clock(3p) — Linux manual page


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

PROLOG         top

       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         top

       clock — report CPU time used

SYNOPSIS         top

       #include <time.h>

       clock_t clock(void);

DESCRIPTION         top

       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         top

       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         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top



       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

       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         top




SEE ALSO         top

       asctime(3p), clock_getres(3p), ctime(3p), difftime(3p),
       gmtime(3p), localtime(3p), mktime(3p), strftime(3p),
       strptime(3p), time(3p), times(3p), utime(3p)

       The Base Definitions volume of POSIX.1‐2017, time.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                         CLOCK(3P)

Pages that refer to this page: time.h(0p)asctime(3p)ctime(3p)difftime(3p)gmtime(3p)localtime(3p)mktime(3p)strftime(3p)time(3p)