CLOCK

CLOCK(3)                  Linux Programmer’s Manual                 CLOCK(3)

NAME

clock – determine processor time

SYNOPSIS

#include <time.h>

clock_t clock(void);

DESCRIPTION

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

RETURN VALUE

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.

CONFORMING TO

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

NOTES

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.

In glibc 2.17 and earlier, clock() was implemented on top of
times(2).  For improved accuracy, since glibc 2.18, it is implemented
on top of clock_gettime(2) (using the CLOCK_PROCESS_CPUTIME_ID
clock).

SEE ALSO

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

COLOPHON

This page is part of release 3.76 of the Linux man-pages project.  A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
http://www.kernel.org/doc/man-pages/.

GNU                              2014-09-21                         CLOCK(3)

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