1CLOCK(3P)                  POSIX Programmer's Manual                 CLOCK(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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NAME

12       clock — report CPU time used
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SYNOPSIS

15       #include <time.h>
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17       clock_t clock(void);
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DESCRIPTION

20       The functionality described on this reference page is aligned with  the
21       ISO C  standard.  Any  conflict between the requirements described here
22       and the ISO C standard is unintentional. This  volume  of  POSIX.1‐2017
23       defers to the ISO C standard.
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25       The  clock() function shall return the implementation's best approxima‐
26       tion to the processor time used by the process since the  beginning  of
27       an implementation-defined era related only to the process invocation.
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RETURN VALUE

30       To  determine the time in seconds, the value returned by clock() should
31       be divided by the value of the macro CLOCKS_PER_SEC.  CLOCKS_PER_SEC is
32       defined  to  be one million in <time.h>.  If the processor time used is
33       not available or its value cannot be represented,  the  function  shall
34       return the value (clock_t)-1.
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ERRORS

37       No errors are defined.
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39       The following sections are informative.
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EXAMPLES

42       None.
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APPLICATION USAGE

45       In  programming environments where clock_t is a 32-bit integer type and
46       CLOCKS_PER_SEC is one million, clock() will start failing in less  than
47       36  minutes  of  processor  time  for signed clock_t, or 72 minutes for
48       unsigned clock_t.  Applications intended to be portable to  such  envi‐
49       ronments   should   use   times()   instead  (or  clock_gettime()  with
50       CLOCK_PROCESS_CPUTIME_ID, if supported).
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52       In order to measure the time spent in  a  program,  clock()  should  be
53       called at the start of the program and its return value subtracted from
54       the value returned by subsequent calls. The value returned  by  clock()
55       is  defined for compatibility across systems that have clocks with dif‐
56       ferent resolutions. The resolution on any particular system need not be
57       to microsecond accuracy.
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RATIONALE

60       None.
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FUTURE DIRECTIONS

63       None.
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SEE ALSO

66       asctime(),  clock_getres(), ctime(), difftime(), gmtime(), localtime(),
67       mktime(), strftime(), strptime(), time(), times(), utime()
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69       The Base Definitions volume of POSIX.1‐2017, <time.h>
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72       Portions of this text are reprinted and reproduced in  electronic  form
73       from  IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
74       table Operating System Interface (POSIX), The Open Group Base  Specifi‐
75       cations  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of
76       Electrical and Electronics Engineers, Inc and The Open Group.   In  the
77       event of any discrepancy between this version and the original IEEE and
78       The Open Group Standard, the original IEEE and The Open Group  Standard
79       is  the  referee document. The original Standard can be obtained online
80       at http://www.opengroup.org/unix/online.html .
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82       Any typographical or formatting errors that appear  in  this  page  are
83       most likely to have been introduced during the conversion of the source
84       files to man page format. To report such errors,  see  https://www.ker‐
85       nel.org/doc/man-pages/reporting_bugs.html .
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89IEEE/The Open Group                  2017                            CLOCK(3P)
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