membarrier.2: Remove redundant mention of return value of MEMBARRIER_CMD_SHARED

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

.\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>
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.TH TIMEGM 3 2015-03-02 "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 ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbw21 lb lb
l l l.
Interface	Attribute	Value
T{
.BR timelocal (),
.BR timegm ()
T}	Thread safety	MT-Safe env locale
.TE
.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");
    if (tz)
        tz = strdup(tz);
    setenv("TZ", "", 1);
    tzset();
    ret = mktime(tm);
    if (tz) {
        setenv("TZ", tz, 1);
        free(tz);
    } else
        unsetenv("TZ");
    tzset();
    return ret;
}
.fi
.in
.SH SEE ALSO
.BR gmtime (3),
.BR localtime (3),
.BR mktime (3),
.BR tzset (3)