mirror of https://github.com/mkerrisk/man-pages
114 lines
3.5 KiB
Groff
114 lines
3.5 KiB
Groff
.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.\"
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.\" Modified Sat Jul 24 21:27:01 1993 by Rik Faith (faith@cs.unc.edu)
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.\" Modified 14 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
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.\" Added notes on differences from other UNIX systems with respect to
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.\" waited-for children.
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.TH CLOCK 3 2015-08-08 "GNU" "Linux Programmer's Manual"
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.SH NAME
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clock \- determine processor time
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.SH SYNOPSIS
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.nf
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.B #include <time.h>
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.PP
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.B clock_t clock(void);
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.fi
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.SH DESCRIPTION
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The
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.BR clock ()
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function returns an approximation of processor time used by the program.
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.SH RETURN VALUE
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The value returned is the CPU time used so far as a
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.IR clock_t ;
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to get the number of seconds used, divide by
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.BR CLOCKS_PER_SEC .
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If the processor time used is not available or its value cannot
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be represented, the function returns the value
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.IR (clock_t)\ \-1 .
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.SH ATTRIBUTES
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For an explanation of the terms used in this section, see
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.BR attributes (7).
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.TS
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allbox;
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lb lb lb
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l l l.
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Interface Attribute Value
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T{
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.BR clock ()
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T} Thread safety MT-Safe
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.TE
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.sp 1
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.SH CONFORMING TO
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POSIX.1-2001, POSIX.1-2008, C89, C99.
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XSI requires that
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.B CLOCKS_PER_SEC
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equals 1000000 independent
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of the actual resolution.
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.SH NOTES
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The C standard allows for arbitrary values at the start of the program;
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subtract the value returned from a call to
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.BR clock ()
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at the start of the program to get maximum portability.
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.PP
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Note that the time can wrap around.
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On a 32-bit system where
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.B CLOCKS_PER_SEC
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equals 1000000 this function will return the same
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value approximately every 72 minutes.
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.PP
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On several other implementations,
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the value returned by
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.BR clock ()
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also includes the times of any children whose status has been
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collected via
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.BR wait (2)
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(or another wait-type call).
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Linux does not include the times of waited-for children in the
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value returned by
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.BR clock ().
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.\" I have seen this behavior on Irix 6.3, and the OSF/1, HP/UX, and
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.\" Solaris manual pages say that clock() also does this on those systems.
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.\" POSIX.1-2001 doesn't explicitly allow this, nor is there an
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.\" explicit prohibition. -- MTK
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The
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.BR times (2)
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function, which explicitly returns (separate) information about the
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caller and its children, may be preferable.
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.PP
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In glibc 2.17 and earlier,
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.BR clock ()
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was implemented on top of
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.BR times (2).
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For improved accuracy,
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since glibc 2.18, it is implemented on top of
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.BR clock_gettime (2)
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(using the
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.BR CLOCK_PROCESS_CPUTIME_ID
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clock).
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.SH SEE ALSO
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.BR clock_gettime (2),
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.BR getrusage (2),
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.BR times (2)
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