Manpage of GETTIMEOFDAY

GETTIMEOFDAY

Section: Linux Programmer's Manual (2)
Updated: 2016-03-15
Index
 

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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

settimeofday():
    Since glibc 2.19:
        _DEFAULT_SOURCE
    Glibc 2.19 and earlier:
        _BSD_SOURCE  

DESCRIPTION

The functions gettimeofday() and settimeofday() can get and set the time as well as a timezone. The tvargument 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 tzargument is a struct timezone:

struct timezone {
    int tz_minuteswest;     /* minutes west of Greenwich */
    int tz_dsttime;         /* type of DST correction */
};

If either tvor tzis NULL, the corresponding structure is not set or returned. (However, compilation warnings will result if tvis NULL.)

The use of the timezonestructure is obsolete; the tzargument should normally be specified as NULL.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 tzargument, the tvargument is NULL and the tz_minuteswestfield is nonzero. (The tz_dsttimefield 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 errnois set appropriately).  

ERRORS

EFAULT
One of tvor tzpointed 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_TIMEcapability 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() isaffected 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 timevalstructures are described in timeradd(3).

Traditionally, the fields of struct timevalwere of type long.  

The tz_dsttime field

On a non-Linux kernel, with glibc, the tz_dsttimefield of struct timezonewill 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_dsttimefield of struct timezonehas never been used by settimeofday() or gettimeofday(). Thus, the following is purely of historical interest.

On old systems, the field tz_dsttimecontains 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.  

SEE ALSO

date(1), adjtimex(2), clock_gettime(2), time(2), ctime(3), ftime(3), timeradd(3), capabilities(7), time(7), vdso(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
The tz_dsttime field
SEE ALSO

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Time: 22:27:40 GMT, June 20, 2016