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

clock — report CPU time used

#include <time.h>
clock_t clock(void);

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 process

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 If the processor time used is not available or its value
cannot be represented, the function shall return the value (

No errors are defined.


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 it wraps after
2147 seconds or 36 minutes.



The Base Definitions volume of POSIX.1‐2008,


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 .

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