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.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
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.\" Modified by Michael Haardt (michael@moria.de)
.\" Modified 1993-07-23 by Rik Faith (faith@cs.unc.edu)
.\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
.\"   Fixed necessary '#include' lines.
.\" Modified 1995-04-15 by Michael Chastain (mec@shell.portal.com):
.\"   Added reference to adjtimex.
.\" Removed some nonsense lines pointed out by Urs Thuermann,
.\"   (urs@isnogud.escape.de), aeb, 950722.
.\" Modified 1997-01-14 by Austin Donnelly (and1000@debian.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.manpages@gmail.com>
.\"   Added notes on capability requirement.
.\"
.TH GETTIMEOFDAY 2 2009-03-25 "Linux" "Linux Programmer's Manual"
.SH NAME
gettimeofday, settimeofday \- get / set time
.SH SYNOPSIS
.nf
.B #include <sys/time.h>

.BI "int gettimeofday(struct timeval *" tv ", struct timezone *" tz );

.BI "int settimeofday(const struct timeval *" tv \
", const struct timezone *" tz );

.fi
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR settimeofday ():
_BSD_SOURCE
.SH DESCRIPTION
The functions
.BR gettimeofday ()
and
.BR settimeofday ()
can get and set the time as well as a timezone.
The
.I tv
argument is a
.I struct timeval
(as specified in
.IR <sys/time.h> ):
.sp
.in +4n
.nf
struct timeval {
    time_t      tv_sec;     /* seconds */
    suseconds_t tv_usec;    /* microseconds */
};
.fi
.in
.sp
and gives the number of seconds and microseconds since the Epoch (see
.BR time (2)).
The
.I tz
argument is a
.IR "struct timezone" :
.sp
.in +4n
.nf
struct timezone {
    int tz_minuteswest;     /* minutes west of Greenwich */
    int tz_dsttime;         /* type of DST correction */
};
.fi
.in
.PP
If either
.I tv
or
.I tz
is NULL, the corresponding structure is not set or returned.
.\" The following is covered under EPERM below:
.\" .PP
.\" Only the superuser may use
.\" .BR settimeofday ().
.PP
The use of the
.I timezone
structure is obsolete; the
.I tz
argument should normally be specified as NULL.
The
.I 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
.I 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 :
.in +4n
.nf

\fBDST_NONE\fP     /* not on dst */
.br
\fBDST_USA\fP      /* USA style dst */
.br
\fBDST_AUST\fP     /* Australian style dst */
.br
\fBDST_WET\fP      /* Western European dst */
.br
\fBDST_MET\fP      /* Middle European dst */
.br
\fBDST_EET\fP      /* Eastern European dst */
.br
\fBDST_CAN\fP      /* Canada */
.br
\fBDST_GB\fP       /* Great Britain and Eire */
.br
\fBDST_RUM\fP      /* Rumania */
.br
\fBDST_TUR\fP      /* Turkey */
.br
\fBDST_AUSTALT\fP  /* Australian style with shift in 1986 */
.fi
.in
.PP
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.
Under Linux, in a call to
.BR settimeofday ()
the
.I tz_dsttime
field should be zero.
.PP
Under Linux there are some peculiar "warp clock" semantics associated
with the
.BR settimeofday ()
system call if on the very first call (after booting)
that has a non-NULL
.I tz
argument, the
.I tv
argument is NULL and the
.I 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.
.PP
Macros for operating on
.I timeval
structures are described in
.BR timeradd (3).
.SH "RETURN VALUE"
.BR gettimeofday ()
and
.BR settimeofday ()
return 0 for success, or \-1 for failure (in which case
.I errno
is set appropriately).
.SH ERRORS
.TP
.B EFAULT
One of
.I tv
or
.I tz
pointed outside the accessible address space.
.TP
.B EINVAL
Timezone (or something else) is invalid.
.TP
.B EPERM
The calling process has insufficient privilege to call
.BR settimeofday ();
under Linux the
.B CAP_SYS_TIME
capability is required.
.SH "CONFORMING TO"
SVr4, 4.3BSD.
POSIX.1-2001 describes
.BR gettimeofday ()
but not
.BR settimeofday ().
POSIX.1-2008 marks
.BR gettimeofday ()
as obsolete, recomending the use of
.BR clock_gettime (2)
instead.
.SH NOTES
.LP
Traditionally, the fields of
.I struct timeval
were of type
.IR long .
.SH "SEE ALSO"
.BR date (1),
.BR adjtimex (2),
.BR time (2),
.BR ctime (3),
.BR ftime (3),
.BR capabilities (7),
.BR time (7)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
