man3/clock.3

   1 .\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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  25 .\" Modified Sat Jul 24 21:27:01 1993 by Rik Faith (faith@cs.unc.edu)
  26 .\" Modified 14 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
  27 .\"     Added notes on differences from other UNIX systems with respect to
  28 .\"     waited-for children.
  29 .TH CLOCK 3  2017-09-15 "GNU" "Linux Programmer's Manual"
  30 .SH NAME
  31 clock \- determine processor time
  32 .SH SYNOPSIS
  33 .nf
  34 .B #include <time.h>
  35 .PP
  36 .B clock_t clock(void);
  37 .fi
  38 .SH DESCRIPTION
  39 The
  40 .BR clock ()
  41 function returns an approximation of processor time used by the program.
  42 .SH RETURN VALUE
  43 The value returned is the CPU time used so far as a
  44 .IR clock_t ;
  45 to get the number of seconds used, divide by
  46 .BR CLOCKS_PER_SEC .
  47 If the processor time used is not available or its value cannot
  48 be represented, the function returns the value
  49 .IR (clock_t)\ \-1 .
  50 .SH ATTRIBUTES
  51 For an explanation of the terms used in this section, see
  52 .BR attributes (7).
  53 .TS
  54 allbox;
  55 lb lb lb
  56 l l l.
  57 Interface       Attribute       Value
  58 T{
  59 .BR clock ()
  60 T}      Thread safety   MT-Safe
  61 .TE
  62 .sp 1
  63 .SH CONFORMING TO
  64 POSIX.1-2001, POSIX.1-2008, C89, C99.
  65 XSI requires that
  66 .B CLOCKS_PER_SEC
  67 equals 1000000 independent
  68 of the actual resolution.
  69 .SH NOTES
  70 The C standard allows for arbitrary values at the start of the program;
  71 subtract the value returned from a call to
  72 .BR clock ()
  73 at the start of the program to get maximum portability.
  74 .PP
  75 Note that the time can wrap around.
  76 On a 32-bit system where
  77 .B CLOCKS_PER_SEC
  78 equals 1000000 this function will return the same
  79 value approximately every 72 minutes.
  80 .PP
  81 On several other implementations,
  82 the value returned by
  83 .BR clock ()
  84 also includes the times of any children whose status has been
  85 collected via
  86 .BR wait (2)
  87 (or another wait-type call).
  88 Linux does not include the times of waited-for children in the
  89 value returned by
  90 .BR clock ().
  91 .\" I have seen this behavior on Irix 6.3, and the OSF/1, HP/UX, and
  92 .\" Solaris manual pages say that clock() also does this on those systems.
  93 .\" POSIX.1-2001 doesn't explicitly allow this, nor is there an
  94 .\" explicit prohibition. -- MTK
  95 The
  96 .BR times (2)
  97 function, which explicitly returns (separate) information about the
  98 caller and its children, may be preferable.
  99 .PP
 100 In glibc 2.17 and earlier,
 101 .BR clock ()
 102 was implemented on top of
 103 .BR times (2).
 104 For improved accuracy,
 105 since glibc 2.18, it is implemented on top of
 106 .BR clock_gettime (2)
 107 (using the
 108 .BR CLOCK_PROCESS_CPUTIME_ID
 109 clock).
 110 .SH SEE ALSO
 111 .BR clock_gettime (2),
 112 .BR getrusage (2),
 113 .BR times (2)