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1 | .\" Hey Emacs! This file is -*- nroff -*- source. |
2 | .\" | |
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 | |
5 | .\" | |
6 | .\" Permission is granted to make and distribute verbatim copies of this | |
7 | .\" manual provided the copyright notice and this permission notice are | |
8 | .\" preserved on all copies. | |
9 | .\" | |
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 | |
13 | .\" permission notice identical to this one. | |
c13182ef | 14 | .\" |
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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16 | .\" manual page may be incorrect or out-of-date.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 | |
19 | .\" have taken the same level of care in the production of this manual, | |
20 | .\" which is licensed free of charge, as they might when working | |
21 | .\" professionally. | |
c13182ef | 22 | .\" |
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23 | .\" Formatted or processed versions of this manual, if unaccompanied by |
24 | .\" the source, must acknowledge the copyright and authors of this work. | |
25 | .\" | |
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 | |
29 | .\" | |
3847be78 | 30 | .TH CLOCK_GETRES 3 2008-11-07 "" "Linux Programmer's Manual" |
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31 | .SH NAME |
32 | clock_getres, clock_gettime, clock_settime \- clock and time functions | |
33 | .SH SYNOPSIS | |
34 | .B #include <time.h> | |
35 | .sp | |
36 | .BI "int clock_getres(clockid_t " clk_id ", struct timespec *" res ); | |
5895e7eb | 37 | |
fea681da | 38 | .BI "int clock_gettime(clockid_t " clk_id ", struct timespec *" tp ); |
5895e7eb | 39 | |
fea681da | 40 | .BI "int clock_settime(clockid_t " clk_id ", const struct timespec *" tp ); |
cc4615cc | 41 | .sp |
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42 | Link with \fI\-lrt\fP. |
43 | .sp | |
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44 | .in -4n |
45 | Feature Test Macro Requirements for glibc (see | |
46 | .BR feature_test_macros (7)): | |
47 | .in | |
48 | .sp | |
49 | .ad l | |
50 | .BR clock_getres (), | |
51 | .BR clock_gettime (), | |
52 | .BR clock_settime (): | |
53 | _POSIX_C_SOURCE\ >=\ 199309L | |
54 | .ad b | |
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55 | .SH DESCRIPTION |
56 | The function | |
57 | .BR clock_getres () | |
58 | finds the resolution (precision) of the specified clock | |
59 | .IR clk_id , | |
60 | and, if | |
61 | .I res | |
0c2ec4f1 | 62 | is non-NULL, stores it in the \fIstruct timespec\fP pointed to by |
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63 | .IR res . |
64 | The resolution of clocks depends on the implementation and cannot be | |
65 | configured by a particular process. | |
66 | If the time value pointed to by the argument | |
67 | .I tp | |
68 | of | |
69 | .BR clock_settime () | |
70 | is not a multiple of | |
71 | .IR res , | |
72 | then it is truncated to a multiple of | |
73 | .IR res . | |
74 | .PP | |
75 | The functions | |
76 | .BR clock_gettime () | |
77 | and | |
78 | .BR clock_settime () | |
79 | retrieve and set the time of the specified clock | |
80 | .IR clk_id . | |
81 | .PP | |
82 | The | |
83 | .I res | |
84 | and | |
85 | .I tp | |
86 | arguments are | |
f19a0f03 | 87 | .I timespec |
a6915393 | 88 | structures, as specified in |
c13182ef | 89 | .IR <time.h> : |
fea681da | 90 | .sp |
3ad4ddcd | 91 | .in +4n |
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92 | .nf |
93 | struct timespec { | |
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94 | time_t tv_sec; /* seconds */ |
95 | long tv_nsec; /* nanoseconds */ | |
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96 | }; |
97 | .fi | |
3ad4ddcd | 98 | .in |
fea681da | 99 | .PP |
c13182ef | 100 | The |
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101 | .I clk_id |
102 | argument is the identifier of the particular clock on which to act. | |
103 | A clock may be system-wide and hence visible for all processes, or | |
104 | per-process if it measures time only within a single process. | |
105 | .LP | |
6f36deb4 | 106 | All implementations support the system-wide real-time clock, |
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107 | which is identified by |
108 | .BR CLOCK_REALTIME . | |
109 | Its time represents seconds and nanoseconds since the Epoch. | |
110 | When its time is changed, timers for a relative interval are | |
111 | unaffected, but timers for an absolute point in time are affected. | |
112 | .LP | |
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113 | More clocks may be implemented. |
114 | The interpretation of the | |
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115 | corresponding time values and the effect on timers is unspecified. |
116 | .LP | |
5260fe08 | 117 | Sufficiently recent versions of glibc and the Linux kernel |
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118 | support the following clocks: |
119 | .TP | |
120 | .B CLOCK_REALTIME | |
6f36deb4 | 121 | System-wide real-time clock. |
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122 | Setting this clock requires appropriate privileges. |
123 | .TP | |
124 | .B CLOCK_MONOTONIC | |
125 | Clock that cannot be set and represents monotonic time since | |
126 | some unspecified starting point. | |
127 | .TP | |
128 | .B CLOCK_PROCESS_CPUTIME_ID | |
129 | High-resolution per-process timer from the CPU. | |
130 | .TP | |
131 | .B CLOCK_THREAD_CPUTIME_ID | |
132 | Thread-specific CPU-time clock. | |
133 | .SH "RETURN VALUE" | |
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134 | .BR clock_gettime (), |
135 | .BR clock_settime () | |
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136 | and |
137 | .BR clock_getres () | |
138 | return 0 for success, or \-1 for failure (in which case | |
139 | .I errno | |
140 | is set appropriately). | |
141 | .SH ERRORS | |
142 | .TP | |
143 | .B EFAULT | |
144 | .I tp | |
145 | points outside the accessible address space. | |
146 | .TP | |
147 | .B EINVAL | |
c13182ef | 148 | The |
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149 | .I clk_id |
150 | specified is not supported on this system. | |
151 | .TP | |
152 | .B EPERM | |
153 | .BR clock_settime () | |
154 | does not have permission to set the clock indicated. | |
2b2581ee MK |
155 | .SH "CONFORMING TO" |
156 | SUSv2, POSIX.1-2001. | |
157 | .SH AVAILABILITY | |
158 | On POSIX systems on which these functions are available, the symbol | |
159 | .B _POSIX_TIMERS | |
c84371c6 | 160 | is defined in \fI<unistd.h>\fP to a value greater than 0. |
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161 | The symbols |
162 | .BR _POSIX_MONOTONIC_CLOCK , | |
163 | .BR _POSIX_CPUTIME , | |
164 | .B _POSIX_THREAD_CPUTIME | |
165 | indicate that | |
166 | .BR CLOCK_MONOTONIC , | |
167 | .BR CLOCK_PROCESS_CPUTIME_ID , | |
168 | .B CLOCK_THREAD_CPUTIME_ID | |
169 | are available. | |
170 | (See also | |
171 | .BR sysconf (3).) | |
19c98696 | 172 | .SH NOTES |
19c98696 | 173 | .SS Note for SMP systems |
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174 | The |
175 | .B CLOCK_PROCESS_CPUTIME_ID | |
176 | and | |
177 | .B CLOCK_THREAD_CPUTIME_ID | |
178 | clocks are realized on many platforms using timers from the CPUs | |
179 | (TSC on i386, AR.ITC on Itanium). | |
180 | These registers may differ between CPUs and as a consequence | |
181 | these clocks may return | |
182 | .B bogus results | |
183 | if a process is migrated to another CPU. | |
184 | .PP | |
185 | If the CPUs in an SMP system have different clock sources then | |
186 | there is no way to maintain a correlation between the timer registers since | |
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187 | each CPU will run at a slightly different frequency. |
188 | If that is the case then | |
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189 | .I clock_getcpuclockid(0) |
190 | will return | |
191 | .B ENOENT | |
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192 | to signify this condition. |
193 | The two clocks will then only be useful if it | |
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194 | can be ensured that a process stays on a certain CPU. |
195 | .PP | |
196 | The processors in an SMP system do not start all at exactly the same | |
197 | time and therefore the timer registers are typically running at an offset. | |
198 | Some architectures include code that attempts to limit these offsets on bootup. | |
199 | However, the code cannot guarantee to accurately tune the offsets. | |
200 | Glibc contains no provisions to deal with these offsets (unlike the Linux | |
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201 | Kernel). |
202 | Typically these offsets are small and therefore the effects may be | |
fea681da | 203 | negligible in most cases. |
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204 | .SH BUGS |
205 | According to POSIX.1-2001, the | |
206 | .B CLOCK_PROCESS_CPUTIME_ID | |
207 | and | |
208 | .B CLOCK_THREAD_CPUTIME_ID | |
209 | clocks should be settable using | |
210 | .BR clock_settime (). | |
211 | However, the clocks currently | |
212 | .\" Linux 2.6.28 | |
213 | are not settable. | |
b3f72461 MK |
214 | .\" FIXME . Track the following bug report |
215 | .\" http://bugzilla.kernel.org/show_bug.cgi?id=11972 | |
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216 | .SH "SEE ALSO" |
217 | .BR date (1), | |
218 | .BR adjtimex (2), | |
219 | .BR gettimeofday (2), | |
220 | .BR settimeofday (2), | |
221 | .BR time (2), | |
edf60acb | 222 | .BR clock_getcpuclockid (3), |
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223 | .BR ctime (3), |
224 | .BR ftime (3), | |
24ad017d | 225 | .BR pthread_getcpuclockid (3), |
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226 | .BR sysconf (3), |
227 | .BR time (7) |