Using IPC_CREAT with an already created shared memory segment

Question

I am trying to figure out what will happen if I use the IPC_CREAT flag with shmget(). I have used a key of an already created shared memory segment from another process. When I did so, the calling process has shared actually this memory segment with the old process ( For sure I have attached this memory segment using shmat()). So can I conclude that if I use IPC_CREAT with a shared memory segment that is already created the effect is that this memory will be shared with the calling process ?

Answer 1

Yes, that's the idea. You use IPC_CREAT to allow the creation, not to force it. To force creation (and failure if the key already exists), IPC_CREAT | IPC_EXCL.