gettimeofday (2)
Leading comments
Copyright (c) 1992 Drew Eckhardt (drew@cs.colorado.edu), March 28, 1992 %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical t...
NAME
gettimeofday, settimeofday - get / set timeSYNOPSIS
#include <sys/time.h> int gettimeofday(struct timeval *tv, struct timezone *tz); int settimeofday(const struct timeval *tv, const struct timezone *tz);Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
settimeofday(): _BSD_SOURCE
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 */
};
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. (However, compilation warnings will result if tv is NULL.)
The use of the
timezone
structure is obsolete; the
tz
argument should normally be specified as NULL.
(See NOTES below.)
Under Linux, there are some peculiar "warp clock" semantics associated
with 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.
(The
tz_dsttime
field should be zero for this case.)
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.
RETURN VALUE
gettimeofday() and settimeofday() return 0 for success, or -1 for failure (in which case errno is set appropriately).ERRORS
- 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.
CONFORMING TO
SVr4, 4.3BSD. POSIX.1-2001 describes gettimeofday() but not settimeofday(). POSIX.1-2008 marks gettimeofday() as obsolete, recommending the use of clock_gettime(2) instead.NOTES
The time returned by gettimeofday() is affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the system time). If you need a monotonically increasing clock, see clock_gettime(2).Macros for operating on timeval structures are described in timeradd(3).
Traditionally, the fields of struct timeval were of type long.
The tz_dsttime field
On a non-Linux kernel, with glibc, the tz_dsttime field of struct timezone will be set to a nonzero value by gettimeofday() if the current timezone has ever had or will have a daylight saving rule applied. In this sense it exactly mirrors the meaning of daylight(3) for the current zone. On Linux, with glibc, the setting of the tz_dsttime field of struct timezone has never been used by settimeofday() or gettimeofday(). Thus, the following is purely of historical interest.On old systems, 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: this 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 /* Romania */
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 timezones has been abandoned.