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GETTIMEOFDAY !!!GETTIMEOFDAY NAME SYNOPSIS DESCRIPTION RETURN VALUE ERRORS NOTE CONFORMING TO SEE ALSO ---- !!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''__);__ !!DESCRIPTION __gettimeofday__ and __settimeofday__ can get and set the time as well as a timezone. ''tv'' is a __timeval__ struct, as specified in /usr/include/sys/time.h: struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ }; The ''tv_sec'' member of the struct is the number of seconds since the Epoch (see time(2)), and ''tv_usec'' is the amount of microseconds past the current ''tv_sec'' value. The ''tz'' argument is a __timezone__ : struct timezone { int tz_minuteswest; /* minutes W of Greenwich */ int tz_dsttime; /* type of dst correction */ }; The use of the timezone struct is obsolete; the ''tz_dsttime'' field has never been used under Linux - it has not been and will not be supported by libc or glibc. Each and every occurrence of this field in the kernel source (other than the declaration) is a bug. Thus, the following is purely of historic interest. The field ''tz_dsttime'' contains a symbolic constant (values are given below) that indicates in which part of the year Daylight Saving Time is in force. (Note: its 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 follows : __DST_NONE__ /* not on dst */__ DST_USA__ /* USA style dst */__ DST_AUST__ /* Australian style dst */__ DST_WET__ /* Western European dst */__ DST_MET__ /* Middle European dst */__ DST_EET__ /* Eastern European dst */__ DST_CAN__ /* Canada */__ DST_GB__ /* Great Britain and Eire */__ DST_RUM__ /* Rumania */__ DST_TUR__ /* Turkey */__ DST_AUSTALT__ /* 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 time zones has been abandoned. Under Linux, in a call to __settimeofday__ the ''tz_dsttime'' field should be zero. Under Linux there is some peculiar `warp clock' semantics associated to 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 ''tz_minuteswest'' field is nonzero. 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. The following macros are defined to operate on a struct timeval : #define timerisset(tvp)\ ((tvp)- If either ''tv'' or ''tz'' is null, the corresponding structure is not set or returned. Only the super user may use __settimeofday__. !!RETURN VALUE __gettimeofday__ and __settimeofday__ return 0 for success, or -1 for failure (in which case ''errno'' is set appropriately). !!ERRORS __EPERM__ __settimeofday__ is called by someone other than the superuser. __EINVAL__ Timezone (or something else) is invalid. __EFAULT__ One of ''tv'' or ''tz'' pointed outside your accessible address space. !!NOTE The prototype for __settimeofday__ and the defines for __timercmp__, __timerisset__, __timerclear__, __timeradd__, __timersub__ are (since glibc2.2.2) only available if ___BSD_SOURCE__ is defined (either explicitly, or implicitly, by not defining _POSIX_SOURCE or compiling with the -ansi flag). !!CONFORMING TO SVr4, BSD 4.3 !!SEE ALSO date(1), adjtimex(2), time(2), ctime(3), ftime(3) ----
5 pages link to
settimeofday(2)
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Man2s
stime(2)
syscalls(2)
adjtimex(2)
hwclock(8)
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