Re: [Boost-users] [interprocess] shared_memory_object opened or created?

Boris wrote:

On Tue, 18 Mar 2008 21:23:39 +0100, Ion Gaztañaga wrote:

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If I instantiate a shared_memory_object with open_or_create how do I know if it was opened or created? I ask as if it was created I need to create a structure in the mapped region. If it was opened though I must not create a structure (as another program created the structure before and I might overwrite data).

I ran into this problem after creating a reader and writer. They exchange data using a shared_memory_object. It would be great if I could start the two programs in any order. But currently the programs don't know which one created and which one opened the shared memory. Do I miss anything or was open_or_create added for another use case?

Boris Since you must create an object to share between both, use find_or_construct<> on both processes (supposing both have enough information to create the shared structure) before doing anything. If

Boris wrote: the object is already created, the function will return a pointer to the already created structure.

Can I use find_or_construct<> also with the shared_memory_object? It seems like I must use managed memory segments then?

Yes. Underlying OS shared memory devices shm_open or similar have no way to tell you if you've created or open the device. Another alternative is to start creating and opening exclusively in a loop. In most cases you won't need to loop more than two times to open or create it with different calls. Imagine you start the server first. Even if I could add a mechanism to shared_memory_object had to tell if you've opened or created it, the client can open it just after the server has created it and the client must wait until the server. So you need some sort of synchronization even if you know who created the segment. Regards, Ion