ADJTIME(3) Linux Programmer's Manual ADJTIME(3)NAMEadjtime - correct the time to synchronise the system clock
SYNOPSIS
#define _BSD_SOURCE
int adjtime(const struct timeval *delta, struct timeval *olddelta);
DESCRIPTION
The adjtime() function gradually adjusts the system clock (as returned
by gettimeofday()). The amount of time by which the clock is to be
adjusted is specified in the structure pointed to by delta. This
structure has the following form:
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
If the adjustment in delta is positive, then the system clock is
speeded up by some small percentage (i.e., by adding a small amount of
time to the clock value in each second) until the adjustment has been
completed. If the adjustment in delta is negative, then the clock is
slowed down in a similar fashion.
If a clock adjustment from an earlier adjtime() call is already in
progress at the time of a later adjtime() call, and delta is not NULL
for the later call, then the earlier adjustment is stopped, but any
already completed part of that adjustment is not undone.
If olddelta is not NULL, then the buffer that it points to is used to
return the amount of time remaining from any previous adjustment that
has not yet been completed.
NOTES
The adjustment that adjtime() makes to the clock is carried out in such
a manner that the clock is always monotonically increasing. Using adj‐
time() to adjust the time prevents the problems that can be caused for
certain applications (e.g., make(1)) by abrupt positive or negative
jumps in the system time.
adjtime() is intended to be used to make small adjustments to the sys‐
tem time. Most systems impose a limit on the adjustment that can be
specified in delta. In the glibc implementation, delta must be less
than or equal to (INT_MAX / 1000000 - 2) and greater than or equal to
(INT_MIN / 1000000 + 2) (respectively 2145 and -2145 seconds on x86).
RETURN VALUE
On success, adjtime() returns 0. On failure, -1 is returned, and errno
is set to indicate the error.
ERRORS
EINVAL The adjustment in delta is outside the permitted range.
EPERM The caller does not have sufficient privilege to adjust the
time. Under Linux the CAP_SYS_TIME capability is required.
CONFORMING TO
4.3BSD, System V.
BUGS
Currently, if delta is specified as NULL, no valid information about
the outstanding clock adjustment is returned in olddelta. (In this
circumstance, adjtime() should return the outstanding clock adjustment,
without changing it.) This is the result of a kernel limitation.
SEE ALSOadjtimex(2), gettimeofday(2), time(7)Linux 2.6.16 2006-05-01 ADJTIME(3)
