TZNAME

TZNAME is a term used in many C library implementations to refer to the two strings that denote the time zone abbreviations used for standard time and daylight saving time. In most systems these abbreviations are exposed as a two-element array of pointers to char named tzname: tzname[0] is the standard time abbreviation, and tzname[1] is the daylight saving time abbreviation. The array is populated by tzset(), which reads the active time zone specification from the TZ environment variable or the system time zone database.

Initialization and usage: When tzset is invoked, it parses the time zone information and fills tzname accordingly. After successful initialization, time conversion functions such as localtime use the tzname values to describe the local zone in formatting operations. The actual abbreviations depend on the configured time zone data; common examples include names like “EST”/“EDT” or “PST”/“PDT”.

Portability and variations: The existence and naming of TZNAME are not guaranteed by the C standard. POSIX does not require tzname; some platforms provide it as extern char *tzname[2], while others may use different symbols (for example, Windows environments may expose _tzname). In practice, most code relies on tzname (lowercase) or uses strftime with %Z to obtain the zone name, which can be more portable across platforms.

Notes: Access to tzname can be affected by calls to tzset, and in some implementations the array may be overwritten by subsequent tzset calls, affecting thread safety. For robust programming, rely on standard time formatting facilities when possible and be aware of platform-specific behavior regarding time zone naming.

See also: TZ environment variable, tzset, localtime, time.h, strftime, timezone.