clock.3p (3539B)
- '\" et
- .TH CLOCK "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH 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.
- .\"
- .SH NAME
- clock
- \(em report CPU time used
- .SH SYNOPSIS
- .LP
- .nf
- #include <time.h>
- .P
- clock_t clock(void);
- .fi
- .SH DESCRIPTION
- 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\(hy2017 defers to the ISO\ C standard.
- .P
- The
- \fIclock\fR()
- 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.
- .SH "RETURN VALUE"
- To determine the time in seconds, the value returned by
- \fIclock\fR()
- should be divided by the value of the macro CLOCKS_PER_SEC.
- CLOCKS_PER_SEC is defined to be one million in
- .IR <time.h> .
- If the processor time used is not available or its value cannot be
- represented, the function shall return the value (\c
- .BR clock_t )\-1.
- .SH ERRORS
- No errors are defined.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- None.
- .SH "APPLICATION USAGE"
- In programming environments where
- .BR clock_t
- is a 32-bit integer type and CLOCKS_PER_SEC is one million,
- \fIclock\fR()
- will start failing in less than 36 minutes of processor time for
- signed
- .BR clock_t ,
- or 72 minutes for unsigned
- .BR clock_t .
- Applications intended to be portable to such environments should use
- \fItimes\fR()
- instead (or
- \fIclock_gettime\fR()
- with CLOCK_PROCESS_CPUTIME_ID, if supported).
- .P
- In order to measure the time spent in a program,
- \fIclock\fR()
- 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
- \fIclock\fR()
- is defined for compatibility across systems that have clocks with
- different resolutions. The resolution on any particular system need
- not be to microsecond accuracy.
- .SH RATIONALE
- None.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- .IR "\fIasctime\fR\^(\|)",
- .IR "\fIclock_getres\fR\^(\|)",
- .IR "\fIctime\fR\^(\|)",
- .IR "\fIdifftime\fR\^(\|)",
- .IR "\fIgmtime\fR\^(\|)",
- .IR "\fIlocaltime\fR\^(\|)",
- .IR "\fImktime\fR\^(\|)",
- .IR "\fIstrftime\fR\^(\|)",
- .IR "\fIstrptime\fR\^(\|)",
- .IR "\fItime\fR\^(\|)",
- .IR "\fItimes\fR\^(\|)",
- .IR "\fIutime\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "\fB<time.h>\fP"
- .\"
- .SH COPYRIGHT
- 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
- http://www.opengroup.org/unix/online.html .
- .PP
- 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.kernel.org/doc/man-pages/reporting_bugs.html .