[Boost-users] Re: Re: Semaphore and barriers for boost-threads?

From: Michael Glassford (glassfordm_at_[hidden])
Date: 2004-02-17 18:01:39

"Craig Rodrigues" <rodrigc_at_[hidden]> wrote in message
news:20040217214636.GA69703_at_crodrigues.org...
> On Mon, Feb 16, 2004 at 08:33:47PM -0500, Michael Glassford wrote:
> > The original designer of the Boost.Thread library and others felt
that
> > semaphores were too easily misused, leading to errors (see
> > http://boost.org/libs/thread/doc/rationale.html#events). Others
have
> > argued the issue on the various Boost mailing lists at some length
> > more than once; some have been convinced that the "too easily
misused"
> > argument has merit, others have not.
>
>
> OK, well I don't do much Windows programming, so I don't
> really understand what event variables are.  I'll have to do
> my homework and read up on them.

Sorry, I confused the semaphore debate with the event debate. What I
said and the reference I gave obviously applies to events, not
semaphores.

> In terms of explaining what a semaphore is,
> my frame of reference is the stuff in POSIX:
>
>
http://www.opengroup.org/onlinepubs/007904975/functions/sem_post.html
>
http://www.opengroup.org/onlinepubs/007904975/functions/sem_wait.html
>
http://www.opengroup.org/onlinepubs/007904975/functions/sem_trywait.html
>
http://www.opengroup.org/onlinepubs/007904975/functions/sem_timedwait.html
>
> Basically, a semaphore is initialized with a count.
> When you call sem_post(), the semaphore count is incremented.
> When you call sem_wait(), if the semaphore count is 0, then the
> thread blocks, else the semaphore count is decremented.

Yes, I have Butenhof's Programming with POSIX Threads as a reference
for this, though I haven't read it all yet.

> It is possible to implement semaphore's with mutexes and
> condition variables.

Possibly this is why they weren't included in Boost.Thread? I'll have
to do some research.

> I looked at the source code for
> FreeBSD 5.2's implementation of semaphores to see how this was
> done:
>
>
http://www.freebsd.org/cgi/cvsweb.cgi/~checkout~/src/lib/libpthread/thread/thr_sem.c
>
> I even took a whack at writing one based on Boost (it may be buggy):
>
> #include <boost/thread.hpp>
> #include <boost/thread/mutex.hpp>
>
> struct mysem {
>    mysem(unsigned int count): count_(count) {}
>
>    void post() {
>       boost::mutex::scoped_lock l(sem_mutex_);
>       ++count_;
>       sem_cond_.notify_all();
>    }
>
>    void wait() {
>       boost::mutex::scoped_lock l(sem_mutex_);
>       for(;;) {
>
>        if( count_ == 0 ) sem_cond_.wait(l);
>        else  { --count_; break; }
>       }
>    }
>
>   private:
>    unsigned int count_;
>    boost::mutex sem_mutex_;
>    boost::condition sem_cond_;
> };
>
>
> What kinds of things can go wrong if I use this class?
>
>
>
> > As far as adding a barrier class, however: one has existed in the
> > thread_dev branch in CVS for quite some time; I hope to move it to
the
> > main branch very soon.
>
> Cool, thanks!
>
> -- 
> Craig Rodrigues
> http://crodrigues.org
> rodrigc_at_[hidden]