CLOCK_SETTIME(2) | System Calls Manual | CLOCK_SETTIME(2)

# NAME

`clock_settime`, `clock_gettime`, `clock_getres` —
clock and timer functions

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <time.h>`

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

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

int
`clock_getres`(clockid_t clock_id, struct timespec *res);

# DESCRIPTION

The `clock_settime`() function sets the clock identified by clock_id to the absolute time specified by tp. If the time specified by tp is not a multiple of the resolution of the clock, tp is truncated to a multiple of the resolution.

The `clock_gettime`() function stores the time of the clock identified by clock_id into the location specified by tp.

The `clock_getres`() function stores the resolution of the clock identified by clock_id into the location specified by res, unless res is `NULL`.

The following clock_id values are supported:

`CLOCK_REALTIME`
  identifies the realtime clock for the system. For this clock, the values specified by `clock_settime`() and obtained by `clock_gettime`() represent the amount of time (in seconds and nanoseconds) since 00:00 Universal Coordinated Time, January 1, 1970.
`CLOCK_MONOTONIC`
  identifies a clock that increases at a steady rate (monotonically). This clock is not affected by calls to adjtime(2) and settimeofday(2) and will fail with an `EINVAL` error if it's the clock specified in a call to `clock_settime`(). The origin of the clock is unspecified.
`CLOCK_VIRTUAL`
  identifies a clock that increments only when the CPU is running in user mode on behalf of the calling process.
`CLOCK_PROF`
  identifies a clock that increments when the CPU is running in user or kernel mode on behalf of the calling process.
`CLOCK_PROCESS_CPUTIME_ID`
  identifies a per process clock based on tick values. This clock is not settable.
`CLOCK_THREAD_CPUTIME_ID`
  identifies a per thread clock based on tick values. This clock is not settable.

If the calling user is not the super-user, the `clock_settime`() system call will fail, and the `clock_settime`() function in the standard C library will try to use the clockctl(4) device if present, thus making it possible for non privileged users to set the system time. If clockctl(4) is not present or not accessible, then `clock_settime`() returns `EPERM`.

# RETURN VALUES

A value of 0 is returned on success. Otherwise, a value of -1 is returned and errno is set to indicate the error.

# ERRORS

The `clock_settime`(), `clock_gettime`() and `clock_getres`() functions will fail if:

[`EINVAL`]
  The clock_id argument does not specify a known clock.
[`ENOSYS`]
  The function is not supported by this implementation.

The `clock_settime`() function will fail if:

[`EINVAL`]
  The tp argument is outside the range for the specified clock, clock_id; or the tp argument specified a nanosecond value less than zero of greater than or equal 1000 million; or the clock_id argument is a clock that can not be adjusted.
[`EPERM`]
  The calling process does not have the appropriate privilege to set the specified clock, clock_id.

The `clock_gettime`() function will fail if:

[`EFAULT`]
  The tp argument specifies an address that is not a valid part of the process address space.

# SEE ALSO

ctime(3), time(3), clockctl(4)

# STANDARDS

The `clock_settime`(), `clock_gettime`() and `clock_getres`() functions conform to IEEE Std 1003.1b-1993 (“POSIX.1b”).

September 23, 2016 | NetBSD 9.5