mirror of https://github.com/mkerrisk/man-pages
235 lines
6.6 KiB
Groff
235 lines
6.6 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
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.\"
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.\" Copyright (c) 2003 Nick Clifford (zaf@nrc.co.nz), Jan 25, 2003
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.\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl), Aug 24, 2003
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.\"
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of this
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.\" manual under the conditions for verbatim copying, provided that the
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.\" entire resulting derived work is distributed under the terms of a
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.\" permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume no
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.\" responsibility for errors or omissions, or for damages resulting from
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.\" the use of the information contained herein. The author(s) may not
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.\" have taken the same level of care in the production of this manual,
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.\" which is licensed free of charge, as they might when working
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\"
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.\" 2003-08-23 Martin Schulze <joey@infodrom.org> improvements
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.\" 2003-08-24 aeb, large parts rewritten
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.\" 2004-08-06 Christoph Lameter <clameter@sgi.com>, SMP note
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.\"
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.TH CLOCK_GETRES 2 2009-02-05 "" "Linux Programmer's Manual"
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.SH NAME
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clock_getres, clock_gettime, clock_settime \- clock and time functions
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.SH SYNOPSIS
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.B #include <time.h>
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.sp
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.BI "int clock_getres(clockid_t " clk_id ", struct timespec *" res );
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.BI "int clock_gettime(clockid_t " clk_id ", struct timespec *" tp );
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.BI "int clock_settime(clockid_t " clk_id ", const struct timespec *" tp );
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.sp
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Link with \fI\-lrt\fP.
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.sp
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.in -4n
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Feature Test Macro Requirements for glibc (see
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.BR feature_test_macros (7)):
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.in
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.sp
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.ad l
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.BR clock_getres (),
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.BR clock_gettime (),
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.BR clock_settime ():
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_POSIX_C_SOURCE\ >=\ 199309L
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.ad b
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.SH DESCRIPTION
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The function
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.BR clock_getres ()
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finds the resolution (precision) of the specified clock
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.IR clk_id ,
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and, if
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.I res
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is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
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.IR res .
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The resolution of clocks depends on the implementation and cannot be
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configured by a particular process.
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If the time value pointed to by the argument
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.I tp
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of
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.BR clock_settime ()
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is not a multiple of
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.IR res ,
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then it is truncated to a multiple of
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.IR res .
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.PP
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The functions
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.BR clock_gettime ()
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and
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.BR clock_settime ()
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retrieve and set the time of the specified clock
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.IR clk_id .
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.PP
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The
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.I res
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and
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.I tp
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arguments are
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.I timespec
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structures, as specified in
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.IR <time.h> :
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.sp
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.in +4n
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.nf
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struct timespec {
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time_t tv_sec; /* seconds */
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long tv_nsec; /* nanoseconds */
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};
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.fi
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.in
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.PP
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The
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.I clk_id
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argument is the identifier of the particular clock on which to act.
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A clock may be system-wide and hence visible for all processes, or
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per-process if it measures time only within a single process.
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.LP
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All implementations support the system-wide real-time clock,
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which is identified by
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.BR CLOCK_REALTIME .
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Its time represents seconds and nanoseconds since the Epoch.
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When its time is changed, timers for a relative interval are
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unaffected, but timers for an absolute point in time are affected.
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.LP
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More clocks may be implemented.
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The interpretation of the
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corresponding time values and the effect on timers is unspecified.
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.LP
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Sufficiently recent versions of glibc and the Linux kernel
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support the following clocks:
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.TP
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.B CLOCK_REALTIME
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System-wide real-time clock.
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Setting this clock requires appropriate privileges.
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.TP
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.B CLOCK_MONOTONIC
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Clock that cannot be set and represents monotonic time since
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some unspecified starting point.
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.TP
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.BR CLOCK_MONOTONIC_RAW " (since Linux 2.6.28; Linux-specific)"
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.\" Added in commit 2d42244ae71d6c7b0884b5664cf2eda30fb2ae68, John Stultz
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Similar to
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.BR CLOCK_MONOTONIC ,
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but provides access to a raw hardware-based time
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that is not subject to NTP adjustments.
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.TP
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.B CLOCK_PROCESS_CPUTIME_ID
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High-resolution per-process timer from the CPU.
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.TP
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.B CLOCK_THREAD_CPUTIME_ID
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Thread-specific CPU-time clock.
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.SH "RETURN VALUE"
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.BR clock_gettime (),
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.BR clock_settime ()
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and
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.BR clock_getres ()
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return 0 for success, or \-1 for failure (in which case
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.I errno
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is set appropriately).
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.SH ERRORS
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.TP
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.B EFAULT
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.I tp
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points outside the accessible address space.
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.TP
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.B EINVAL
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The
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.I clk_id
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specified is not supported on this system.
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.TP
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.B EPERM
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.BR clock_settime ()
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does not have permission to set the clock indicated.
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.SH "CONFORMING TO"
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SUSv2, POSIX.1-2001.
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.SH AVAILABILITY
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On POSIX systems on which these functions are available, the symbol
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.B _POSIX_TIMERS
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is defined in \fI<unistd.h>\fP to a value greater than 0.
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The symbols
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.BR _POSIX_MONOTONIC_CLOCK ,
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.BR _POSIX_CPUTIME ,
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.B _POSIX_THREAD_CPUTIME
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indicate that
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.BR CLOCK_MONOTONIC ,
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.BR CLOCK_PROCESS_CPUTIME_ID ,
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.B CLOCK_THREAD_CPUTIME_ID
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are available.
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(See also
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.BR sysconf (3).)
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.SH NOTES
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.SS Note for SMP systems
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The
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.B CLOCK_PROCESS_CPUTIME_ID
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and
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.B CLOCK_THREAD_CPUTIME_ID
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clocks are realized on many platforms using timers from the CPUs
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(TSC on i386, AR.ITC on Itanium).
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These registers may differ between CPUs and as a consequence
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these clocks may return
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.B bogus results
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if a process is migrated to another CPU.
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.PP
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If the CPUs in an SMP system have different clock sources then
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there is no way to maintain a correlation between the timer registers since
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each CPU will run at a slightly different frequency.
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If that is the case then
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.I clock_getcpuclockid(0)
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will return
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.B ENOENT
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to signify this condition.
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The two clocks will then only be useful if it
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can be ensured that a process stays on a certain CPU.
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.PP
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The processors in an SMP system do not start all at exactly the same
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time and therefore the timer registers are typically running at an offset.
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Some architectures include code that attempts to limit these offsets on bootup.
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However, the code cannot guarantee to accurately tune the offsets.
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Glibc contains no provisions to deal with these offsets (unlike the Linux
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Kernel).
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Typically these offsets are small and therefore the effects may be
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negligible in most cases.
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.SH BUGS
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According to POSIX.1-2001, the
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.B CLOCK_PROCESS_CPUTIME_ID
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and
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.B CLOCK_THREAD_CPUTIME_ID
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clocks should be settable using
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.BR clock_settime ().
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However, the clocks currently
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.\" Linux 2.6.28
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are not settable.
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.\" FIXME . Track the following bug report
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.\" http://bugzilla.kernel.org/show_bug.cgi?id=11972
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.SH "SEE ALSO"
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.BR date (1),
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.BR adjtimex (2),
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.BR gettimeofday (2),
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.BR settimeofday (2),
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.BR time (2),
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.BR clock_getcpuclockid (3),
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.BR ctime (3),
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.BR ftime (3),
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.BR pthread_getcpuclockid (3),
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.BR sysconf (3),
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.BR time (7)
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