shm::shmget_id!

macro_rules! shmget_id {
    ($key: expr, $size: expr) => ({
        match shmget! (
            $key,
            0o0666 | shm::ffi::Ipc::CREAT as i32
                   | shm::ffi::Ipc::EXCL as i32,
            $size
        ) {
            Some(id) => Some(id),
            None => shmget! (
                $key,
                0o0666,
                $size
            ),
        }
    });
}

The shmget_id returns and creates or gets the memory segment.