GETTIMEOFDAY
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
gettimeofday, settimeofday - get / set time
SYNOPSIS
#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
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structure is obsolete; the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, or -1 for failure (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
settimeofday();
under Linux the
CAP_SYS_TIME
capability is required.
CONFORMING TO
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.
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
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.
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)
COLOPHON
This page is part of release 4.13 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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- The tz_dsttime field
-
- SEE ALSO
-
- COLOPHON
-
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