What is the lifetime of shared memory on Linux

Question

I am creating, writing to and reading from a shared segment on Linux using the ftok/shmget/shmat/shmdt functions.

If I write to the segment in one program which then exits, and subsequently read the segment from another program later, I am surprised to find the data still there.

I would have expected that when the last process sharing a segment does a shmdt, the segment would be free'd.

Can I rely on this behavior? Or is it analogous to continuing to use a pointer after free()'ing it?

Answer 1

The shared memory area remains until it is deleted via shmctl(shmid,IPC_RMID,...) [or system reboot]. This will take effect after every process has done shmdt [or terminated].

The semantics are very similar to a file. shmat is like open. shmdt is like close and the [above] shmctl is like unlink.

If you close a file, you'd like it to persist until specifically deleted, wouldn't you? So, shared memory segments operate similarly