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



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

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 .



IEEE/The Open Group                  2017                             CLOCK(3)
