clock — determine processor time


#include <time.h>
clock_t clock( void);  


The clock() function returns an approximation of processor time used by the program.


The value returned is the CPU time used so far as a clock_t; to get the number of seconds used, divide by CLOCKS_PER_SEC. If the processor time used is not available or its value cannot be represented, the function returns the value (clock_t) −1.


C89, C99, POSIX.1-2001. POSIX requires that CLOCKS_PER_SEC equals 1000000 independent of the actual resolution.


The C standard allows for arbitrary values at the start of the program; subtract the value returned from a call to clock() at the start of the program to get maximum portability.

Note that the time can wrap around. On a 32-bit system where CLOCKS_PER_SEC equals 1000000 this function will return the same value approximately every 72 minutes.

On several other implementations, the value returned by clock() also includes the times of any children whose status has been collected via wait(2) (or another wait-type call). Linux does not include the times of waited-for children in the value returned by clock(). The times(2) function, which explicitly returns (separate) information about the caller and its children, may be preferable.


clock_gettime(2), getrusage(2), times(2)


This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (c) 1993 by Thomas Koenig (

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Modified Sat Jul 24 21:27:01 1993 by Rik Faith (
Modified 14 Jun 2002, Michael Kerrisk <>
Added notes on differences from other UNIX systems with respect to
waited-for children.