Ubuntu Manpage: gettimeofday, settimeofday

NAME

       gettimeofday, settimeofday - get / set time

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/time.h>

       int gettimeofday(struct timeval *restrict tv,
                        struct timezone *_Nullable restrict tz);
       int settimeofday(const struct timeval *tv,
                        const struct timezone *_Nullable 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       The  use  of  the  timezone  structure  is  obsolete;  the  tz argument should normally be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.  On error, -1 is returned and
       errno is set to indicate the error.

ERRORS

       EFAULT One of tv or tz pointed outside the accessible address space.

       EINVAL (settimeofday()): timezone is invalid.

       EINVAL (settimeofday()): tv.tv_sec is negative or tv.tv_usec  is  outside  the  range  [0,
              999,999].

       EINVAL (since Linux 4.3)
              (settimeofday()):  An  attempt  was  made  to set the time to a value less than the
              current value of the CLOCK_MONOTONIC clock (see clock_gettime(2)).

       EPERM  The calling process has insufficient privilege to call settimeofday(); under  Linux
              the CAP_SYS_TIME capability is required.

STANDARDS

       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.