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.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\"
.\" 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.\" manual page may be incorrect or out-of-date.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
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" License.
.\" Modified Sat Jul 24 21:27:01 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified 14 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\" Added notes on differences from other Unix systems with respect to
.\" waited-for children.
.TH CLOCK 3 2008-08-28 "GNU" "Linux Programmer's Manual"
.SH NAME
clock \- Determine processor time
.SH SYNOPSIS
.nf
.B #include <time.h>
.sp
.B clock_t clock(void);
.fi
.SH DESCRIPTION
The
.BR clock ()
function returns an approximation of processor time used by the program.
.SH "RETURN VALUE"
The value returned is the CPU time used so far as a
.IR clock_t ;
to get the number of seconds used, divide by
.BR CLOCKS_PER_SEC .
If the processor time used is not available or its value cannot
be represented, the function returns the value
.IR (clock_t)\ \-1 .
.SH "CONFORMING TO"
C89, C99, POSIX.1-2001.
POSIX requires that
.B CLOCKS_PER_SEC
equals 1000000 independent
of the actual resolution.
.SH NOTES
The C standard allows for arbitrary values at the start of the program;
subtract the value returned from a call to
.BR clock ()
at the start of the program to get maximum portability.
.PP
Note that the time can wrap around.
On a 32-bit system where
.B CLOCKS_PER_SEC
equals 1000000 this function will return the same
value approximately every 72 minutes.
.PP
On several other implementations,
the value returned by
.BR clock ()
also includes the times of any children whose status has been
collected via
.BR wait (2)
(or another wait-type call).
Linux does not include the times of waited-for children in the
value returned by
.BR clock ().
.\" I have seen this behavior on Irix 6.3, and the OSF/1, HP/UX, and
.\" Solaris manual pages say that clock() also does this on those systems.
.\" POSIX.1-2001 doesn't explicitly allow this, nor is there an
.\" explicit prohibition. -- MTK
The
.BR times (2)
function, which explicitly returns (separate) information about the
caller and its children, may be preferable.
.SH "SEE ALSO"
.BR clock_gettime (2),
.BR getrusage (2),
.BR times (2)
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