adjtime − correct the time to synchronize the system clock
int adjtime(const struct timeval *delta, struct timeval *olddelta);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
The adjtime() function gradually adjusts the system clock (as returned by gettimeofday(2)). 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:
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.
On success, adjtime() returns 0. On failure, −1 is returned, and errno is set to indicate the error.
The adjustment in delta is outside the permitted range.
The caller does not have sufficient privilege to adjust the time. Under Linux the CAP_SYS_TIME capability is required.
For an explanation of the terms used in this section, see attributes(7).
4.3BSD, System V.
The adjustment that adjtime() makes to the clock is carried out in such a manner that the clock is always monotonically increasing. Using adjtime() 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 system 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 i386).
A longstanding bug meant that if delta was specified as NULL, no valid information about the outstanding clock adjustment was returned in olddelta. (In this circumstance, adjtime() should return the outstanding clock adjustment, without changing it.) This bug is fixed on systems with glibc 2.8 or later and Linux kernel 2.6.26 or later.
adjtimex(2), gettimeofday(2), time(7)
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