-.\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
+.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" 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.
.\" 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.\" manual page may be incorrect or out-of-date.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
.\" 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.
+.\" %%%LICENSE_END
+.\"
.\" Modified Sat Jul 24 21:27:01 1993 by Rik Faith (faith@cs.unc.edu)
-.\" Modified 14 Jun 2002, Michael Kerrisk <mtk-manpages@gmx.net>
-.\" Added notes on differences from other Unices with respect to
+.\" 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 2002-06-14 "GNU" "Linux Programmer's Manual"
+.TH CLOCK 3 2017-09-15 "GNU" "Linux Programmer's Manual"
.SH NAME
-clock \- Determine processor time
+clock \- determine processor time
.SH SYNOPSIS
.nf
.B #include <time.h>
-.sp
+.PP
.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"
+.SH RETURN VALUE
The value returned is the CPU time used so far as a
-.BR clock_t ;
+.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 (clock_t)\-1.
-.SH "CONFORMING TO"
-C89, C99, POSIX.1-2001.
-POSIX requires that CLOCKS_PER_SEC equals 1000000 independent
+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;
+lb lb lb
+l l l.
+Interface Attribute Value
+T{
+.BR clock ()
+T} Thread safety MT-Safe
+.TE
+.sp 1
+.SH CONFORMING TO
+POSIX.1-2001, POSIX.1-2008, C89, C99.
+XSI 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;
.BR clock ()
at the start of the program to get maximum portability.
.PP
-Note that the time can wrap around. On a 32bit system where
-CLOCKS_PER_SEC equals 1000000 this function will return the same
+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
+the value returned by
.BR clock ()
also includes the times of any children whose status has been
collected via
-.BR wait ()
+.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 behaviour on Irix 6.3, and the OSF/1, HP/UX, and
+.\" 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 ()
-function, which explicitly returns (separate) information about the
+.BR times (2)
+function, which explicitly returns (separate) information about the
caller and its children, may be preferable.
-.SH "SEE ALSO"
+.PP
+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
+.BR CLOCK_PROCESS_CPUTIME_ID
+clock).
+.SH SEE ALSO
+.BR clock_gettime (2),
.BR getrusage (2),
.BR times (2)
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