.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992

.\"

.\" %%%LICENSE_START(VERBATIM)

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.\" manual provided the copyright notice and this permission notice are

.\" preserved on all copies.

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.\" 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.

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.\" 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.\" manual page may be incorrect or out-of-date.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 (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 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

gettimeofday, settimeofday \- get / set time

.SH SYNOPSIS

.nf

.B #include <sys/time.h>

.PP

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

.PP

.BI "int settimeofday(const struct timeval *" tv \

", const struct timezone *" tz );

.fi

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.in

.PP

.BR settimeofday ():

    Since glibc 2.19:

        _DEFAULT_SOURCE

    Glibc 2.19 and earlier:

        _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> ):

.PP

.in +4n

.EX

struct timeval {

    time_t      tv_sec;     /* seconds */

    suseconds_t tv_usec;    /* microseconds */

};

.EE

.in

.PP

and gives the number of seconds and microseconds since the Epoch (see

.BR time (2)).

The

.I tz

argument is a

.IR "struct timezone" :

.PP

.in +4n

.EX

struct timezone {

    int tz_minuteswest;     /* minutes west of Greenwich */

    int tz_dsttime;         /* type of DST correction */

};

.EE

.in

.PP

If either

.I tv

or

.I tz

is NULL, the corresponding structure is not set or returned.

.\" FIXME . The compilation warning looks to be going away in 2.17

.\" see glibc commit 4b7634a5e03b0da6f8875de9d3f74c1cf6f2a6e8

(However, compilation warnings will result if

.I tv

is NULL.)

.\" 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.

(See NOTES below.)

.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 nonzero.

(The

.I 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.

.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, recommending the use of

.BR clock_gettime (2)

instead.

.SH NOTES

The time returned by

.BR gettimeofday ()

.I 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

.BR clock_gettime (2).

.PP

Macros for operating on

.I timeval

structures are described in

.BR timeradd (3).

.PP

Traditionally, the fields of

.I struct timeval

were of type

.IR long .

.\"

.SS C library/kernel differences

On some architectures, an implementation of

.BR gettimeofday ()

is provided in the

.BR vdso (7).

.\"

.SS The tz_dsttime field

On a non-Linux kernel, with glibc, the

.I tz_dsttime

field of

.I struct timezone

will be set to a nonzero value by

.BR 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

.BR daylight (3)

for the current zone.

On Linux, with glibc, the setting of the

.I tz_dsttime

field of

.I struct timezone

has never been used by

.BR settimeofday ()

or

.BR gettimeofday ().

.\" 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 historical interest.

.PP

On old systems, 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: 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:

.PP

.in +4n

.EX

\fBDST_NONE\fP     /* not on DST */

\fBDST_USA\fP      /* USA style DST */

\fBDST_AUST\fP     /* Australian style DST */

\fBDST_WET\fP      /* Western European DST */

\fBDST_MET\fP      /* Middle European DST */

\fBDST_EET\fP      /* Eastern European DST */

\fBDST_CAN\fP      /* Canada */

\fBDST_GB\fP       /* Great Britain and Eire */

\fBDST_RUM\fP      /* Romania */

\fBDST_TUR\fP      /* Turkey */

\fBDST_AUSTALT\fP  /* Australian style with shift in 1986 */

.EE

.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.

.SH SEE ALSO

.BR date (1),

.BR adjtimex (2),

.BR clock_gettime (2),

.BR time (2),

.BR ctime (3),

.BR ftime (3),

.BR timeradd (3),

.BR capabilities (7),

.BR time (7),

.BR vdso (7),

.BR hwclock (8)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.