man3/clock.3

   1 .\" (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
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  23 .\" Modified Sat Jul 24 21:27:01 1993 by Rik Faith (faith@cs.unc.edu)
  24 .\" Modified 14 Jun 2002, Michael Kerrisk <mtk-manpages@gmx.net>
  25 .\"     Added notes on differences from other Unix systems with respect to
  26 .\"     waited-for children.
  27 .TH CLOCK 3  2002-06-14 "GNU" "Linux Programmer's Manual"
  28 .SH NAME
  29 clock \- Determine processor time
  30 .SH SYNOPSIS
  31 .nf
  32 .B #include <time.h>
  33 .sp
  34 .B clock_t clock(void);
  35 .fi
  36 .SH DESCRIPTION
  37 The
  38 .BR clock ()
  39 function returns an approximation of processor time used by the program.
  40 .SH "RETURN VALUE"
  41 The value returned is the CPU time used so far as a
  42 .IR clock_t ;
  43 to get the number of seconds used, divide by
  44 .BR CLOCKS_PER_SEC .
  45 If the processor time used is not available or its value cannot
  46 be represented, the function returns the value
  47 .BR (clock_t)\-1 .
  48 .SH "CONFORMING TO"
  49 C89, C99, POSIX.1-2001.
  50 POSIX requires that CLOCKS_PER_SEC equals 1000000 independent
  51 of the actual resolution.
  52 .SH NOTES
  53 The C standard allows for arbitrary values at the start of the program;
  54 subtract the value returned from a call to
  55 .BR clock ()
  56 at the start of the program to get maximum portability.
  57 .PP
  58 Note that the time can wrap around.
  59 On a 32bit system where
  60 CLOCKS_PER_SEC equals 1000000 this function will return the same
  61 value approximately every 72 minutes.
  62 .PP
  63 On several other implementations,
  64 the value returned by
  65 .BR clock ()
  66 also includes the times of any children whose status has been
  67 collected via
  68 .BR wait (2)
  69 (or another wait-type call).
  70 Linux does not include the times of waited-for children in the
  71 value returned by
  72 .BR clock ().
  73 .\" I have seen this behavior on Irix 6.3, and the OSF/1, HP/UX, and
  74 .\" Solaris manual pages say that clock() also does this on those systems.
  75 .\" POSIX.1-2001 doesn't explicitly allow this, nor is there an
  76 .\" explicit prohibition. -- MTK
  77 The
  78 .BR times (2)
  79 function, which explicitly returns (separate) information about the
  80 caller and its children, may be preferable.
  81 .SH "SEE ALSO"
  82 .BR getrusage (2),
  83 .BR times (2)