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21.4.2 High-Resolution Calendar

The time_t data type used to represent simple times has a resolution of only one second. Some applications need more precision.

So, the GNU C Library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in sys/time.h.

— Data Type: struct timezone

The struct timezone structure is used to hold minimal information about the local time zone. It has the following members:

int tz_minuteswest
This is the number of minutes west of UTC.
int tz_dsttime
If nonzero, Daylight Saving Time applies during some part of the year.

The struct timezone type is obsolete and should never be used. Instead, use the facilities described in Time Zone Functions.

— Function: int gettimeofday (struct timeval *tp, struct timezone *tzp)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The gettimeofday function returns the current calendar time as the elapsed time since the epoch in the struct timeval structure indicated by tp. (see Elapsed Time for a description of struct timeval). Information about the time zone is returned in the structure pointed at tzp. If the tzp argument is a null pointer, time zone information is ignored.

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

ENOSYS
The operating system does not support getting time zone information, and tzp is not a null pointer. GNU systems do not support using struct timezone to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in Time Zone Functions.

— Function: int settimeofday (const struct timeval *tp, const struct timezone *tzp)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

The settimeofday function sets the current calendar time in the system clock according to the arguments. As for gettimeofday, the calendar time is represented as the elapsed time since the epoch. As for gettimeofday, time zone information is ignored if tzp is a null pointer.

You must be a privileged user in order to use settimeofday.

Some kernels automatically set the system clock from some source such as a hardware clock when they start up. Others, including Linux, place the system clock in an “invalid” state (in which attempts to read the clock fail). A call of stime removes the system clock from an invalid state, and system startup scripts typically run a program that calls stime.

settimeofday causes a sudden jump forwards or backwards, which can cause a variety of problems in a system. Use adjtime (below) to make a smooth transition from one time to another by temporarily speeding up or slowing down the clock.

With a Linux kernel, adjtimex does the same thing and can also make permanent changes to the speed of the system clock so it doesn't need to be corrected as often.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EPERM
This process cannot set the clock because it is not privileged.
ENOSYS
The operating system does not support setting time zone information, and tzp is not a null pointer.

— Function: int adjtime (const struct timeval *delta, struct timeval *olddelta)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

This function speeds up or slows down the system clock in order to make a gradual adjustment. This ensures that the calendar time reported by the system clock is always monotonically increasing, which might not happen if you simply set the clock.

The delta argument specifies a relative adjustment to be made to the clock time. If negative, the system clock is slowed down for a while until it has lost this much elapsed time. If positive, the system clock is speeded up for a while.

If the olddelta argument is not a null pointer, the adjtime function returns information about any previous time adjustment that has not yet completed.

This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it.

With a Linux kernel, you can use the adjtimex function to permanently change the clock speed.

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

EPERM
You do not have privilege to set the time.

Portability Note: The gettimeofday, settimeofday, and adjtime functions are derived from BSD.

Symbols for the following function are declared in sys/timex.h.

— Function: int adjtimex (struct timex *timex)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

adjtimex is functionally identical to ntp_adjtime. See High Accuracy Clock.

This function is present only with a Linux kernel.