gettimeofday, settimeofday — get / set time
#include <sys/time.h>
int
gettimeofday( |
struct timeval * | tv, |
struct timezone * | tz) ; |
int
settimeofday( |
const struct timeval * | tv, |
const struct timezone * | tz) ; |
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
<sys/time.h>
):
struct timeval { time_t tv_sec
;/* seconds */ suseconds_t tv_usec
;/* 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 tz_minuteswest
;/* minutes west of Greenwich */ int tz_dsttime
;/* type of DST correction */ };
If either tv
or
tz
is NULL, the
corresponding structure is not set or returned.
The use of the timezone
structure is
obsolete; the tz
argument should normally be specified as NULL. 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.
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Note |
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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 non-zero. 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)−>tv_sec || (tvp)−>tv_usec) #define timercmp(tvp, uvp, cmp)\ ((tvp)−>tv_sec cmp (uvp)−>tv_sec ||\ (tvp)−>tv_sec == (uvp)−>tv_sec &&\ (tvp)−>tv_usec cmp (uvp)−>tv_usec) #define timerclear(tvp)\ ((tvp)−>tv_sec = (tvp)−>tv_usec = 0)
gettimeofday
() and
settimeofday
() return 0 for
success, or −1 for failure (in which case errno
is set appropriately).
One of tv
or
tz
pointed
outside the accessible address space.
Timezone (or something else) is invalid.
The calling process has insufficient privilege to
call settimeofday
();
under Linux the CAP_SYS_TIME
capability is
required.
The prototype for settimeofday
() and the defines for
timercmp
(), timerisset
(), timerclear
(), timeradd
(), timersub
() are (since glibc 2.2.2) only
available if _BSD_SOURCE
is
defined.
Traditionally, the fields of struct timeval were longs.
date(1), adjtimex(2), time(2), ctime(3), ftime(3), capabilities(7), time(7)
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