intro.1, _exit.2, access.2, alarm.2, alloc_hugepages.2, arch_prctl.2, bind.2, chdir...

[thirdparty/man-pages.git] / man3 / timegm.3

.\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>
.\"
.\" %%%LICENSE_START(verbatim)
.\" 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.\" manual page may be incorrect or out-of-date.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.
.\" %%%LICENSE_END
.\"
.TH TIMEGM 3 2007-07-26 "GNU" "Linux Programmer's Manual"
.SH NAME
timegm, timelocal \- inverses of gmtime and localtime
.SH SYNOPSIS
.nf
.B #include <time.h>
.sp
.BI "time_t timelocal(struct tm *" tm );
.sp
.BI "time_t timegm(struct tm *" tm );
.sp
.fi
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR timelocal (),
.BR timegm ():
_BSD_SOURCE || _SVID_SOURCE
.SH DESCRIPTION
The functions
.BR timelocal ()
and
.BR timegm ()
are the inverses of
.BR localtime (3)
and
.BR gmtime (3).
.SH CONFORMING TO
These functions are nonstandard GNU extensions
that are also present on the BSDs.
Avoid their use; see NOTES.
.SH NOTES
The
.BR timelocal ()
function is equivalent to the POSIX standard function
.BR mktime (3).
There is no reason to ever use it.
.LP
For a portable version of
.BR timegm (),
set the
.B TZ
environment variable to UTC, call
.BR mktime (3)
and restore the value of
.BR TZ .
Something like

.in +4n
.nf
#include <time.h>
#include <stdlib.h>

time_t
my_timegm(struct tm *tm)
{
    time_t ret;
    char *tz;

    tz = getenv("TZ");
    setenv("TZ", "", 1);
    tzset();
    ret = mktime(tm);
    if (tz)
        setenv("TZ", tz, 1);
    else
        unsetenv("TZ");
    tzset();
    return ret;
}
.fi
.in
.SH SEE ALSO
.BR gmtime (3),
.BR localtime (3),
.BR mktime (3),
.BR tzset (3)