Question

I have a class which opens a posix shared memory using shm_open. Now I have to create objects of this class multiple times (5-10 objects total), and that means the same shared memory is going to be opened that many times.

The shared memory has variable size, and can be from 1k to 500k.

The questions regarding this are :
1. Is this going to cause increased memory usage? (I do not see this happening, so my guess is no)
2. Is this going to cause bad performances when accessing shared memory?
3. Anything else that can go wrong?

Answer 1

The answer is on the page you linked shm_open to:

shm_open() creates and opens a new, or opens an existing, POSIX shared memory object. A POSIX shared memory object is in effect a handle which can be used by unrelated processes to mmap(2) the same region of shared memory.

To access a shared memory object, you need a handle to it. shm_open() can be used both to open (create) new, and reopen existing shared memory object. No memory increase nor bad performances occur when reopening an existing object.