Re: [boost] Re: Boost.Threads: Do we need all those mutexes?

From: Peter Dimov (pdimov_at_[hidden])
Date: 2004-07-11 12:25:00

Howard Hinnant wrote:
>
> Speaking of which, I've never been happy with "t" in the
> ScopedTimedLock docs.  I've always felt it was ambiguous.  Does t
> represent an absolute time, or a change in time?  Both interpretations
> are useful.

It represents an absolute time. The main rationale for using absolute times
is condition::timed_wait, which needs to be called in a loop. A relative
time would need to be recalculated for each iteration. But relative times
are useful, too.

> In the Metrowerks::threads lib I have:
>
>      scoped_timed_lock(mutex_type& m, const universal_time& unv_time);
>      scoped_timed_lock(mutex_type& m, const elapsed_time& elps_time);
>
> Where universal_time and elapsed_time are simplistic structs (like the
> boost time), but represent absolute and delta times respectively. They
> can also be added and subtracted as makes sense (e.g. you can add a
> universal_time and an elapsed_time, or two elapsed_time's, but you
> can't add two universal_time's).  I took the Posix route and defined
> universal_time as the number of seconds and nanoseconds since midnight
> Jan. 1, 1970, UTC. The universal_time default constructor returns the
> current time.

One problem with the UTC time is that it can jump backwards, if the system
time changes. Maybe we should consider using (or adding) a monotonic time.