settimeofday(2)

1gettimeofday(2)               System Calls Manual              gettimeofday(2)
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NAME

6       gettimeofday, settimeofday - get / set time
7

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <sys/time.h>
13
14       int gettimeofday(struct timeval *restrict tv,
15                        struct timezone *_Nullable restrict tz);
16       int settimeofday(const struct timeval *tv,
17                        const struct timezone *_Nullable tz);
18
19   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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21       settimeofday():
22           Since glibc 2.19:
23               _DEFAULT_SOURCE
24           glibc 2.19 and earlier:
25               _BSD_SOURCE
26

DESCRIPTION

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51       The use of the timezone structure is obsolete; the tz  argument  should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

63       gettimeofday()  and  settimeofday() return 0 for success.  On error, -1
64       is returned and errno is set to indicate the error.
65

ERRORS

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

83   C library/kernel differences
84       On some architectures, an implementation of gettimeofday() is  provided
85       in the vdso(7).
86

STANDARDS

88       gettimeofday()
89              POSIX.1-2008 (obsolete).
90
91       settimeofday()
92              None.
93

HISTORY

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
99       Traditionally, the fields of struct timeval were of type long.
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101   The tz_dsttime field
102       On a non-Linux kernel, with glibc, the tz_dsttime field of struct time‐
103       zone will be set to a nonzero value by gettimeofday()  if  the  current
104       timezone  has ever had or will have a daylight saving rule applied.  In
105       this sense it exactly mirrors the meaning of daylight(3) for  the  cur‐
106       rent  zone.   On Linux, with glibc, the setting of the tz_dsttime field
107       of struct timezone has never been used by settimeofday() or  gettimeof‐
108       day().  Thus, the following is purely of historical interest.
109
110       On old systems, the field tz_dsttime contains a symbolic constant (val‐
111       ues are given below) that indicates in which part of the year  Daylight
112       Saving  Time is in force.  (Note: this value is constant throughout the
113       year: it does not indicate that DST is in force, it just selects an al‐
114       gorithm.)  The daylight saving time algorithms defined are as follows:
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116           DST_NONE     /* not on DST */
117           DST_USA      /* USA style DST */
118           DST_AUST     /* Australian style DST */
119           DST_WET      /* Western European DST */
120           DST_MET      /* Middle European DST */
121           DST_EET      /* Eastern European DST */
122           DST_CAN      /* Canada */
123           DST_GB       /* Great Britain and Eire */
124           DST_RUM      /* Romania */
125           DST_TUR      /* Turkey */
126           DST_AUSTALT  /* Australian style with shift in 1986 */
127
128       Of  course  it turned out that the period in which Daylight Saving Time
129       is in force cannot be given by a simple algorithm, one per country; in‐
130       deed,  this  period is determined by unpredictable political decisions.
131       So this method of representing timezones has been abandoned.
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NOTES

134       The time returned by gettimeofday() is affected by discontinuous  jumps
135       in  the system time (e.g., if the system administrator manually changes
136       the system time).  If you need a monotonically  increasing  clock,  see
137       clock_gettime(2).
138
139       Macros  for  operating  on  timeval  structures are described in timer‐
140       add(3).
141