weak_ptr and thread safety
Guys, could you please clarify whether the following usage of weak_ptr is thread safe: //thread A: shared_ptr<Foo> p = wp.lock(); //thread B: p.reset(); I believe it's not thread safe as it looks very similar to the example 3 from the shared_ptr documentation: // thread A p = p3; // reads p3, writes p // thread B p3.reset(); // writes p3; undefined, simultaneous read/write Am I right in my assumption? And if yes, could you please give any recommendations for the following use case. I have a networking thread which owns a set of net session objects stored as shared_ptrs. I also have another thread where some entities have weak_ptrs members pointing to net session objects from networking thread. From time to time net sessions are erased and I think when at the very this moment entity object from another thread tries to lock() the net session and access it seg. fault happens. Thanks in advance. -- Best regards, Pavel
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Wednesday 22 July 2009, Pavel Shevaev wrote:
Guys, could you please clarify whether the following usage of weak_ptr is thread safe:
//thread A: shared_ptr<Foo> p = wp.lock();
//thread B: p.reset();
It's safe, presuming your "p" in thread A is a different shared_ptr object from the "p" reset in thread B. They can safely be copies sharing ownership of the same object.
I believe it's not thread safe as it looks very similar to the example 3 from the shared_ptr documentation:
// thread A p = p3; // reads p3, writes p
// thread B p3.reset(); // writes p3; undefined, simultaneous read/write
In this unsafe case, thread A and B are manipulating the same shared_ptr object. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEARECAAYFAkpnZ2gACgkQ5vihyNWuA4VUPACfXSYjZHiK9dkNnjAziUHFK5S9 KAAAnRYrm4HRILt05OBE/9iw2NaKJSqa =nfKj -----END PGP SIGNATURE-----
It's safe, presuming your "p" in thread A is a different shared_ptr object from the "p" reset in thread B. They can safely be copies sharing ownership of the same object.
Oh, sorry for the confusing example, it should be more clear: //thread A: shared_ptr<Foo> a = wp.lock(); //wp is a weak pointer for p from thread B //thread B: p.reset(); What I'm actually asking about whether it's thread safe to call simultaneously wp.lock() in one thread and p.reset() in another one. -- Best regards, Pavel
Pavel Shevaev wrote:
Guys, could you please clarify whether the following usage of weak_ptr is thread safe:
//thread A: shared_ptr<Foo> p = wp.lock();
//thread B: p.reset();
p is local to thread A, so B can't reset it. // thread B: wp.reset(); is not safe (A reads wp, B writes wp). // before A and B shared_ptr<X> p( new X ); weak_ptr<X> wp( p ); // A shared_ptr<X> p2 = wp.lock(); // B p.reset(); is safe (A reads wp, B writes p).
// before A and B shared_ptr<X> p( new X ); weak_ptr<X> wp( p );
// A shared_ptr<X> p2 = wp.lock();
// B p.reset();
is safe (A reads wp, B writes p).
This is my case, thanks for the clarification(what a relief, btw). Hm...guess at least it's not what causing my app to seg.fault :) -- Best regards, Pavel
Is Http Server 3 a good approach for this? http://www.boost.org/doc/libs/1_39_0/doc/html/boost_asio/examples.html#boost... I've got a server, that listens on a socket. It receives requests, and does CPU-bound work, then writes out a response. So I'd like it to handle N requests at a time, where N is the number of cores. HTTP Server 3 looks like it does this, but questions: - is this the best approach? - what (practically speaking) will happen to HTTP Server 3 if all of its threads are busy processing requests, and say 10 requests arrive. Will they fail? Will the O/S queue them up? - Alex
-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users-bounces@lists.boost.org] On Behalf Of Pavel Shevaev Sent: Wednesday, July 22, 2009 3:44 PM To: boost-users@lists.boost.org Subject: Re: [Boost-users] weak_ptr and thread safety
// before A and B shared_ptr<X> p( new X ); weak_ptr<X> wp( p );
// A shared_ptr<X> p2 = wp.lock();
// B p.reset();
is safe (A reads wp, B writes p).
This is my case, thanks for the clarification(what a relief, btw). Hm...guess at least it's not what causing my app to seg.fault :)
-- Best regards, Pavel _______________________________________________ Boost-users mailing list Boost-users@lists.boost.org http://lists.boost.org/mailman/listinfo.cgi/boost-users
participants (3)
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Alex Black -
Frank Mori Hess -
Pavel Shevaev -
Peter Dimov