[Boost-users] [Interprocess]: using condition variables with readers-writer lock (named_sharable_mutex)

[Chris Evans]

Hi,

I am trying to use condition variables to signify updated data in a managed_shared_memory segment. I have one "writer" and multiple "readers" of the shared state, so I am using a readers-writer lock as follows:

using SharedMutex           = bi::named_sharable_mutex;
using ReadLock              = bi::sharable_lock<SharedMutex>;
using WriteLock             = bi::scoped_lock<SharedMutex>;
using NewEntryCondition     = bi::named_condition_any;

// With a "writer" block as follows:

{
    WriteLock w_lock( sh_mut_ );

    // Update the shared memory object

    sh_cond_.notify_all();
}

// And a "reader" block as follows (many processes call the following simultaneously):

{
    ReadLock r_lock( sh_mut_ );

    sh_cond_.wait(
            r_lock,
            []() {

                std::cout << "checking predicate..." << std::endl;

                // predicate logic goes here...

            });

    // Access object in shared memory here

}


Unfortunately, although the code compiles, the reader processes are never awakened from wait() and block forever.

According to the following thread from 2011:

http://lists.boost.org/boost-users/2011/02/66053.php

"POSIX does not support condition
variables with read-write locks (although I think Windows Vista does) so
Interprocess does not offer any support for this."

At the time, it appears named_condition_any didn't yet exist. Has the status of this changed in the last 5 years? Is there a suggested way of implementing readers-writer locks with condition variables using boost::interprocess?

Thanks!

--

Chris Evans
Systems Engineer
Azure Summit Technology, Inc.
3050 Chain Bridge Road, Suite 600
Fairfax, VA 22030

[Ion Gaztañaga]

El 20/09/2016 a las 16:42, Chris Evans escribió:
> Hi,
>
> I am trying to use condition variables to signify updated data in a
> managed_shared_memory segment. I have one "writer" and multiple
> "readers" of the shared state, so I am using a readers-writer lock as
> follows:
>

> ...

> Unfortunately, although the code compiles, the reader processes are
> never awakened from wait() and block forever.

Looks like a bug. named_condition_any should work with
named_sharable_mutex. Please open a ticket.

Best,

Ion
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[Chris Evans]

Thanks for the suggestion. For anyone interested: I've opened a ticket and included a small example demonstrating the behavior.

https://svn.boost.org/trac/boost/ticket/12476