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.
clock — 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
approximation 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
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 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.
asctime(), ctime(), difftime(),
gmtime(), localtime(), mktime(),
strftime(), strptime(), time(),
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 http://www.unix.org/online.html .
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