1GETTIMEOFDAY(2)            Linux Programmer's Manual           GETTIMEOFDAY(2)
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NAME

6       gettimeofday, settimeofday - get / set time
7

SYNOPSIS

9       #include <sys/time.h>
10
11       int gettimeofday(struct timeval *restrict tv,
12                        struct timezone *restrict tz);
13       int settimeofday(const struct timeval *tv,
14                        const struct timezone *tz);
15
16   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
17
18       settimeofday():
19           Since glibc 2.19:
20               _DEFAULT_SOURCE
21           Glibc 2.19 and earlier:
22               _BSD_SOURCE
23

DESCRIPTION

25       The  functions  gettimeofday()  and  settimeofday() can get and set the
26       time as well as a timezone.
27
28       The tv argument is a struct timeval (as specified in <sys/time.h>):
29
30           struct timeval {
31               time_t      tv_sec;     /* seconds */
32               suseconds_t tv_usec;    /* microseconds */
33           };
34
35       and gives the number of seconds and microseconds since the  Epoch  (see
36       time(2)).
37
38       The tz argument is a struct timezone:
39
40           struct timezone {
41               int tz_minuteswest;     /* minutes west of Greenwich */
42               int tz_dsttime;         /* type of DST correction */
43           };
44
45       If  either  tv or tz is NULL, the corresponding structure is not set or
46       returned.  (However, compilation warnings will result if tv is NULL.)
47
48       The use of the timezone structure is obsolete; the tz  argument  should
49       normally be specified as NULL.  (See NOTES below.)
50
51       Under  Linux, there are some peculiar "warp clock" semantics associated
52       with the settimeofday() system call if on the very  first  call  (after
53       booting)  that  has a non-NULL tz argument, the tv argument is NULL and
54       the tz_minuteswest field is nonzero.  (The tz_dsttime field  should  be
55       zero  for this case.)  In such a case it is assumed that the CMOS clock
56       is on local time, and that it has to be incremented by this  amount  to
57       get UTC system time.  No doubt it is a bad idea to use this feature.
58

RETURN VALUE

60       gettimeofday()  and  settimeofday() return 0 for success.  On error, -1
61       is returned and errno is set to indicate the error.
62

ERRORS

64       EFAULT One of tv or tz pointed outside the accessible address space.
65
66       EINVAL (settimeofday()): timezone is invalid.
67
68       EINVAL (settimeofday()): tv.tv_sec is negative or tv.tv_usec is outside
69              the range [0..999,999].
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71       EINVAL (since Linux 4.3)
72              (settimeofday()): An attempt was made to set the time to a value
73              less than the current value of the  CLOCK_MONOTONIC  clock  (see
74              clock_gettime(2)).
75
76       EPERM  The  calling process has insufficient privilege to call settime‐
77              ofday(); under Linux the CAP_SYS_TIME capability is required.
78

CONFORMING TO

80       SVr4, 4.3BSD.  POSIX.1-2001 describes gettimeofday() but not settimeof‐
81       day().  POSIX.1-2008 marks gettimeofday() as obsolete, recommending the
82       use of clock_gettime(2) instead.
83

NOTES

85       The time returned by gettimeofday() is affected by discontinuous  jumps
86       in  the system time (e.g., if the system administrator manually changes
87       the system time).  If you need a monotonically  increasing  clock,  see
88       clock_gettime(2).
89
90       Macros  for  operating  on  timeval  structures are described in timer‐
91       add(3).
92
93       Traditionally, the fields of struct timeval were of type long.
94
95   C library/kernel differences
96       On some architectures, an implementation of gettimeofday() is  provided
97       in the vdso(7).
98
99   The tz_dsttime field
100       On a non-Linux kernel, with glibc, the tz_dsttime field of struct time‐
101       zone will be set to a nonzero value by gettimeofday()  if  the  current
102       timezone  has ever had or will have a daylight saving rule applied.  In
103       this sense it exactly mirrors the meaning of daylight(3) for  the  cur‐
104       rent  zone.   On Linux, with glibc, the setting of the tz_dsttime field
105       of struct timezone has never been used by settimeofday() or  gettimeof‐
106       day().  Thus, the following is purely of historical interest.
107
108       On old systems, the field tz_dsttime contains a symbolic constant (val‐
109       ues are given below) that indicates in which part of the year  Daylight
110       Saving  Time is in force.  (Note: this value is constant throughout the
111       year: it does not indicate that DST is in force, it just selects an al‐
112       gorithm.)  The daylight saving time algorithms defined are as follows:
113
114           DST_NONE     /* not on DST */
115           DST_USA      /* USA style DST */
116           DST_AUST     /* Australian style DST */
117           DST_WET      /* Western European DST */
118           DST_MET      /* Middle European DST */
119           DST_EET      /* Eastern European DST */
120           DST_CAN      /* Canada */
121           DST_GB       /* Great Britain and Eire */
122           DST_RUM      /* Romania */
123           DST_TUR      /* Turkey */
124           DST_AUSTALT  /* Australian style with shift in 1986 */
125
126       Of  course  it turned out that the period in which Daylight Saving Time
127       is in force cannot be given by a simple algorithm, one per country; in‐
128       deed,  this  period is determined by unpredictable political decisions.
129       So this method of representing timezones has been abandoned.
130

SEE ALSO

132       date(1), adjtimex(2), clock_gettime(2),  time(2),  ctime(3),  ftime(3),
133       timeradd(3), capabilities(7), time(7), vdso(7), hwclock(8)
134

COLOPHON

136       This  page  is  part of release 5.12 of the Linux man-pages project.  A
137       description of the project, information about reporting bugs,  and  the
138       latest     version     of     this    page,    can    be    found    at
139       https://www.kernel.org/doc/man-pages/.
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143Linux                             2021-03-22                   GETTIMEOFDAY(2)
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