GETTIMEOFDAY(2) Linux Programmers Manual GETTIMEOFDAY(2)
NAME
gettimeofday, settimeofday - get / set time
SYNOPSIS
#include
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(): _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 ):
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.
The use of the timezone structure is obsolete; the tz argument should
normally be specified as NULL. The tz_dsttime field has never been
used under Linux; it has not been and will not be supported by libc or
glibc. Each and every occurrence of this field in the kernel source
(other than the declaration) is a bug. Thus, the following is purely
of historic interest.
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: its 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 /* Rumania */
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 time zones has been abandoned. Under
Linux, in a call to settimeofday() the tz_dsttime field should be zero.
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 non-zero. 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.
Macros for operating on timeval structures are described in timer
add(3).
RETURN VALUE
gettimeofday() and settimeofday() return 0 for success, or -1 for fail
ure (in which case errno is set appropriately).
ERRORS
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 settime
ofday(); under Linux the CAP_SYS_TIME capability is required.
CONFORMING TO
SVr4, 4.3BSD. POSIX.1-2001 describes gettimeofday() but not settimeof
day().
NOTES
Traditionally, the fields of struct timeval were of type long.
SEE ALSO
date(1), adjtimex(2), time(2), ctime(3), ftime(3), capabilities(7),
time(7)
COLOPHON
This page is part of release 3.05 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2007-07-27 GETTIMEOFDAY(2)
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