Originální popis anglicky:
gettimeofday, settimeofday - get / set time
Návod, kniha: Linux Programmer's Manual
#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
timeval struct,
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
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The use of the timezone struct is obsolete; the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)->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)
If either
tv or
tz is null, the corresponding structure is not set
or returned.
Only the super user may use
settimeofday.
gettimeofday and
settimeofday return 0 for success, or -1 for
failure (in which case
errno is set appropriately).
- 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
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 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).
Traditionally, the fields of struct timeval were longs.
SVr4, BSD 4.3. POSIX 1003.1-2001 describes gettimeofday() but not
settimeofday().
date(1),
adjtimex(2),
time(2),
ctime(3),
ftime(3),
capabilities(7)
