man3/timegm.3

   1 .\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>
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  23 .TH TIMEGM 3 2001-12-26 "GNU" "Linux Programmer's Manual"
  24 .SH NAME
  25 timegm, timelocal \- inverses for gmtime and localtime
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <time.h>
  29 .sp
  30 .BI "time_t timelocal (struct tm *" tm );
  31 .sp
  32 .BI "time_t timegm (struct tm *" tm );
  33 .SH DESCRIPTION
  34 The functions
  35 .BR timelocal ()
  36 and
  37 .BR timegm ()
  38 are the inverses to
  39 .BR localtime (3)
  40 and
  41 .BR gmtime (3).
  42 .SH NOTES
  43 These functions are GNU extensions.
  44 The
  45 .BR timelocal ()
  46 function is equivalent to the POSIX standard function
  47 .BR mktime (3).
  48 There is no reason to ever use it.
  49 .LP
  50 For a portable version of
  51 .BR timegm (),
  52 set the
  53 .B TZ
  54 environment variable to UTC, call
  55 .BR mktime ()
  56 and restore the value of
  57 .BR TZ .
  58 Something like
  59
  60 .RS
  61 .nf
  62 #include <time.h>
  63 #include <stdlib.h>
  64
  65 time_t my_timegm (struct tm *tm) {
  66     time_t ret;
  67     char *tz;
  68
  69     tz = getenv("TZ");
  70     setenv("TZ", "", 1);
  71     tzset();
  72     ret = mktime(tm);
  73     if (tz)
  74         setenv("TZ", tz, 1);
  75     else
  76         unsetenv("TZ");
  77     tzset();
  78     return ret;
  79 }
  80 .fi
  81 .RE
  82 .SH "SEE ALSO"
  83 .BR gmtime (3),
  84 .BR localtime (3),
  85 .BR mktime (3),
  86 .BR tzset (3)