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.1‐2008 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 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.

       The value returned by clock() may wrap around on some implementations. For example,  on  a
       machine with 32-bit values for clock_t, it wraps after 2147 seconds or 36 minutes.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

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

       The Base Definitions volume of POSIX.1‐2008, <time.h>

COPYRIGHT

       Portions of this text are reprinted and  reproduced  in  electronic  form  from  IEEE  Std
       1003.1,  2013  Edition,  Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013  by  the
       Institute  of  Electrical  and  Electronics  Engineers,  Inc and The Open Group.  (This is
       POSIX.1-2008 with the  2013  Technical  Corrigendum  1  applied.)  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.unix.org/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 .