clock man page


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

clock — report CPU time used


#include <time.h>

clock_t clock(void);


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. 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.


No errors are defined.

The following sections are informative.



Application Usage

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 implementations. For example, on a machine with 32-bit values for clock_t, it wraps after 2147 seconds or 36 minutes.



Future Directions


See Also

asctime(), ctime(), difftime(), gmtime(), localtime(), mktime(), strftime(), strptime(), time(), utime()

The Base Definitions volume of POSIX.1‐2008, <time.h>


2013 IEEE/The Open Group POSIX Programmer's Manual