gettimeofday(2) System Calls Manual gettimeofday(2)
NAME
gettimeofday, settimeofday - get / set time
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/time.h>
int gettimeofday(struct timeval *restrict tv,
struct timezone *_Nullable restrict tz);
int settimeofday(const struct timeval *tv,
const struct timezone *_Nullable 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
DESCRIPTION
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The use of the timezone structure is obsolete; the tz argument should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.
RETURN VALUE
gettimeofday() and settimeofday() return 0 for success. On error, -1
is returned and errno is set to indicate the error.
ERRORS
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 settime-
ofday(); under Linux the CAP_SYS_TIME capability is required.
VERSIONS
C library/kernel differences
On some architectures, an implementation of gettimeofday() is provided
in the vdso(7).
The kernel accepts NULL for both tv and tz. The timezone argument is
ignored by glibc and musl, and not passed to/from the kernel. An-
droid's bionic passes the timezone argument to/from the kernel, but An-
droid does not update the kernel timezone based on the device timezone
in Settings, so the kernel's timezone is typically UTC.
STANDARDS
gettimeofday()
POSIX.1-2008 (obsolete).
settimeofday()
None.
HISTORY
SVr4, 4.3BSD. POSIX.1-2001 describes gettimeofday() but not settimeof-
day(). POSIX.1-2008 marks gettimeofday() as obsolete, recommending the
use of clock_gettime(2) instead.
Traditionally, the fields of struct timeval were of type long.
The tz_dsttime field
On a non-Linux kernel, with glibc, the tz_dsttime field of struct time-
zone 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 cur-
rent zone. On Linux, with glibc, the setting of the tz_dsttime field
of struct timezone has never been used by settimeofday() or gettimeof-
day(). Thus, the following is purely of historical interest.
On old systems, the field tz_dsttime contains a symbolic constant (val-
ues 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 al-
gorithm.) 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; in-
deed, this period is determined by unpredictable political decisions.
So this method of representing timezones has been abandoned.
NOTES
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 timer-
add(3).
SEE ALSO
date(1), adjtimex(2), clock_gettime(2), time(2), ctime(3), ftime(3),
timeradd(3), capabilities(7), time(7), vdso(7), hwclock(8)
Linux man-pages 6.7 2023-10-31 gettimeofday(2)
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