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[thirdparty/man-pages.git] / man2 / gettimeofday.2

.\" Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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 2021-03-22 "Linux man-pages (unreleased)"
.SH NAME
gettimeofday, settimeofday \- get / set time
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/time.h>
.PP
.BI "int gettimeofday(struct timeval *restrict " tv ,
.BI "                 struct timezone *restrict " tz );
.BI "int settimeofday(const struct timeval *" tv ,
.BI "                 const struct timezone *" tz );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR settimeofday ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Glibc 2.19 and earlier:
        _BSD_SOURCE
.fi
.SH DESCRIPTION
The functions
.BR gettimeofday ()
and
.BR settimeofday ()
can get and set the time as well as a timezone.
.PP
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)).
.PP
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.
On error, \-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
One of
.I tv
or
.I tz
pointed outside the accessible address space.
.TP
.B EINVAL
.RB ( settimeofday ()):
.I timezone
is invalid.
.TP
.B EINVAL
.RB ( settimeofday ()):
.I tv.tv_sec
is negative or
.I tv.tv_usec
is outside the range [0..999,999].
.TP
.BR EINVAL " (since Linux 4.3)"
.\" commit e1d7ba8735551ed79c7a0463a042353574b96da3
.RB ( settimeofday ()):
An attempt was made to set the time to a value less than
the current value of the
.B CLOCK_MONOTONIC
clock (see
.BR clock_gettime (2)).
.TP
.B EPERM
The calling process has insufficient privilege to call
.BR settimeofday ();
under Linux the
.B CAP_SYS_TIME
capability is required.
.SH STANDARDS
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)