Shared memory addresses

Question

I am using C on Linux, and allocating 2 shared memory segments.

The first segment is created in a main program, then I call a subprocess and create the second segment.

In the subprocess, I place the address of the second segment in a pointer I set aside in the first segment.

Upon returning to the main program, when I attach to the second segment and compare the pointers (the one returned from shmat, and the one previously stored by the subprocess) I find they are different.

Is this expected?

Thanks, Mark.

Answer 1

Yes, this is expected. Mapping to a common address in the virtual space of the two processes would be a very constraining limitation. Among others, the memory manager would have to know simultaneously which processes are willing to map, so that it finds a common free area. This would defeat the very principle of virtual memory (every process sees a blank address space), and cause configurations impossible to arbitrate.

Sharing at common addresses is indeed possible, but only makes sense when the mapping is to some reserved section of the address space, so that it doesn't get mapped elsehow.