TZSET(3)                   POSIX Programmer's Manual                  TZSET(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       daylight, timezone, tzname, tzset -- set timezone conversion
       information

SYNOPSIS
       #include <time.h>

       extern int daylight;
       extern long timezone;
       extern char *tzname[2];
       void tzset(void);

DESCRIPTION
       The tzset() function shall use the value of the environment variable TZ
       to set time conversion information used by ctime(), localtime(),
       mktime(), and strftime().  If TZ is absent from the environment,
       implementation-defined default timezone information shall be used.

       The tzset() function shall set the external variable tzname as follows:


           tzname[0] = "std";
           tzname[1] = "dst";

       where std and dst are as described in the Base Definitions volume of
       POSIX.1-2017, Chapter _8, Environment Variables.

       The tzset() function also shall set the external variable daylight to 0
       if Daylight Savings Time conversions should never be applied for the
       timezone in use; otherwise, non-zero. The external variable timezone
       shall be set to the difference, in seconds, between Coordinated
       Universal Time (UTC) and local standard time.

       If a thread accesses tzname, daylight, or timezone directly while
       another thread is in a call to tzset(), or to any function that is
       required or allowed to set timezone information as if by calling
       tzset(), the behavior is undefined.

RETURN VALUE
       The tzset() function shall not return a value.

ERRORS
       No errors are defined.

       The following sections are informative.

EXAMPLES
       Example TZ variables and their timezone differences are given in the
       table below:

                              +-----------+------------+
                              |    TZ     |  timezone  |
                              +-----------+------------+
                              |EST5EDT    | 5*60*60    |
                              |GMT0       | 0*60*60    |
                              |JST-9      | -9*60*60   |
                              |MET-1MEST  | -1*60*60   |
                              |MST7MDT    | 7*60*60    |
                              |PST8PDT    | 8*60*60    |
                              +-----------+------------+
APPLICATION USAGE
       Since the ctime(), localtime(), mktime(), strftime(), and strftime___l()
       functions are required to set timezone information as if by calling
       tzset(), there is no need for an explicit tzset() call before using
       these functions. However, portable applications should call tzset()
       explicitly before using ctime___r() or localtime___r() because setting
       timezone information is optional for those functions.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       ctime(), localtime(), mktime(), strftime()

       The Base Definitions volume of POSIX.1-2017, Chapter _8, Environment
       Variables, <time.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                             TZSET(3)
