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



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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 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  Defi-
       nitions volume of IEEE Std 1003.1-2001, <time.h>

COPYRIGHT
       Portions  of  this  text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
       Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     CLOCK(3P)