PROLOG

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

NAME

       clock — report CPU time used

SYNOPSIS

       #include <time.h>

       clock_t clock(void);

DESCRIPTION

       The functionality described on this reference page is aligned with the ISO C standard. Any
       conflict between the requirements described here and the ISO C standard is  unintentional.
       This volume of POSIX.1‐2017 defers to the ISO C standard.

       The clock() function shall return the implementation's best approximation to the processor
       time used by the process since the beginning of an implementation-defined era related only
       to the process invocation.

RETURN VALUE

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

ERRORS

       No errors are defined.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       In programming environments where clock_t is a 32-bit integer type and  CLOCKS_PER_SEC  is
       one  million,  clock()  will  start  failing in less than 36 minutes of processor time for
       signed clock_t, or 72 minutes for unsigned clock_t.  Applications intended to be  portable
       to   such   environments   should   use   times()   instead   (or   clock_gettime()   with
       CLOCK_PROCESS_CPUTIME_ID, if supported).

       In order to measure the time spent in a program, clock() should be called at the start  of
       the  program  and its return value subtracted from the value returned by subsequent calls.
       The value returned by clock() is defined for compatibility across systems that have clocks
       with  different  resolutions.  The  resolution  on  any  particular  system need not be to
       microsecond accuracy.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       asctime(),  clock_getres(),  ctime(),   difftime(),   gmtime(),   localtime(),   mktime(),
       strftime(), strptime(), time(), times(), utime()

       The Base Definitions volume of POSIX.1‐2017, <time.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1-2017, Standard for Information Technology -- Portable  Operating  System  Interface
       (POSIX),  The  Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
       the Institute of Electrical and Electronics Engineers, Inc and The  Open  Group.   In  the
       event  of  any  discrepancy  between this version and the original IEEE and The Open Group
       Standard, the original IEEE and The Open Group  Standard  is  the  referee  document.  The
       original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or  formatting errors that appear in this page are most likely to have
       been introduced during the conversion of the source files to man page  format.  To  report
       such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .