The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


clock - report CPU time used


#include <time.h>

clock_t clock(void);


The clock() function returns the implementation's best approximation to the processor time used by the process since the beginning of an implementation-dependent time related only to the process invocation.


To determine the time in seconds, the value returned by clock() should be divided by the value of the macro CLOCKS_PER_SEC. 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 returns the value (clock_t)-1.


No errors are defined.




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.

The value returned by clock() may wrap around on some systems. For example, on a machine with 32-bit values for clock_t it will wrap after 2147 seconds or 36 minutes.




asctime(), ctime(), difftime(), gmtime(), localtime(), mktime(), strftime(), strptime(), time(), utime(), <time.h>.


Derived from Issue 1 of the SVID.

UNIX ® is a registered Trademark of The Open Group.
Copyright © 1997 The Open Group
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