gettimeofday(2) - Linux manual page (original) (raw)

gettimeofday(2) System Calls Manual gettimeofday(2)

NAME top

   gettimeofday, settimeofday - get / set time

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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 top

   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)](../man2/time.2.html)).

   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   The use of the _timezone_ structure is obsolete; the _tz_ argument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 VERSIONS.

   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 _tzminuteswest_ field is nonzero.  (The
   _tzdsttime_ 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 top

   **gettimeofday**() and **settimeofday**() return 0 for success.  On error,
   -1 is returned and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   **EFAULT** One of _tv_ or _tz_ pointed outside the accessible address
          space.

   **EINVAL** (**settimeofday**()): _timezone_ is invalid.

   **EINVAL** (**settimeofday**()): _tv.tvsec_ is negative or _tv.tvusec_ 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)](../man2/clock%5Fgettime.2.html)).

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

VERSIONS top

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

   The kernel accepts NULL for both _tv_ and _tz_.  The timezone argument
   is ignored by glibc and musl, and not passed to/from the kernel.
   Android's bionic passes the timezone argument to/from the kernel,
   but Android does not update the kernel timezone based on the
   device timezone in Settings, so the kernel's timezone is typically
   UTC.

STANDARDS top

   **gettimeofday**()
          POSIX.1-2008 (obsolete).

   **settimeofday**()
          None.

HISTORY top

   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)](../man2/clock%5Fgettime.2.html) instead.

   Traditionally, the fields of _struct timeval_ were of type _long_.

The tz_dsttime field On a non-Linux kernel, with glibc, the tzdsttime 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 tzdsttime 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 _tzdsttime_ 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.

NOTES top

   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)](../man2/clock%5Fgettime.2.html).

   Macros for operating on _timeval_ structures are described in
   [timeradd(3)](../man3/timeradd.3.html).

SEE ALSO top

   [date(1)](../man1/date.1.html), [adjtimex(2)](../man2/adjtimex.2.html), [clock_gettime(2)](../man2/clock%5Fgettime.2.html), [time(2)](../man2/time.2.html), [ctime(3)](../man3/ctime.3.html),
   [ftime(3)](../man3/ftime.3.html), [timeradd(3)](../man3/timeradd.3.html), [capabilities(7)](../man7/capabilities.7.html), [time(7)](../man7/time.7.html), [vdso(7)](../man7/vdso.7.html),
   [hwclock(8)](../man8/hwclock.8.html)

COLOPHON top

   This page is part of the _man-pages_ (Linux kernel and C library
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   man-pages@man7.org

Linux man-pages 6.10 2024-07-23 gettimeofday(2)

Pages that refer to this page:adjtimex(2), alarm(2), clock_getres(2), getitimer(2), seccomp(2), stime(2), syscalls(2), time(2), timerfd_create(2), adjtime(3), ctime(3), difftime(3), ftime(3), pmdaeventarray(3), pmtimeval(3), pthread_cond_init(3), timeradd(3), timeval(3type), tzset(3), uuid_time(3), rtc(4), systemd.exec(5), capabilities(7), time(7), vdso(7), hwclock(8), mount(8)