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cunixshared-memoryftok

What is the point of having a key_t if what will be the key to access shared memory is the return value of shmget()?

When using shared memory, why should we care about creating a key

key_t ftok(const char *path, int id);

in the following bit of code?

key_t key;
int shmid;

key = ftok("/home/beej/somefile3", 'R');
shmid = shmget(key, 1024, 0644 | IPC_CREAT);

From what I've come to understand, what is needed to access a given shared memory is the shmid, not the key. Or am I wrong? If what we need is the shmid, what is the point in not just creating a random key every time?

Edit

@Beej's Guide to Unix IPC one can read:

What about this key nonsense? How do we create one? Well, since the type key_t is actually just a long, you can use any number you want. But what if you hard-code the number and some other unrelated program hardcodes the same number but wants another queue? The solution is to use the ftok() function which generates a key from two arguments.

Reading this, it gives me the impression that what one needs to attach to a shared-memory block is the key. But this isn't true, is it?

Solution

  • Yes, you need to use the shmid to access the shared memory (using shmat()) after you've opened it using shmget(). But the specific block of shared memory that you'll be accessing is based on the key that you are using i.e. different process wishing to communicate via the shm will need to use the same key. If you just used a random number as a key, you might get a clash with some other unrelated program.

    I was going to suggest taking a look at Beej's Guide to IPC but I see you've already found it :)