NAME

       clock_gettime, clock_settime, clock_getres — get/set/calibrate date and time

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <time.h>

       int
       clock_gettime(clockid_t clock_id, struct timespec *tp);

       int
       clock_settime(clockid_t clock_id, const struct timespec *tp);

       int
       clock_getres(clockid_t clock_id, struct timespec *tp);

DESCRIPTION

       The  clock_gettime()  and  clock_settime()  system calls allow the calling process to retrieve or set the
       value used by a clock which is specified by clock_id.

       The clock_id argument can be a value obtained from clock_getcpuclockid(3) or pthread_getcpuclockid(3)  as
       well as the following values:

       CLOCK_REALTIME
       CLOCK_REALTIME_PRECISE
       CLOCK_REALTIME_FAST
               Increments as a wall clock should.
       CLOCK_MONOTONIC
       CLOCK_MONOTONIC_PRECISE
       CLOCK_MONOTONIC_FAST
               Increments in SI seconds.
       CLOCK_UPTIME
       CLOCK_UPTIME_PRECISE
       CLOCK_UPTIME_FAST
               Starts at zero when the kernel boots and increments monotonically in SI seconds while the machine
               is running.
       CLOCK_VIRTUAL
               Increments only when the CPU is running in user mode on behalf of the calling process.
       CLOCK_PROF
               Increments when the CPU is running in user or kernel mode.
       CLOCK_SECOND
               Returns  the  current  second  without  performing  a full time counter query, using an in-kernel
               cached value of the current second.
       CLOCK_PROCESS_CPUTIME_ID
               Returns the execution time of the calling process.
       CLOCK_THREAD_CPUTIME_ID
               Returns the execution time of the calling thread.

       The clock IDs CLOCK_REALTIME_FAST, CLOCK_MONOTONIC_FAST, CLOCK_UPTIME_FAST are analogs  of  corresponding
       IDs  without  _FAST  suffix  but do not perform a full time counter query, so their accuracy is one timer
       tick.  Similarly, CLOCK_REALTIME_PRECISE, CLOCK_MONOTONIC_PRECISE, CLOCK_UPTIME_PRECISE are used  to  get
       the most exact value as possible, at the expense of execution time.

       The structure pointed to by tp is defined in <sys/timespec.h> as:

       struct timespec {
               time_t  tv_sec;         /* seconds */
               long    tv_nsec;        /* and nanoseconds */
       };

       Only  the  super-user  may  set the time of day, using only CLOCK_REALTIME.  If the system securelevel is
       greater than 1 (see init(8)), the time may only be advanced.  This limitation is  imposed  to  prevent  a
       malicious  super-user from setting arbitrary time stamps on files.  The system time can still be adjusted
       backwards using the adjtime(2) system call even when the system is secure.

       The resolution (granularity) of a clock is returned by the clock_getres() system  call.   This  value  is
       placed in a (non-NULL) *tp.

RETURN VALUES

       Upon  successful  completion,  the value 0 is returned; otherwise the value -1 is returned and the global
       variable errno is set to indicate the error.

ERRORS

       The following error codes may be set in errno:

       [EINVAL]           The clock_id or timespec argument was not a valid value.

       [EPERM]            A user other than the super-user attempted to set the time.

SEE ALSO

       date(1), adjtime(2), clock_getcpuclockid(3), ctime(3), pthread_getcpuclockid(3), timed(8)

STANDARDS

       The clock_gettime(), clock_settime(), and clock_getres() system calls conform to  IEEE  Std  1003.1b-1993
       (“POSIX.1”).    The   clock   IDs   CLOCK_REALTIME_FAST,   CLOCK_REALTIME_PRECISE,  CLOCK_MONOTONIC_FAST,
       CLOCK_MONOTONIC_PRECISE, CLOCK_UPTIME, CLOCK_UPTIME_FAST, CLOCK_UPTIME_PRECISE, CLOCK_SECOND are  FreeBSD
       extensions to the POSIX interface.

HISTORY

       The clock_gettime(), clock_settime(), and clock_getres() system calls first appeared in FreeBSD 3.0.

Debian                                          December 7, 2019                                CLOCK_GETTIME(2)