Ubuntu Manpage: gettimeofday, settimeofday

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 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       The use of the timezone  structure  is  obsolete;  the  tz  argument  should  normally  be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.

C library/kernel differences
On some architectures, an implementation of gettimeofday() is provided in the vdso(7).

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.

COLOPHON

       This  page  is  part of release 4.15 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at https://www.kernel.org/doc/man-pages/.