GETTIMEOFDAY (2)
get / set time
SYNOPSIS
#include <sys/time.h>
#include <unistd.h>
int gettimeofday(struct timeval * tv , struct timezone * tz );
int settimeofday(const struct timeval * tv
, const struct timezone * tz );
DESCRIPTION
gettimeofday
and
settimeofday
can set the time as well as a timezone.
tv
is a
timeval
struct, as specified in /usr/include/sys/time.h:
struct timeval {
+8
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
-8
};
and
tz
is a
timezone
:
struct timezone {
+8
int tz_minuteswest; /* minutes W of Greenwich */
int tz_dsttime; /* type of dst correction */
-8
};
The use of the timezone struct is obsolete; the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 :
14
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)->tv_sec || (tvp)->tv_usec)
#define timercmp(tvp, uvp, cmp)\\
+8
((tvp)->tv_sec cmp (uvp)->tv_sec ||\\
(tvp)->tv_sec == (uvp)->tv_sec &&\\
(tvp)->tv_usec cmp (uvp)->tv_usec)
-8
#define timerclear(tvp)\\
((tvp)->tv_sec = (tvp)->tv_usec = 0)
If either
tv
or
tz
is null, the corresponding structure is not set or returned.
Only the super user may use
settimeofday .
RETURN VALUES
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.
CONFORMING TO
SEE ALSO
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