As an illustrative example from Linux we can take a part from a manual page. The method with other operating systems is pretty the same.
An application can ask shared memory with an other application from OS.
shmget - allocates a shared memory segment int shmget(key_t key, size_t size, int shmflg);

shmget() returns the identifier of the shared memory segment associated with the value of the argument key. A new shared memory segment, with size equal to the value of size rounded up to a multiple of PAGE_SIZE, is created if key has the value IPC_PRIVATE or key isn't IPC_PRIVATE, no shared memory segment corresponding to key exists, and IPC_CREAT is specified in shmflg.