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 (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEclock — report CPU time used
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 approxima‐
tion to the processor time used by the process since the beginning of
an implementation-defined era related only to the process invocation.
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.
No errors are defined.
The following sections are informative.
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 dif‐
ferent 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.
SEE ALSOasctime(), ctime(), difftime(), gmtime(), localtime(), mktime(), strf‐
time(), strptime(), time(), utime()
The Base Definitions volume of POSIX.1‐2008, <time.h>
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 Electri‐
cal 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.ker‐
IEEE/The Open Group 2013 CLOCK(3P)