ADJTIME(2)

ADJTIME(2) | System Calls Manual | ADJTIME(2)

# NAME

`adjtime` —
correct the time to allow synchronization of the system clock

# LIBRARY

Standard C Library (libc, -lc)

# SYNOPSIS

`#include <sys/time.h>`

int
`adjtime`(const struct timeval *delta, struct timeval *olddelta);

# DESCRIPTION

`adjtime`() makes small adjustments to the system time, as returned by gettimeofday(2), advancing or retarding it by the time specified by the timeval delta. If delta is negative, the clock is slowed down by incrementing it more slowly than normal until the correction is complete. If delta is positive, a larger increment than normal is used. The skew used to perform the correction is generally a fraction of one percent. Thus, the time is always a monotonically increasing function. A time correction from an earlier call to `adjtime`() may not be finished when `adjtime`() is called again. If olddelta is non-nil, the structure pointed to will contain, upon return, the number of microseconds still to be corrected from the earlier call.

This call may be used by time servers that synchronize the clocks of computers in a local area network. Such time servers would slow down the clocks of some machines and speed up the clocks of others to bring them to the average network time.

If the calling user is not the super user, the `adjtime`() system call will fail, and the `adjtime`() 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 adjust the system time. If clockctl(4) is not present or not accessible, then `adjtime`() returns `EPERM`.

# RETURN VALUES

A return value of 0 indicates that the call succeeded. A return value of -1 indicates that an error occurred, and in this case an error code is stored in the global variable errno.

# ERRORS

`adjtime`() will fail if:

[`EFAULT`]
  An argument points outside the process's allocated address space.
[`EPERM`]
  The process's effective user ID is not that of the super user.

# SEE ALSO

date(1), gettimeofday(2), clockctl(4), timed(8), timedc(8)

R. Gusella and S. Zatti, TSP: The Time Synchronization Protocol for UNIX 4.3BSD.

# HISTORY

The `adjtime`() function call appeared in 4.3BSD.

December 8, 2015 | NetBSD 9.0