path: root/man2/gettimeofday.2

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.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
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.\" 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 <mtk16@ext.canterbury.ac.nz>
.\"   Added notes on capability requirement.
.\"
.TH GETTIMEOFDAY 2 2004-05-27 "Linux 2.6.6" "Linux Programmer's Manual"
.SH NAME
gettimeofday, settimeofday \- get / set time
.SH SYNOPSIS
.B #include <sys/time.h>
.sp
.BI "int gettimeofday(struct timeval *" tv ", struct timezone *" tz );
.br
.BI "int settimeofday(const struct timeval *" tv
.BI ", const struct timezone *" tz );
.SH DESCRIPTION
The functions
.B gettimeofday
and
.B settimeofday
can get and set the time as well as a timezone.
The    
.I tv
argument is a 
.B timeval 
struct, as specified  in <sys/time.h>:
.sp
.nf
struct timeval {
.in +8
time_t		tv_sec;		/* seconds */
suseconds_t	tv_usec;	/* microseconds */
.in -8
};
.fi
.sp
and gives the number of seconds and microseconds since the Epoch (see
.BR time (2)).
The 
.I tz
argument is a 
.B timezone 
:
.sp
.nf
struct timezone {
.in +8
int	tz_minuteswest; /* minutes W of Greenwich */
int	tz_dsttime;     /* type of dst correction */
.in -8
};
.fi
.PP
The use of the timezone struct is obsolete; the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 : 
.PP
.ta 14
\fB DST_NONE\fP	/* not on dst */
.br
\fB DST_USA\fP	/* USA style dst */
.br
\fB DST_AUST\fP	/* Australian style dst */
.br
\fB DST_WET\fP	/* Western European dst */
.br
\fB DST_MET\fP	/* Middle European dst */
.br
\fB DST_EET\fP	/* Eastern European dst */
.br
\fB DST_CAN\fP	/* Canada */
.br
\fB DST_GB\fP	/* Great Britain and Eire */
.br
\fB DST_RUM\fP	/* Rumania */
.br
\fB DST_TUR\fP	/* Turkey */
.br
\fB DST_AUSTALT\fP	/* Australian style with shift in 1986 */
.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 time zones
has been abandoned. Under Linux, in a call to
.B settimeofday
the
.I tz_dsttime
field should be zero.
.PP
Under Linux there is some peculiar `warp clock' semantics associated
to the
.B 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. 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
The following macros are defined to operate on a struct timeval :
.br
.nf
#define	timerisset(tvp)\\
.ti +8
((tvp)->tv_sec || (tvp)->tv_usec)
#define	timercmp(tvp, uvp, cmp)\\
.in +8
((tvp)->tv_sec cmp (uvp)->tv_sec ||\\
(tvp)->tv_sec == (uvp)->tv_sec &&\\
(tvp)->tv_usec cmp (uvp)->tv_usec)
.in -8
#define	timerclear(tvp)\\
.ti +8
((tvp)->tv_sec = (tvp)->tv_usec = 0)
.fi
.PP
If either
.I tv
or 
.I tz
is null, the corresponding structure is not set or returned.
.PP
Only the super user may use
.BR settimeofday .
.SH "RETURN VALUE"
.B gettimeofday
and
.B 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 NOTE
The prototype for
.B settimeofday
and the defines for
.BR timercmp ,
.BR timerisset ,
.BR timerclear ,
.BR timeradd ,
.BR timersub
are (since glibc2.2.2) only available if
.B _BSD_SOURCE
is defined (either explicitly, or implicitly, by not defining
_POSIX_SOURCE or compiling with the -ansi flag).
.LP
Traditionally, the fields of struct timeval were longs.
.SH "CONFORMING TO"
SVr4, BSD 4.3. POSIX 1003.1-2001 describes gettimeofday()
but not settimeofday().
.SH "SEE ALSO"
.BR date (1),
.BR adjtimex (2),
.BR time (2),
.BR ctime (3),
.BR ftime (3),
.BR capabilities (7)