Note: time zone information is no longer provided by this interface. See
localtime(3) for information on how to retrieve it.
The system's notion of the current UTC time is obtained with the
gettimeofday() call, and set with the
settimeofday() call. The time is expressed in seconds and microseconds since midnight (0 hour), January 1, 1970. The resolution of the system clock is hardware dependent, and the time may be updated continuously or in “ticks”.
If
tp is NULL, the time will not be returned or set. Despite being declared
void *, the objects pointed to by
tzp shall be of type
struct timezone.
The structures pointed to by
tp and
tzp are defined in
<sys/time.h>. The first one is described in
timeval(3) and the latter legacy structure is defined as:
struct timezone {
int tz_minuteswest; /* of Greenwich */
int tz_dsttime; /* type of dst correction to apply */
};
The
timezone structure is provided only for source compatibility. It is ignored by
settimeofday(), and
gettimeofday() will always return zeroes.
If the calling user is not the super-user, then the
settimeofday() function in the standard C library will try to use the
clockctl(4) device if present, thus making possible for non privileged users to set the system time. If
clockctl(4) is not present or not accessible, then
settimeofday() reverts to the
settimeofday() system call, which is restricted to the super user.