Access scope define the scope that shm/lock to be allow to acccess.
Can be accessed between threads in 1 process.
Can be accessed between child processes that forked by 1 mother process who init it.
Can be accessed in the whole system, owner/group/access-mode is needed, or default. This is the only scope that attach() is required.
Due to the memory layout in difference between processes, shared memory may be allocated at different base addresses, So no pointer can be stored if shared memory is gonna shared system wide.