1 .\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
3 .\" %%%LICENSE_START(VERBATIM)
4 .\" Permission is granted to make and distribute verbatim copies of this
5 .\" manual provided the copyright notice and this permission notice are
6 .\" preserved on all copies.
8 .\" Permission is granted to copy and distribute modified versions of this
9 .\" manual under the conditions for verbatim copying, provided that the
10 .\" entire resulting derived work is distributed under the terms of a
11 .\" permission notice identical to this one.
13 .\" Since the Linux kernel and libraries are constantly changing, this
14 .\" manual page may be incorrect or out-of-date. The author(s) assume no
15 .\" responsibility for errors or omissions, or for damages resulting from
16 .\" the use of the information contained herein. The author(s) may not
17 .\" have taken the same level of care in the production of this manual,
18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
22 .\" the source, must acknowledge the copyright and authors of this work.
25 .\" Modified by Michael Haardt (michael@moria.de)
26 .\" Modified 1993-07-23 by Rik Faith (faith@cs.unc.edu)
27 .\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
28 .\" Fixed necessary '#include' lines.
29 .\" Modified 1995-04-15 by Michael Chastain (mec@shell.portal.com):
30 .\" Added reference to adjtimex.
31 .\" Removed some nonsense lines pointed out by Urs Thuermann,
32 .\" (urs@isnogud.escape.de), aeb, 950722.
33 .\" Modified 1997-01-14 by Austin Donnelly (and1000@debian.org):
34 .\" Added return values section, and bit on EFAULT
35 .\" Added clarification on timezone, aeb, 971210.
36 .\" Removed "#include <unistd.h>", aeb, 010316.
37 .\" Modified, 2004-05-27 by Michael Kerrisk <mtk.manpages@gmail.com>
38 .\" Added notes on capability requirement.
40 .TH GETTIMEOFDAY 2 2021-03-22 "Linux" "Linux Programmer's Manual"
42 gettimeofday, settimeofday \- get / set time
45 .B #include <sys/time.h>
47 .BI "int gettimeofday(struct timeval *restrict " tv ,
48 .BI " struct timezone *restrict " tz );
49 .BI "int settimeofday(const struct timeval *" tv ,
50 .BI " const struct timezone *" tz );
54 Feature Test Macro Requirements for glibc (see
55 .BR feature_test_macros (7)):
62 Glibc 2.19 and earlier:
70 can get and set the time as well as a timezone.
82 time_t tv_sec; /* seconds */
83 suseconds_t tv_usec; /* microseconds */
88 and gives the number of seconds and microseconds since the Epoch (see
94 .IR "struct timezone" :
99 int tz_minuteswest; /* minutes west of Greenwich */
100 int tz_dsttime; /* type of DST correction */
109 is NULL, the corresponding structure is not set or returned.
110 .\" FIXME . The compilation warning looks to be going away in 2.17
111 .\" see glibc commit 4b7634a5e03b0da6f8875de9d3f74c1cf6f2a6e8
112 (However, compilation warnings will result if
115 .\" The following is covered under EPERM below:
117 .\" Only the superuser may use
118 .\" .BR settimeofday ().
122 structure is obsolete; the
124 argument should normally be specified as NULL.
127 Under Linux, there are some peculiar "warp clock" semantics associated
130 system call if on the very first call (after booting)
135 argument is NULL and the
140 field should be zero for this case.)
141 In such a case it is assumed that the CMOS clock
142 is on local time, and that it has to be incremented by this amount
143 to get UTC system time.
144 No doubt it is a bad idea to use this feature.
149 return 0 for success.
150 On error, \-1 is returned and
152 is set to indicate the error.
160 pointed outside the accessible address space.
163 .RB ( settimeofday ()):
168 .RB ( settimeofday ()):
172 is outside the range [0..999,999].
174 .BR EINVAL " (since Linux 4.3)"
175 .\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
176 .RB ( settimeofday ()):
177 An attempt was made to set the time to a value less than
178 the current value of the
181 .BR clock_gettime (2)).
184 The calling process has insufficient privilege to call
188 capability is required.
191 POSIX.1-2001 describes
197 as obsolete, recommending the use of
198 .BR clock_gettime (2)
204 affected by discontinuous jumps in the system time
205 (e.g., if the system administrator manually changes the system time).
206 If you need a monotonically increasing clock, see
207 .BR clock_gettime (2).
209 Macros for operating on
211 structures are described in
214 Traditionally, the fields of
219 .SS C library/kernel differences
220 On some architectures, an implementation of
225 .SS The tz_dsttime field
226 On a non-Linux kernel, with glibc, the
230 will be set to a nonzero value by
232 if the current timezone has ever had or will have a daylight saving
234 In this sense it exactly mirrors the meaning of
236 for the current zone.
237 On Linux, with glibc, the setting of the
241 has never been used by
246 .\" been and will not be supported by libc or glibc.
247 .\" Each and every occurrence of this field in the kernel source
248 .\" (other than the declaration) is a bug.
249 Thus, the following is purely of historical interest.
251 On old systems, the field
253 contains a symbolic constant (values are given below)
254 that indicates in which part of the year Daylight Saving Time
256 (Note: this value is constant throughout the year:
257 it does not indicate that DST is in force, it just selects an
259 The daylight saving time algorithms defined are as follows:
263 \fBDST_NONE\fP /* not on DST */
264 \fBDST_USA\fP /* USA style DST */
265 \fBDST_AUST\fP /* Australian style DST */
266 \fBDST_WET\fP /* Western European DST */
267 \fBDST_MET\fP /* Middle European DST */
268 \fBDST_EET\fP /* Eastern European DST */
269 \fBDST_CAN\fP /* Canada */
270 \fBDST_GB\fP /* Great Britain and Eire */
271 \fBDST_RUM\fP /* Romania */
272 \fBDST_TUR\fP /* Turkey */
273 \fBDST_AUSTALT\fP /* Australian style with shift in 1986 */
277 Of course it turned out that the period in which
278 Daylight Saving Time is in force cannot be given
279 by a simple algorithm, one per country; indeed,
280 this period is determined by unpredictable political
282 So this method of representing timezones
287 .BR clock_gettime (2),
292 .BR capabilities (7),