gettimeofday(2) — Linux manual pages

gettimeofday
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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)`;

Note

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 <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.

The use of the timezone structure is obsolete; the tz argument should normally be specified as NULL.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 nonzero. 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 timeradd(3).

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().

NOTES

Traditionally, the fields of struct timeval were of type long.

COLOPHON

This page is part of release 2.79 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/.

Copyright (c) 1992 Drew Eckhardt (drewcs.colorado.edu), March 28, 1992

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.

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.

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struct	timeval {
	time_t	`tv_sec`;	/* seconds */
	suseconds_t	`tv_usec`;	/* microseconds */
};

struct	timezone {
	int	`tz_minuteswest`;	/* minutes west of Greenwich */
	int	`tz_dsttime`;	/* type of DST correction */
};