This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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.

       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 .

IEEE/The Open Group                                   2017                                         CLOCK(3POSIX)

       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.

       No errors are defined.

       Thefollowingsectionsareinformative.

       None.

       None.

       clock — report CPU time used

       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.

       None.

       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.

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

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

       #include <time.h>

       clock_t clock(void);