7
IEEE Std 1003.1-2008, 2016 Edition
Copyright © 2001-2016 The IEEE and The Open Group

#### 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.1-2008 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

> No errors are defined.

The following sections are informative.

#### 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, gmtime, localtime, mktime, strftime, strptime, time, times, utime

> 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.

End of informative text.

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