New instance of class in a shared memory segment C++

Question

I'm actually trying to step-up in thread, process and shared memory in C++ but I'm actually stuck with my Inter Process Communication.

I decided to use a message queue cause i already used some in past project but never with multiple process.

Here is my problem, I can't get register events from different process. I think that initialize my message queue in a share memory segment would fix the problem but I can't figure out how to "force" the new instance of my class to be in my shared memory segment

Because I start with shared memory in C++, I use boost Simple Exemple

Ideally what i would like to have would be something like :

int main (void)
{
    shared_memory_object shm (open_or_create, "MySharedMemory", read_write);

    // initialise shm using boost Simple Exemple

    shmPtr = region.get_address();

    // initialise message queue named msgQueue in my shared memory segment

    msgQueuePtr = &msgQueue;

}

At the end, shmPtr would be equal to msgQueuePtr.
Any help would be appreciate.

The usual problem with shared memory is you can't store pointers as the address space of the two process will differ. So what's a valid pointer in one process will be an invalid pointer in the other. — Richard Critten, May 08 '20 at 16:45
Does this answer your question? [Create new C++ object at specific memory address?](https://stackoverflow.com/questions/1554774/create-new-c-object-at-specific-memory-address/1554796) — François Andrieux, May 08 '20 at 16:46
@MikeVine the point of Boost Interprocess is to hide those mechanics — sehe, May 08 '20 at 20:46
@FrançoisAndrieux it should, thanks for the link, because i'm a bit hurry (the end of my project is on sunday), i'll check that just after. — Dzious, May 09 '20 at 09:25

score 1 · Accepted Answer · answered May 08 '20 at 21:02

I would use a managed memory segment so you can "just" construct your types in shared memory (note: be sure they are POD or take a shared-memory allocator to properly handle any dynamic allocation/reference).

#include <boost/interprocess/managed_shared_memory.hpp>
#include <iostream>

struct MyQueue { /* */ };

namespace bip = boost::interprocess;

int main() {
    bip::managed_shared_memory msm(bip::open_or_create, "test", 10ul<<30); // 10 MiB
    MyQueue& q = *msm.find_or_construct<MyQueue>("queue_instance")();
}

Here's a bigger demo complete with a queue implementation + locking etc. Boost shared memory and synchronized queue issue/crash in consumer process

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Boost Interprocess has a complete message_queue implementation: https://www.boost.org/doc/libs/1_73_0/doc/html/boost/interprocess/message_queue.html

So, when in doubt, I suggest to use that.

I didn't know that boost has a message_queue already, thanks for the info, i'll use this one for the moment. I might come back later on (when my project will have ended) to give a try at the other method you gave me. Thanks a lot — Dzious, May 09 '20 at 09:28

New instance of class in a shared memory segment C++

1 Answers1

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