Share pointer between subprocesses

Question

I have a 64 bit application that creates 2 subprocesses (32 bit) via a popen2 implementation. Everything is written in C++.

I need the 2 subprocesses to access the same object in memory and I don't have a good idea about how to do this.

If I understand correctly, each subprocess will have a different memory map and therefore I can't just pass a memory address between the two.

Additional information: The target platform is Mac but I'm looking for an answer that is as platform independent as possible Mac specific answers are fine, I probably won't use this approach on other platforms. I simply don't know enough about using threads; I came down this route because the subprocesses must be 32 bit.

Answer 1

You can use shared memory concept. It means, that you allocate (using OS services) a memory, that will be visible by both subprocesses.

As wiki recoomends, you can use boost.interprocess to use shared memory on platform-independent level.

Answer 2

It's a difficult problem.

You are correct that each process has its own address space. Objects created by one process can't be accessed by another process.

It is possible to use shared memory, and place the objects there. One complication is that in general the shared memory segment will be mapped at different addresses into each process's address space. This means that you can't use pointers inside those object. This can be alleviated by working with indices instead of pointers.

Furthermore, if process A is 32-bit and process B is 64-bit, primitive types such as long can have different width. Thus when sharing data in such a scenario you need to use fixed-width types such as int32_t.

One final complication is synchronization: if a process can modify an object while another process is reading or modifying it, you'll need to introduces inter-process synchronization.