Shared memory: what's the difference between the key and the id?

Question

When invoking ipcs -a, what is the difference between the key column and the id column?

Here is an example output for ip

Answer 1

See the documentation for shmget(2). The key is IPC_PRIVATE or 0 on creation. The shmid is the value returned for that specific shm segment so that other processes or system calls can reference the segment.