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'\" t
.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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 (date) "Linux man-pages (unreleased)"
.SH NAME
clock \- determine processor time
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <time.h>
.P
.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 ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface Attribute Value
T{
.na
.nh
.BR clock ()
T} Thread safety MT-Safe
.TE
.SH VERSIONS
XSI requires that
.B CLOCKS_PER_SEC
equals 1000000 independent
of the actual resolution.
.P
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 STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, C89.
.P
In glibc 2.17 and earlier,
.BR clock ()
was implemented on top of
.BR times (2).
For improved accuracy,
since glibc 2.18, it is implemented on top of
.BR clock_gettime (2)
(using the
.B CLOCK_PROCESS_CPUTIME_ID
clock).
.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.
.P
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.
.SH SEE ALSO
.BR clock_gettime (2),
.BR getrusage (2),
.BR times (2)
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