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
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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
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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16 .\" manual page may be incorrect or out-of-date.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
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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 .TH CLOCK_GETRES 2 2010-02-03 "" "Linux Programmer's Manual"
32 clock_getres, clock_gettime, clock_settime \- clock and time functions
36 .BI "int clock_getres(clockid_t " clk_id ", struct timespec *" res );
38 .BI "int clock_gettime(clockid_t " clk_id ", struct timespec *" tp );
40 .BI "int clock_settime(clockid_t " clk_id ", const struct timespec *" tp );
42 Link with \fI\-lrt\fP.
45 Feature Test Macro Requirements for glibc (see
46 .BR feature_test_macros (7)):
54 _POSIX_C_SOURCE\ >=\ 199309L
60 finds the resolution (precision) of the specified clock
64 is non-NULL, stores it in the \fIstruct timespec\fP pointed to by
66 The resolution of clocks depends on the implementation and cannot be
67 configured by a particular process.
68 If the time value pointed to by the argument
74 then it is truncated to a multiple of
81 retrieve and set the time of the specified clock
90 structures, as specified in
96 time_t tv_sec; /* seconds */
97 long tv_nsec; /* nanoseconds */
104 argument is the identifier of the particular clock on which to act.
105 A clock may be system-wide and hence visible for all processes, or
106 per-process if it measures time only within a single process.
108 All implementations support the system-wide real-time clock,
109 which is identified by
111 Its time represents seconds and nanoseconds since the Epoch.
112 When its time is changed, timers for a relative interval are
113 unaffected, but timers for an absolute point in time are affected.
115 More clocks may be implemented.
116 The interpretation of the
117 corresponding time values and the effect on timers is unspecified.
119 Sufficiently recent versions of glibc and the Linux kernel
120 support the following clocks:
123 System-wide real-time clock.
124 Setting this clock requires appropriate privileges.
127 Clock that cannot be set and represents monotonic time since
128 some unspecified starting point.
130 .BR CLOCK_MONOTONIC_RAW " (since Linux 2.6.28; Linux-specific)"
131 .\" Added in commit 2d42244ae71d6c7b0884b5664cf2eda30fb2ae68, John Stultz
133 .BR CLOCK_MONOTONIC ,
134 but provides access to a raw hardware-based time
135 that is not subject to NTP adjustments.
137 .B CLOCK_PROCESS_CPUTIME_ID
138 High-resolution per-process timer from the CPU.
140 .B CLOCK_THREAD_CPUTIME_ID
141 Thread-specific CPU-time clock.
143 .BR clock_gettime (),
147 return 0 for success, or \-1 for failure (in which case
149 is set appropriately).
154 points outside the accessible address space.
159 specified is not supported on this system.
160 .\" Linux also gives this error on attempts to set CLOCK_PROCESS_CPUTIME_ID
161 .\" and CLOCK_THREAD_CPUTIME_ID, when probably the proper error should be
166 does not have permission to set the clock indicated.
170 On POSIX systems on which these functions are available, the symbol
172 is defined in \fI<unistd.h>\fP to a value greater than 0.
174 .BR _POSIX_MONOTONIC_CLOCK ,
176 .B _POSIX_THREAD_CPUTIME
178 .BR CLOCK_MONOTONIC ,
179 .BR CLOCK_PROCESS_CPUTIME_ID ,
180 .B CLOCK_THREAD_CPUTIME_ID
185 .SS Note for SMP systems
187 .B CLOCK_PROCESS_CPUTIME_ID
189 .B CLOCK_THREAD_CPUTIME_ID
190 clocks are realized on many platforms using timers from the CPUs
191 (TSC on i386, AR.ITC on Itanium).
192 These registers may differ between CPUs and as a consequence
193 these clocks may return
195 if a process is migrated to another CPU.
197 If the CPUs in an SMP system have different clock sources then
198 there is no way to maintain a correlation between the timer registers since
199 each CPU will run at a slightly different frequency.
200 If that is the case then
201 .I clock_getcpuclockid(0)
204 to signify this condition.
205 The two clocks will then only be useful if it
206 can be ensured that a process stays on a certain CPU.
208 The processors in an SMP system do not start all at exactly the same
209 time and therefore the timer registers are typically running at an offset.
210 Some architectures include code that attempts to limit these offsets on bootup.
211 However, the code cannot guarantee to accurately tune the offsets.
212 Glibc contains no provisions to deal with these offsets (unlike the Linux
214 Typically these offsets are small and therefore the effects may be
215 negligible in most cases.
217 According to POSIX.1-2001, a process with "appropriate privileges" may set the
218 .B CLOCK_PROCESS_CPUTIME_ID
220 .B CLOCK_THREAD_CPUTIME_ID
222 .BR clock_settime ().
223 On Linux, these clocks are not settable
224 (i.e., no process has "appropriate privileges").
225 .\" See http://bugzilla.kernel.org/show_bug.cgi?id=11972
229 .BR gettimeofday (2),
230 .BR settimeofday (2),
232 .BR clock_getcpuclockid (3),
235 .BR pthread_getcpuclockid (3),