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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
GETTIMEOFDAY(2) Linux Programmer's Manual GETTIMEOFDAY(2)
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The use of the timezone structure is obsolete; the tz argument should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 Timezone (or something else) is invalid.
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.
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.
date(1), adjtimex(2), clock_gettime(2), time(2), ctime(3), ftime(3),
timeradd(3), capabilities(7), time(7), vdso(7), hwclock(8)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-12-12 GETTIMEOFDAY(2)
Pages that refer to this page: adjtimex(2), alarm(2), clock_getres(2), getitimer(2), stime(2), syscalls(2), time(2), timerfd_create(2), adjtime(3), ctime(3), difftime(3), ftime(3), pmdaeventarray(3), timeradd(3), tzset(3), uuid_time(3), rtc(4), systemd.exec(5), capabilities(7), time(7), vdso(7), hwclock(8), mount(8)