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.
39 The use of the timezone structure is obsolete; the tz argument should
41 normally be specified as NULL. The tz_dsttime field has never been
42 used under Linux; it has not been and will not be supported by libc or
43 glibc. Each and every occurrence of this field in the kernel source
44 (other than the declaration) is a bug. Thus, the following is purely
45 of historic interest.
46 The field tz_dsttime contains a symbolic constant (values are given
48 below) that indicates in which part of the year Daylight Saving Time is
49 in force. (Note: its value is constant throughout the year: it does
50 not indicate that DST is in force, it just selects an algorithm.) The
51 daylight saving time algorithms defined are as follows :
52 DST_NONE /* not on dst */
54 DST_USA /* USA style dst */
55 DST_AUST /* Australian style dst */
56 DST_WET /* Western European dst */
57 DST_MET /* Middle European dst */
58 DST_EET /* Eastern European dst */
59 DST_CAN /* Canada */
60 DST_GB /* Great Britain and Eire */
61 DST_RUM /* Rumania */
62 DST_TUR /* Turkey */
63 DST_AUSTALT /* Australian style with shift in 1986 */
64 Of course it turned out that the period in which Daylight Saving Time
66 is in force cannot be given by a simple algorithm, one per country;
67 indeed, this period is determined by unpredictable political decisions.
68 So this method of representing timezones has been abandoned. Under
69 Linux, in a call to settimeofday() the tz_dsttime field should be zero.
70 Under Linux there are some peculiar "warp clock" semantics associated
72 with the settimeofday() system call if on the very first call (after
73 booting) that has a non-NULL tz argument, the tv argument is NULL and
74 the tz_minuteswest field is nonzero. In such a case it is assumed that
75 the CMOS clock is on local time, and that it has to be incremented by
76 this amount to get UTC system time. No doubt it is a bad idea to use
77 this feature.
78 Macros for operating on timeval structures are described in timer‐
80 add(3).
81
83 gettimeofday() and settimeofday() return 0 for success, or -1 for fail‐
84 ure (in which case errno is set appropriately).
85
87 EFAULT One of tv or tz pointed outside the accessible address space.
88 EINVAL Timezone (or something else) is invalid.
90 EPERM The calling process has insufficient privilege to call settime‐
92 ofday(); under Linux the CAP_SYS_TIME capability is required.
93
95 SVr4, 4.3BSD. POSIX.1-2001 describes gettimeofday() but not settimeof‐
96 day(). POSIX.1-2008 marks gettimeofday() as obsolete, recommending the
97 use of clock_gettime(2) instead.
98
100 Traditionally, the fields of struct timeval were of type long.
101
103 date(1), adjtimex(2), time(2), ctime(3), ftime(3), capabilities(7),
104 time(7)
105
107 This page is part of release 3.25 of the Linux man-pages project. A
108 description of the project, and information about reporting bugs, can
109 be found at http://www.kernel.org/doc/man-pages/.
110Linux 2009-03-25 GETTIMEOFDAY(2)