gettimeofday() - Unix, Linux System Call
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NAME
gettimeofday, settimeofday - get / set time
SYNOPSIS
#include <sys/time.h>
int gettimeofday(struct timeval *tv, struct timezone *tz);
int settimeofday(const struct timeval *tv , const struct timezone *tz); 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
<sys/time.h>):
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
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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 */
};
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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timezone structure is obsolete; the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. (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 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)
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RETURN VALUE
gettimeofday() and
settimeofday() return 0 for success, or -1 for failure (in which case
errno is set appropriately).
ERRORS
Tag | Description |
EFAULT |
One of
tv or
tz pointed outside the accessible address space.
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EINVAL |
Timezone (or something else) is invalid.
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EPERM |
The calling process has insufficient privilege to call
settimeofday(); under Linux the
CAP_SYS_TIME capability is required.
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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.
Traditionally, the fields of
struct timeval were longs.
CONFORMING TO
SVr4, 4.3BSD.
POSIX.1-2001 describes
gettimeofday() but not
settimeofday(). SEE ALSO
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