NAME
gettimeofday, settimeofday - get / set time SYNOPSIS
#include
int gettimeofday(struct timeval *tv, struct timezone *tz); int settimeofday(const struct timeval *tv, const struct timezone *tz); Feature Test Macro Requirements for glibc (see featuretestmacros(7)): settimeofday(): BSDSOURCE DESCRIPTION The functions gettimeofday() and settimeofday() can get and set the time as well as a timezone. The tv argument is a struct timeval (as specified in ): struct timeval { timet tvsec; /* seconds */ susecondst tvusec; /* microseconds */ }; and gives the number of seconds and microseconds since the Epoch (see time(2)). The tz argument is a struct timezone: struct timezone { int tzminuteswest; /* minutes west of Greenwich */ int tzdsttime; /* type of DST correction */ }; If either tv or tz is NULL, the corresponding structure is not set or returned. (However, compilation warnings will result if tv is NULL.) The use of the timezone structure is obsolete; the tz argument should normally be specified as NULL. (See NOTES below.) Under Linux there are some peculiar "warp clock" semantics associated with the settimeofday() system call if on the very first call (after booting) that has a non-NULL tz argument, the tv argument is NULL and the tzminuteswest field is nonzero. (The tzdsttime field should be zero for this case.) In such a case it is assumed that the CMOS clock is on local time, and that it has to be incremented by this amount to get UTC system time. No doubt it is a bad idea to use this feature. RETURN VALUE
gettimeofday() and settimeofday() return 0 for success, or -1 for fail‐ ure (in which case errno is set appropriately). ERRORS EFAULT One of tv or tz pointed outside the accessible address space. EINVAL Timezone (or something else) is invalid. EPERM The calling process has insufficient privilege to call settime‐ ofday(); under Linux the CAPSYSTIME capability is required. CONFORMING TO
SVr4, 4.3BSD. POSIX.1-2001 describes gettimeofday() but not settimeof‐
day(). POSIX.1-2008 marks gettimeofday() as obsolete, recommending the use of clockgettime(2) instead. NOTES The time returned by gettimeofday() is affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the system time). If you need a monotonically increasing clock, see clockgettime(2). Macros for operating on timeval structures are described in timer‐ add(3). Traditionally, the fields of struct timeval were of type long. The tzdsttime field has never been used under Linux. Thus, the fol‐ lowing is purely of historic interest. On old systems, the field tzdsttime contains a symbolic constant (val‐ ues are given below) that indicates in which part of the year Daylight Saving Time is in force. (Note: this value is constant throughout the year: it does not indicate that DST is in force, it just selects an algorithm.) The daylight saving time algorithms defined are as fol‐ lows: DSTNONE /* not on DST */ DSTUSA /* USA style DST */ DSTAUST /* Australian style DST */ DSTWET /* Western European DST */ DSTMET /* Middle European DST */ DSTEET /* Eastern European DST */ DSTCAN /* Canada */ DSTGB /* Great Britain and Eire */ DSTRUM /* Romania */ DSTTUR /* Turkey */ DSTAUSTALT /* Australian style with shift in 1986 */ Of course it turned out that the period in which Daylight Saving Time is in force cannot be given by a simple algorithm, one per country; indeed, this period is determined by unpredictable political decisions. So this method of representing timezones has been abandoned. SEE ALSO date(1), adjtimex(2), clockgettime(2), time(2), ctime(3), ftime(3), timeradd(3), capabilities(7), time(7) COLOPHON
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Linux 2012-04-26 GETTIMEOFDAY(2)