man3/clock.3

   1 .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
   2 .\"
   3 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   4 .\"
   5 .\" Modified Sat Jul 24 21:27:01 1993 by Rik Faith (faith@cs.unc.edu)
   6 .\" Modified 14 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
   7 .\"     Added notes on differences from other UNIX systems with respect to
   8 .\"     waited-for children.
   9 .TH clock 3 (date) "Linux man-pages (unreleased)"
  10 .SH NAME
  11 clock \- determine processor time
  12 .SH LIBRARY
  13 Standard C library
  14 .RI ( libc ", " \-lc )
  15 .SH SYNOPSIS
  16 .nf
  17 .B #include <time.h>
  18 .PP
  19 .B clock_t clock(void);
  20 .fi
  21 .SH DESCRIPTION
  22 The
  23 .BR clock ()
  24 function returns an approximation of processor time used by the program.
  25 .SH RETURN VALUE
  26 The value returned is the CPU time used so far as a
  27 .IR clock_t ;
  28 to get the number of seconds used, divide by
  29 .BR CLOCKS_PER_SEC .
  30 If the processor time used is not available or its value cannot
  31 be represented, the function returns the value
  32 .IR (clock_t)\ \-1 .
  33 .SH ATTRIBUTES
  34 For an explanation of the terms used in this section, see
  35 .BR attributes (7).
  36 .ad l
  37 .nh
  38 .TS
  39 allbox;
  40 lbx lb lb
  41 l l l.
  42 Interface       Attribute       Value
  43 T{
  44 .BR clock ()
  45 T}      Thread safety   MT-Safe
  46 .TE
  47 .hy
  48 .ad
  49 .sp 1
  50 .SH STANDARDS
  51 POSIX.1-2001, POSIX.1-2008, C89, C99.
  52 XSI requires that
  53 .B CLOCKS_PER_SEC
  54 equals 1000000 independent
  55 of the actual resolution.
  56 .SH NOTES
  57 The C standard allows for arbitrary values at the start of the program;
  58 subtract the value returned from a call to
  59 .BR clock ()
  60 at the start of the program to get maximum portability.
  61 .PP
  62 Note that the time can wrap around.
  63 On a 32-bit system where
  64 .B CLOCKS_PER_SEC
  65 equals 1000000 this function will return the same
  66 value approximately every 72 minutes.
  67 .PP
  68 On several other implementations,
  69 the value returned by
  70 .BR clock ()
  71 also includes the times of any children whose status has been
  72 collected via
  73 .BR wait (2)
  74 (or another wait-type call).
  75 Linux does not include the times of waited-for children in the
  76 value returned by
  77 .BR clock ().
  78 .\" I have seen this behavior on Irix 6.3, and the OSF/1, HP/UX, and
  79 .\" Solaris manual pages say that clock() also does this on those systems.
  80 .\" POSIX.1-2001 doesn't explicitly allow this, nor is there an
  81 .\" explicit prohibition. -- MTK
  82 The
  83 .BR times (2)
  84 function, which explicitly returns (separate) information about the
  85 caller and its children, may be preferable.
  86 .PP
  87 In glibc 2.17 and earlier,
  88 .BR clock ()
  89 was implemented on top of
  90 .BR times (2).
  91 For improved accuracy,
  92 since glibc 2.18, it is implemented on top of
  93 .BR clock_gettime (2)
  94 (using the
  95 .B CLOCK_PROCESS_CPUTIME_ID
  96 clock).
  97 .SH SEE ALSO
  98 .BR clock_gettime (2),
  99 .BR getrusage (2),
 100 .BR times (2)