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); |
![[Note]](../stylesheet/note.png) |
Note | ||||
|---|---|---|---|---|---|
|
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;suseconds_t tv_usec;};
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;int tz_dsttime;};
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structure is obsolete; the tz argument should normally be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
tvortzpointed outside the accessible address space. - EINVAL
-
Timezone (or something else) is invalid.
- EPERM
-
The calling process has insufficient privilege to call
settimeofday(); under Linux theCAP_SYS_TIMEcapability 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)
COLOPHON
This page is part of release 4.07 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/.
|
Copyright (c) 1992 Drew Eckhardt (drewcs.colorado.edu), March 28, 1992 %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified by Michael Haardt (michaelmoria.de) Modified 1993-07-23 by Rik Faith (faithcs.unc.edu) Modified 1994-08-21 by Michael Chastain (mecshell.portal.com): Fixed necessary '#include' lines. Modified 1995-04-15 by Michael Chastain (mecshell.portal.com): Added reference to adjtimex. Removed some nonsense lines pointed out by Urs Thuermann, (ursisnogud.escape.de), aeb, 950722. Modified 1997-01-14 by Austin Donnelly (and1000debian.org): Added return values section, and bit on EFAULT Added clarification on timezone, aeb, 971210. Removed "#include <unistd.h>", aeb, 010316. Modified, 2004-05-27 by Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirement. |