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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO |
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gettimeofday(2) System Calls Manual gettimeofday(2)
gettimeofday, settimeofday - get / set time
Standard C library (libc, -lc)
#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
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The use of the timezone structure is obsolete; the tz argument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. On
error, -1 is returned and errno is set to indicate the error.
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.
C library/kernel differences
On some architectures, an implementation of gettimeofday() is
provided in the vdso(7).
gettimeofday()
POSIX.1-2008 (obsolete).
settimeofday()
None.
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.
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
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.
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).
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 6.04 2023-03-30 gettimeofday(2)
Pages that refer to this page: adjtimex(2), alarm(2), clock_getres(2), getitimer(2), seccomp(2), stime(2), syscalls(2), time(2), timerfd_create(2), adjtime(3), ctime(3), difftime(3), ftime(3), pmdaeventarray(3), pmtimeval(3), timeradd(3), timeval(3type), tzset(3), uuid_time(3), rtc(4), systemd.exec(5), capabilities(7), time(7), vdso(7), hwclock(8), mount(8)
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HTML rendering created 2023-06-24 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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