Understanding the posix interprocess semaphore

Question

According to my understanding, a semaphore should be usable across related processes without it being placed in shared memory. If so, why does the following code deadlock?

Answer 1

An excellent article on this topic, for future passers-by:

http://blog.superpat.com/2010/07/14/semaphores-on-linux-sem_init-vs-sem_open/

Answer 2

The wording in the manual page is kind of ambiguous.

If pshared is nonzero, then the semaphore is shared between processes, and should be located in a region of shared memory.

Since a child created by fork(2) inherits its parent's memory mappings, it can also access the semaphore.

Yes, but it still has to be in a shared region. Otherwise the memory simply gets copied with the usual CoW and that's that.

You can solve this in at least two ways:

• Use sem_open("my_sem", ...)
• Use shm_open and mmap to create a shared region