1GETTIMEOFDAY(2) Linux Programmer's Manual GETTIMEOFDAY(2)
2
6 gettimeofday, settimeofday - get / set time
7
9 #include <sys/time.h>
10 int gettimeofday(struct timeval *tv, struct timezone *tz);
12 int settimeofday(const struct timeval *tv, const struct timezone *tz);
14 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
16 settimeofday(): _BSD_SOURCE
18
20 The functions gettimeofday() and settimeofday() can get and set the
21 time as well as a timezone. The tv argument is a struct timeval (as
22 specified in <sys/time.h>):
23 struct timeval {
25 time_t tv_sec; /* seconds */
26 suseconds_t tv_usec; /* microseconds */
27 };
28 and gives the number of seconds and microseconds since the Epoch (see
30 time(2)). The tz argument is a struct timezone:
31 struct timezone {
33 int tz_minuteswest; /* minutes west of Greenwich */
34 int tz_dsttime; /* type of DST correction */
35 };
36 If either tv or tz is NULL, the corresponding structure is not set or
38 returned. (However, compilation warnings will result if tv is NULL.)
39 The use of the timezone structure is obsolete; the tz argument should
41 normally be specified as NULL. (See NOTES below.)
42 Under Linux there are some peculiar "warp clock" semantics associated
44 with the settimeofday() system call if on the very first call (after
45 booting) that has a non-NULL tz argument, the tv argument is NULL and
46 the tz_minuteswest field is nonzero. (The tz_dsttime field should be
47 zero for this case.) In such a case it is assumed that the CMOS clock
48 is on local time, and that it has to be incremented by this amount to
49 get UTC system time. No doubt it is a bad idea to use this feature.
50
52 gettimeofday() and settimeofday() return 0 for success, or -1 for fail‐
53 ure (in which case errno is set appropriately).
54
56 EFAULT One of tv or tz pointed outside the accessible address space.
57 EINVAL Timezone (or something else) is invalid.
59 EPERM The calling process has insufficient privilege to call settime‐
61 ofday(); under Linux the CAP_SYS_TIME capability is required.
62
64 SVr4, 4.3BSD. POSIX.1-2001 describes gettimeofday() but not settimeof‐
65 day(). POSIX.1-2008 marks gettimeofday() as obsolete, recommending the
66 use of clock_gettime(2) instead.
67
69 The time returned by gettimeofday() is affected by discontinuous jumps
70 in the system time (e.g., if the system administrator manually changes
71 the system time). If you need a monotonically increasing clock, see
72 clock_gettime(2).
73 Macros for operating on timeval structures are described in timer‐
75 add(3).
76 Traditionally, the fields of struct timeval were of type long.
78 The tz_dsttime field has never been used under Linux. Thus, the fol‐
80 lowing is purely of historic interest.
81 On old systems, the field tz_dsttime contains a symbolic constant (val‐
83 ues are given below) that indicates in which part of the year Daylight
84 Saving Time is in force. (Note: this value is constant throughout the
85 year: it does not indicate that DST is in force, it just selects an
86 algorithm.) The daylight saving time algorithms defined are as fol‐
87 lows:
88 DST_NONE /* not on DST */
90 DST_USA /* USA style DST */
91 DST_AUST /* Australian style DST */
92 DST_WET /* Western European DST */
93 DST_MET /* Middle European DST */
94 DST_EET /* Eastern European DST */
95 DST_CAN /* Canada */
96 DST_GB /* Great Britain and Eire */
97 DST_RUM /* Romania */
98 DST_TUR /* Turkey */
99 DST_AUSTALT /* Australian style with shift in 1986 */
100 Of course it turned out that the period in which Daylight Saving Time
102 is in force cannot be given by a simple algorithm, one per country;
103 indeed, this period is determined by unpredictable political decisions.
104 So this method of representing timezones has been abandoned.
105
107 date(1), adjtimex(2), clock_gettime(2), time(2), ctime(3), ftime(3),
108 timeradd(3), capabilities(7), time(7)
109
111 This page is part of release 3.53 of the Linux man-pages project. A
112 description of the project, and information about reporting bugs, can
113 be found at http://www.kernel.org/doc/man-pages/.
114Linux 2012-04-26 GETTIMEOFDAY(2)