Re: [boost] Boost Threads Condition may block forever on clock change

From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2007-11-26 11:36:20

ak1mbox-boost_at_[hidden] wrote:
> One quick note, another cross OS library that
> implements a lot of threading primitives is APR
> (Apache Runtime Library). I have been using APR
> heavily for the last few month and my testing showed
> that APR does not have this problem of being affected
> by clock changes. I am not sure about their internal
> implementation so can't comment on differences between
> APR and BOOST condition but this might be something
> interesting to look at.
>   
The APR condition object uses relative timeouts. If you're interested,
the source is in locks/unix/thread_cond.c and locks/win32/thread_cond.c,
respectively. For the pthread condition object, the library simply adds
the offset to the current time to receive the end time.

    apr_time_t then;
    struct timespec abstime;

    then = apr_time_now() + timeout;
    abstime.tv_sec = apr_time_sec(then);
    abstime.tv_nsec = apr_time_usec(then) * 1000; /* nanoseconds */

    rv = pthread_cond_timedwait(&cond->cond, &mutex->mutex, &abstime);

Because it uses relative timeouts, it's not subject to the clock change
problem on Win32. However, it also shows nicely how easy it is to get a
relative offset based on an absolute system. As mentioned, the opposite
is very difficult.

Sebastian Redl