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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 dif-
       fer (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  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 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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  Issue  6,  Copyright (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 Stan-
       dard,  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 .



IEEE/The Open Group                  2003                            CLOCK(3P)