gettimeofday, settimeofday - get / set time
#include <sys/time.h>
int gettimeofday(struct timeval *tv, struct timezone *tz);
int settimeofday(const struct timeval *tv, const struct timezone *tz);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
settimeofday():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.19 and earlier:
_BSD_SOURCE
The functions
gettimeofday() and
settimeofday() can get and set
the time as well as a timezone.
The
tv argument is a
struct timeval (as specified in
<sys/time.h>):
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
and gives the number of seconds and microseconds since the Epoch (see
time(2)).
The
tz argument is a
struct timezone:
struct timezone {
int tz_minuteswest; /* minutes west of Greenwich */
int tz_dsttime; /* type of DST correction */
};
If either
tv or
tz is NULL, the corresponding structure is not set
or returned. (However, compilation warnings will result if
tv is NULL.)
The use of the
timezone structure is obsolete; the
tz argument
should normally be specified as NULL. (See NOTES below.)
Under Linux, there are some peculiar "warp clock" semantics associated
with the
settimeofday() system call if on the very first call (after
booting) that has a non-NULL
tz argument, the
tv argument is
NULL and the
tz_minuteswest field is nonzero. (The
tz_dsttime
field should be zero for this case.) In such a case it is assumed that the
CMOS clock is on local time, and that it has to be incremented by this amount
to get UTC system time. No doubt it is a bad idea to use this feature.
gettimeofday() and
settimeofday() return 0 for success, or -1 for
failure (in which case
errno is set appropriately).
- EFAULT
- One of tv or tz pointed outside the accessible address
space.
- EINVAL
- (settimeofday()): timezone is invalid.
- EINVAL
- (settimeofday()): tv.tv_sec is negative or tv.tv_usec
is outside the range [0..999,999].
- EINVAL (since Linux 4.3)
- (settimeofday()): An attempt was made to set the time to a value
less than the current value of the CLOCK_MONOTONIC clock (see
clock_gettime(2)).
- EPERM
- The calling process has insufficient privilege to call
settimeofday(); under Linux the CAP_SYS_TIME capability is
required.
SVr4, 4.3BSD. POSIX.1-2001 describes
gettimeofday() but not
settimeofday(). POSIX.1-2008 marks
gettimeofday() as obsolete,
recommending the use of
clock_gettime(2) instead.
The time returned by
gettimeofday()
is affected by discontinuous
jumps in the system time (e.g., if the system administrator manually changes
the system time). If you need a monotonically increasing clock, see
clock_gettime(2).
Macros for operating on
timeval structures are described in
timeradd(3).
Traditionally, the fields of
struct timeval were of type
long.
On some architectures, an implementation of
gettimeofday() is provided in
the
vdso(7).
On a non-Linux kernel, with glibc, the
tz_dsttime field of
struct
timezone will be set to a nonzero value by
gettimeofday() if the
current timezone has ever had or will have a daylight saving rule applied. In
this sense it exactly mirrors the meaning of
daylight(3) for the
current zone. On Linux, with glibc, the setting of the
tz_dsttime field
of
struct timezone has never been used by
settimeofday() or
gettimeofday(). Thus, the following is purely of historical interest.
On old systems, the field
tz_dsttime contains a symbolic constant (values
are given below) that indicates in which part of the year Daylight Saving Time
is in force. (Note: this value is constant throughout the year: it does not
indicate that DST is in force, it just selects an algorithm.) The daylight
saving time algorithms defined are as follows:
DST_NONE /* not on DST */
DST_USA /* USA style DST */
DST_AUST /* Australian style DST */
DST_WET /* Western European DST */
DST_MET /* Middle European DST */
DST_EET /* Eastern European DST */
DST_CAN /* Canada */
DST_GB /* Great Britain and Eire */
DST_RUM /* Romania */
DST_TUR /* Turkey */
DST_AUSTALT /* Australian style with shift in 1986 */
Of course it turned out that the period in which Daylight Saving Time is in
force cannot be given by a simple algorithm, one per country; indeed, this
period is determined by unpredictable political decisions. So this method of
representing timezones has been abandoned.
date(1),
adjtimex(2),
clock_gettime(2),
time(2),
ctime(3),
ftime(3),
timeradd(3),
capabilities(7),
time(7),
vdso(7),
hwclock(8)