ADJTIME(2) — SYSTEM CALLS

NAME

adjtime − correct the time to allow synchronization of the system clock

SYNOPSIS

#include <sys/time.h>

adjtime(delta, olddelta)
struct timeval ∗delta;
struct timeval ∗olddelta;

DESCRIPTION

adjtime adjusts the system’s notion of the current time, as returned by gettimeofday(2), advancing or retarding it by the amount of time specified in the struct timeval ∗delta.

The adjustment is effected by speeding up (if ∗delta is positive) or slowing down (if ∗delta is negative) the system’s clock by a fixed percentage, currently 10%. 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-zero, then the structure pointed to will contain, upon return, the number of microseconds still to be corrected from the earlier call.

The structures pointed to by delta and olddelta are defined in <sys/time.h> as:

struct timeval {
u_longtv_sec;/∗ seconds since Jan. 1, 1970 ∗/
longtv_usec;/∗ and microseconds ∗/
};

If olddelta is a NULL pointer, the corresponding information will not be returned.

This call may be used in 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.

Only the super-user may adjust the time of day.

The adjustment value will be silently rounded to the resolution of the system clock.

RETURN

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

ERRORS

The following error codes may be set in errno:

EFAULT delta or olddelta points outside the process’s allocated address space, or olddelta points to a region of the process’ allocated address space which is not writable.

EPERM The process’s effective user ID is not that of the super-user.

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