For compatibility with POSIX, the GNU C Library defines global state
variables that depend on time zone rules specified by the TZ
environment variable. However, these state variables are obsolescent
and are planned to be removed in a future version of POSIX,
and programs generally should avoid them because they are not
thread-safe and their values are specified only when TZ uses the
proleptic format. See Specifying the Time Zone with TZ.
Programs should instead use the tm_gmtoff and
tm_zone members of struct tm. See Broken-down Time.
void tzset (void) ¶Preliminary: | MT-Safe env locale | AS-Unsafe heap lock | AC-Unsafe lock mem fd | See POSIX Safety Concepts.
The tzset function initializes the state variables from
the value of the TZ environment variable.
It is not usually necessary for your program to call this function,
partly because your program should not use the state variables,
and partly because this function is called automatically
when you use the time conversion functions localtime,
mktime, strftime, strftime_l, and
wcsftime, or the deprecated function ctime.
Behavior is undefined if one thread accesses any of these variables directly
while another thread is calling tzset or any other function
that is required or allowed to behave as if it called tzset.
char * tzname [2] ¶The array tzname contains two strings, which are
abbreviations of time zones (standard and Daylight
Saving) that the user has selected. tzname[0] abbreviates
a standard time zone (for example, "EST"), and tzname[1]
abbreviates a time zone when daylight saving time is in use (for
example, "EDT"). These correspond to the std and dst
strings (respectively) when the TZ environment variable
uses the proleptic format.
The string values are unspecified if TZ uses the geographical format,
so it is generally better to use the broken-down time structure’s
tm_zone member instead.
In the GNU C Library, the strings have a storage lifetime that lasts indefinitely;
on some other platforms, the lifetime lasts only until TZ is changed.
The tzname array is initialized by tzset.
Though the strings are declared as char *
the user must refrain from modifying them.
Modifying the strings will almost certainly lead to trouble.
long int timezone ¶This contains the difference between UTC and local standard
time, in seconds west of the Prime Meridian.
For example, in the U.S. Eastern time
zone, the value is 5*60*60. Unlike the tm_gmtoff member
of the broken-down time structure, this value is not adjusted for
daylight saving, and its sign is reversed.
The value is unspecified if TZ uses the geographical format,
so it is generally better to use the broken-down time structure’s
tm_gmtoff member instead.
int daylight ¶This variable is nonzero if daylight saving time rules apply. A nonzero value does not necessarily mean that daylight saving time is now in effect; it means only that daylight saving time is sometimes in effect. This variable has little or no practical use; it is present for POSIX compatibility.