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.
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
approximation to the processor time used by the process since the
beginning of an implementation-defined era related only to the
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.
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.
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 Electrical 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
IEEE/The Open Group 2013 CLOCK(3P)