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():
18 Since glibc 2.19:
19 _DEFAULT_SOURCE
20 Glibc 2.19 and earlier:
21 _BSD_SOURCE
22
24 The functions gettimeofday() and settimeofday() can get and set the
25 time as well as a timezone. The tv argument is a struct timeval (as
26 specified in <sys/time.h>):
27 struct timeval {
29 time_t tv_sec; /* seconds */
30 suseconds_t tv_usec; /* microseconds */
31 };
32 and gives the number of seconds and microseconds since the Epoch (see
34 time(2)). The tz argument is a struct timezone:
35 struct timezone {
37 int tz_minuteswest; /* minutes west of Greenwich */
38 int tz_dsttime; /* type of DST correction */
39 };
40 If either tv or tz is NULL, the corresponding structure is not set or
42 returned. (However, compilation warnings will result if tv is NULL.)
43 The use of the timezone structure is obsolete; the tz argument should
45 normally be specified as NULL. (See NOTES below.)
46 Under Linux, there are some peculiar "warp clock" semantics associated
48 with the settimeofday() system call if on the very first call (after
49 booting) that has a non-NULL tz argument, the tv argument is NULL and
50 the tz_minuteswest field is nonzero. (The tz_dsttime field should be
51 zero for this case.) In such a case it is assumed that the CMOS clock
52 is on local time, and that it has to be incremented by this amount to
53 get UTC system time. No doubt it is a bad idea to use this feature.
54
56 gettimeofday() and settimeofday() return 0 for success, or -1 for fail‐
57 ure (in which case errno is set appropriately).
58
60 EFAULT One of tv or tz pointed outside the accessible address space.
61 EINVAL Timezone (or something else) is invalid.
63 EPERM The calling process has insufficient privilege to call settime‐
65 ofday(); under Linux the CAP_SYS_TIME capability is required.
66
68 SVr4, 4.3BSD. POSIX.1-2001 describes gettimeofday() but not settimeof‐
69 day(). POSIX.1-2008 marks gettimeofday() as obsolete, recommending the
70 use of clock_gettime(2) instead.
71
73 The time returned by gettimeofday() is affected by discontinuous jumps
74 in the system time (e.g., if the system administrator manually changes
75 the system time). If you need a monotonically increasing clock, see
76 clock_gettime(2).
77 Macros for operating on timeval structures are described in timer‐
79 add(3).
80 Traditionally, the fields of struct timeval were of type long.
82 C library/kernel differences
84 On some architectures, an implementation of gettimeofday() is provided
85 in the vdso(7).
86 The tz_dsttime field
88 On a non-Linux kernel, with glibc, the tz_dsttime field of struct time‐
89 zone will be set to a nonzero value by gettimeofday() if the current
90 timezone has ever had or will have a daylight saving rule applied. In
91 this sense it exactly mirrors the meaning of daylight(3) for the cur‐
92 rent zone. On Linux, with glibc, the setting of the tz_dsttime field
93 of struct timezone has never been used by settimeofday() or gettimeof‐
94 day(). Thus, the following is purely of historical interest.
95 On old systems, the field tz_dsttime contains a symbolic constant (val‐
97 ues are given below) that indicates in which part of the year Daylight
98 Saving Time is in force. (Note: this value is constant throughout the
99 year: it does not indicate that DST is in force, it just selects an
100 algorithm.) The daylight saving time algorithms defined are as fol‐
101 lows:
102 DST_NONE /* not on DST */
104 DST_USA /* USA style DST */
105 DST_AUST /* Australian style DST */
106 DST_WET /* Western European DST */
107 DST_MET /* Middle European DST */
108 DST_EET /* Eastern European DST */
109 DST_CAN /* Canada */
110 DST_GB /* Great Britain and Eire */
111 DST_RUM /* Romania */
112 DST_TUR /* Turkey */
113 DST_AUSTALT /* Australian style with shift in 1986 */
114 Of course it turned out that the period in which Daylight Saving Time
116 is in force cannot be given by a simple algorithm, one per country;
117 indeed, this period is determined by unpredictable political decisions.
118 So this method of representing timezones has been abandoned.
119
121 date(1), adjtimex(2), clock_gettime(2), time(2), ctime(3), ftime(3),
122 timeradd(3), capabilities(7), time(7), vdso(7), hwclock(8)
123
125 This page is part of release 4.15 of the Linux man-pages project. A
126 description of the project, information about reporting bugs, and the
127 latest version of this page, can be found at
128 https://www.kernel.org/doc/man-pages/.
129Linux 2017-09-15 GETTIMEOFDAY(2)