[zeromq-dev] Perennial thread safety question

[Pieter Hintjens]

The scary text emerged as the best way to keep people from skimming
the docs, sharing sockets between threads, and getting crashes. Anyone
who knows what a full memory barrier is can migrate sockets. The rest
of us don't do that, and stick to simpler, safer patterns (each thread
creates the sockets it needs).

-Pieter


On Tue, Apr 9, 2013 at 12:04 AM, Matthew Woehlke
<matthew.woehlke at kitware.com> wrote:
> On 2013-04-08 16:12, Charles Remes wrote:
>> On Apr 8, 2013, at 12:40 PM, Matthew Woehlke wrote:
>>> Is it safe to create and set up, and tear down, my 0MQ socket in the
>>> ctor/dtor (i.e. in thread A, and poll/recv/etc. in thread B)? Is there
>>> something I should be doing to make it safe?
>>
>> This is answered in the FAQ (http://www.zeromq.org/area:faq). Short answer is: Yes, go ahead and do this. If it crashes, protect the socket with some kind of memory barrier (e.g. mutex).
>
> Ah... I think I may have seen that at some point. However I was
> specifically remembering reading this in the guide:
>
> "Don't share ØMQ sockets between threads. ØMQ sockets are not
> threadsafe. Technically it's possible to migrate a socket from one
> thread to another but it demands skill. The only place where it's
> remotely sane to share sockets between threads are in language bindings
> that need to do magic like garbage collection on sockets."
>
> ...which seems to contradict the FAQ. Maybe some less scary text should
> be used? (As I read the FAQ, it's perfectly safe to migrate a socket
> just as long as the migration is 'clean', e.g. memory gets synchronized
> between the last access by thread A and before the first access by
> thread B.)
>
> --
> Matthew
>
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