adjtime - correct the time to synchronize the system clock
#include <sys/time.h>
int adjtime(const struct timeval *delta, struct timeval *olddelta);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
adjtime():
Since glibc 2.19:
_DEFAULT_SOURCE
Glibc 2.19 and earlier:
_BSD_SOURCE
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:
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.
On success,
adjtime() returns 0. On failure, -1 is returned, and
errno is set to indicate the error.
- 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.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| adjtime () |
Thread safety |
MT-Safe |
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)