tzset

tzset is a C standard library function that initializes the program’s time zone information from the environment. It reads the TZ environment variable (if set) and updates global variables used by time-related functions such as localtime and mktime. After a call to tzset, these functions will interpret times according to the configured local time zone and daylight saving rules.

Prototype and scope: The function is declared in time.h as void tzset(void). It affects global state for the process, including the tzname array (which holds the standard and daylight names), the timezone offset from UTC, and the daylight flag. Because it alters global state, tzset is generally not considered thread-safe in all C library implementations; repeated or concurrent calls can lead to race conditions in multi-threaded programs unless proper synchronization is used.

TZ environment variable and formats: tzset reads TZ to determine the local time zone. If TZ is unset or empty, the system default is used. The TZ value is interpreted using a POSIX-style format describing standard time, offset from UTC, and optional daylight saving rules. In many implementations, TZ can also take a path-style form beginning with a colon, followed by a zone description file path from the zoneinfo database (for example, TZ=:/usr/share/zoneinfo/America/New_York). Examples of simple forms include EST5EDT or PST8PDT, while more explicit forms may specify start and end rules for DST.

Portability and related functions: tzset is available on POSIX-compliant systems and, on Windows, a variant named _tzset may be provided by the C runtime. It relates to functions such as localtime, gmtime, and mktime, which rely on the configured time zone. For modern code requiring locale-aware time data, consider using established time zone databases and safe, thread-aware approaches where possible.