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What I am trying to do is have application A send application B a pointer to an object which A has allocated on shared memory ( using boost::interprocess ). For that pointer transfer I intend to use boost::interprocess::message_queue. Obviously a direct raw pointer from A is not valid in B so I try to transfer an offset_ptr allocated on the shared memory. However that also does not seem to work.

Process A does this:

typedef offset_ptr<MyVector> MyVectorPtr;
MyVectorPtr * myvector;    
myvector = segment->construct<MyVectorPtr>( boost::interprocess::anonymous_instance )();

*myvector = segment->construct<MyVector>( boost::interprocess::anonymous_instance )
        (*alloc_inst_vec); ;

// myvector gets filled with data here

//Send on the message queue
mq->send(myvector, sizeof(MyVectorPtr), 0);

Process B does this:

// Create a "buffer" on this side of the queue
MyVectorPtr * myvector; 
myvector = segment->construct<MyVectorPtr>( boost::interprocess::anonymous_instance )();
mq->receive( myvector, sizeof(MyVectorPtr), recvd_size, priority);

As I see it, in this way a do a bit copy of the offset pointer which invalidates him in process B. How do I do this right?

koopajah
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Yordan Pavlov
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2 Answers2

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It seems you can address it as described in this post on the boost mailing list.

Alp Mestanogullari
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I agree there is some awkwardness here and offset_ptr doesn't really work for what you are trying to do. offset_ptr is useful if the pointer itself is stored inside of another class/struct which also is allocated in your shared memory segment, but generally you have some top-level item which is not a member of some object allocated in shared memory.

You'll notice the offset_ptr example kindof glosses over this - it just has a comment "Communicate list to other processes" with no details. In some cases you may have a single named top-level object and that name can be how you communicate it, but if you have an arbitrary number of top-level objects to communicate, it seems like just sending the offset from the shared memory's base address is the best you can do.

You calculate the offset on the sending in, send it, and then add to the base adddress on the receiving end. If you want to be able to send nullptr as well, you could do like offset_ptr does and agree that 1 is an offset that is sufficiently unlikely to be used, or pick another unlikely sentinel value.

aggieNick02
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