1 .\" Hey Emacs! This file is -*- nroff -*- source.
3 .\" Copyright (c) 2003 Nick Clifford (zaf@nrc.co.nz), Jan 25, 2003
4 .\" Copyright (c) 2003 Andries Brouwer (aeb@cwi.nl), Aug 24, 2003
6 .\" Permission is granted to make and distribute verbatim copies of this
7 .\" manual provided the copyright notice and this permission notice are
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10 .\" Permission is granted to copy and distribute modified versions of this
11 .\" manual under the conditions for verbatim copying, provided that the
12 .\" entire resulting derived work is distributed under the terms of a
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15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
17 .\" responsibility for errors or omissions, or for damages resulting from
18 .\" the use of the information contained herein. The author(s) may not
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20 .\" which is licensed free of charge, as they might when working
23 .\" Formatted or processed versions of this manual, if unaccompanied by
24 .\" the source, must acknowledge the copyright and authors of this work.
26 .\" 2003-08-23 Martin Schulze <joey@infodrom.org> improvements
27 .\" 2003-08-24 aeb, large parts rewritten
28 .\" 2004-08-06 Christoph Lameter <clameter@sgi.com>, SMP note
30 .\" FIXME: Linux 2.6.39 adds CLOCK_BOOTTIME
32 .TH CLOCK_GETRES 2 2010-02-03 "" "Linux Programmer's Manual"
34 clock_getres, clock_gettime, clock_settime \- clock and time functions
38 .BI "int clock_getres(clockid_t " clk_id ", struct timespec *" res );
40 .BI "int clock_gettime(clockid_t " clk_id ", struct timespec *" tp );
42 .BI "int clock_settime(clockid_t " clk_id ", const struct timespec *" tp );
44 Link with \fI\-lrt\fP.
47 Feature Test Macro Requirements for glibc (see
48 .BR feature_test_macros (7)):
56 _POSIX_C_SOURCE\ >=\ 199309L
62 finds the resolution (precision) of the specified clock
66 is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
68 The resolution of clocks depends on the implementation and cannot be
69 configured by a particular process.
70 If the time value pointed to by the argument
76 then it is truncated to a multiple of
83 retrieve and set the time of the specified clock
92 structures, as specified in
98 time_t tv_sec; /* seconds */
99 long tv_nsec; /* nanoseconds */
106 argument is the identifier of the particular clock on which to act.
107 A clock may be system-wide and hence visible for all processes, or
108 per-process if it measures time only within a single process.
110 All implementations support the system-wide real-time clock,
111 which is identified by
113 Its time represents seconds and nanoseconds since the Epoch.
114 When its time is changed, timers for a relative interval are
115 unaffected, but timers for an absolute point in time are affected.
117 More clocks may be implemented.
118 The interpretation of the
119 corresponding time values and the effect on timers is unspecified.
121 Sufficiently recent versions of glibc and the Linux kernel
122 support the following clocks:
125 System-wide real-time clock.
126 Setting this clock requires appropriate privileges.
129 Clock that cannot be set and represents monotonic time since
130 some unspecified starting point.
132 .BR CLOCK_MONOTONIC_RAW " (since Linux 2.6.28; Linux-specific)"
133 .\" Added in commit 2d42244ae71d6c7b0884b5664cf2eda30fb2ae68, John Stultz
135 .BR CLOCK_MONOTONIC ,
136 but provides access to a raw hardware-based time
137 that is not subject to NTP adjustments.
139 .B CLOCK_PROCESS_CPUTIME_ID
140 High-resolution per-process timer from the CPU.
142 .B CLOCK_THREAD_CPUTIME_ID
143 Thread-specific CPU-time clock.
145 .BR clock_gettime (),
149 return 0 for success, or \-1 for failure (in which case
151 is set appropriately).
156 points outside the accessible address space.
161 specified is not supported on this system.
162 .\" Linux also gives this error on attempts to set CLOCK_PROCESS_CPUTIME_ID
163 .\" and CLOCK_THREAD_CPUTIME_ID, when probably the proper error should be
168 does not have permission to set the clock indicated.
172 On POSIX systems on which these functions are available, the symbol
174 is defined in \fI<unistd.h>\fP to a value greater than 0.
176 .BR _POSIX_MONOTONIC_CLOCK ,
178 .B _POSIX_THREAD_CPUTIME
180 .BR CLOCK_MONOTONIC ,
181 .BR CLOCK_PROCESS_CPUTIME_ID ,
182 .B CLOCK_THREAD_CPUTIME_ID
187 .SS Note for SMP systems
189 .B CLOCK_PROCESS_CPUTIME_ID
191 .B CLOCK_THREAD_CPUTIME_ID
192 clocks are realized on many platforms using timers from the CPUs
193 (TSC on i386, AR.ITC on Itanium).
194 These registers may differ between CPUs and as a consequence
195 these clocks may return
197 if a process is migrated to another CPU.
199 If the CPUs in an SMP system have different clock sources then
200 there is no way to maintain a correlation between the timer registers since
201 each CPU will run at a slightly different frequency.
202 If that is the case then
203 .I clock_getcpuclockid(0)
206 to signify this condition.
207 The two clocks will then only be useful if it
208 can be ensured that a process stays on a certain CPU.
210 The processors in an SMP system do not start all at exactly the same
211 time and therefore the timer registers are typically running at an offset.
212 Some architectures include code that attempts to limit these offsets on bootup.
213 However, the code cannot guarantee to accurately tune the offsets.
214 Glibc contains no provisions to deal with these offsets (unlike the Linux
216 Typically these offsets are small and therefore the effects may be
217 negligible in most cases.
219 According to POSIX.1-2001, a process with "appropriate privileges" may set the
220 .B CLOCK_PROCESS_CPUTIME_ID
222 .B CLOCK_THREAD_CPUTIME_ID
224 .BR clock_settime ().
225 On Linux, these clocks are not settable
226 (i.e., no process has "appropriate privileges").
227 .\" See http://bugzilla.kernel.org/show_bug.cgi?id=11972
231 .BR gettimeofday (2),
232 .BR settimeofday (2),
234 .BR clock_getcpuclockid (3),
237 .BR pthread_getcpuclockid (3),