Section: POSIX Programmer's Manual (3P)
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.
--- 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.1-2017 defers to the ISO C standard.
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
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 (clock_t
No errors are defined.
The following sections are informative.
In programming environments where
is a 32-bit integer type and CLOCKS_PER_SEC is one million,
will start failing in less than 36 minutes of processor time for
or 72 minutes for unsigned
Applications intended to be portable to such environments should use
with CLOCK_PROCESS_CPUTIME_ID, if supported).
In order to measure the time spent in a program,
should be called at the start of the program and its return value
subtracted from the value returned by subsequent calls. The value
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 Base Definitions volume of POSIX.1-2017,
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
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