Is it possible for two processes to use the same shared-memory without resorting to a file to obtain it, be it a memory-mapped file or /dev/shm file?

Question

I'm curious because today the only way I know how to give two different processes the same shared-memory is through a memory-mapped file, in other words, both processes open the same memory-mapped file and write/read to/from it.

That has penalties / drawbacks as the operating system needs to swap between disk and memory.

Apologies in advance if that is a silly question, but is there such a thing as a pure shared memory between processes, not backed by a file. If yes, how would the processes get a hold of it if not using a memory-mapped file or /dev/shm file?

Answer 1

Apologies in advance if that is a silly question, but is there such a thing as a pure shared memory between processes, not backed by a file.

Not a silly question!

There is, it's the default way of getting it; (SYSV) shmget is the function you use to get these shared memory buffers. You assign a string name to it, a key, and another process can use that key with shmget to get access to the same. The POSIX way is shm_open with very similar semantics: you give your segment a name, which might look a lot like a file name, but isn't backed by some hard drive, and subsequent shm_open calls on the same name (with compatible/no flags) will grant access to the same memory.