Simply adding a mutex lock in every accessor wouldn't really make them
thread-safe. Consider:
if (my_message.has_foo()) {
DoSomething(my_message.foo());
}
This is not thread-safe if my_message can be modified in a separate thread
*even if* each accessor locked a mutex, because "foo" could be cleared in
another thread between the call to has_foo() and the call to foo().
Here's a more dangerous example:
for (int i = 0; i < my_message.foo_size(); i++) {
DoSomething(my_message.foo(i);
}
Here, if "foo" is cleared between the call to foo_size() and the call to
foo(i), then the program will crash. Having a mutex protecting each
individual accessor does not help.
Another problem is that strings are returned by reference. So even if the
accessor itself locks a mutex, the string's contents may be accessed after
the accessor returns, and if they are simultaneously modified in another
thread you'll get a crash.
What you really need to do is maintain your own mutex at a higher level.
Any thread which reads the message object needs to take out a reader lock
before it starts reading and hold that lock until it is completely done
accessing the object. Similarly, any thread writing to a message object
needs to take a writer lock on that mutex at a higher level.
The protobuf library can't really do anything to help here, since the
correct mutex usage depends entirely on your application code.
On Mon, Jun 29, 2009 at 10:01 AM, Jes <damealpi...@hotmail.com> wrote:
>
> Hi everybody,
>
> we are working on a distributed environment that uses PB, where
> different threads will access to the contents of messages that can be
> updated at any moment through the network.
>
> I wonder if there is an easy way to transform the (derived) Messages
> into a thread-safe version. Maybe the rough solution could be to
> include a Mutex in the Message class and a MutexLock on each method of
> the generated pb.h and pb.cc classes, but perhaps there are issues
> that can break the safety of this approach (such as existing friends
> or similar).
>
> Could you have any suggestion on this? :-)
>
> Thanks in advance!
>
> Jes
>
>
> >
>
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