Thread safe static variables objective c

Question

Is there a way in objective C that I can define a static int that is thread safe?

for example if I have class called Session that has:

static unsigned int session_id = 1000;

- (int) generateSessionID{
        return session_id++;
}

I am constructing session objects from different threads, each session object should have a unique id.

Answer 1

If you're talking Cocoa, the mutex functionality there is provided by NSLock and NSRecursiveLock.

In order to properly protect non-atomic resource, you need these mutexes, lest multiple threads may try to change the data at the same time (leading to corruption) or use the data in a half-changed state (leading to invalid data).

Your code would look something like this:

static NSLock session_id_lock;
static unsigned int session_id = 1000;

- (int) generateSessionID{
    int new_id;
    [myLock lock];
    new_id = session_id++;
    [myLock unlock];
    return new_id;
}

If you're not using Cocoa (or what little Cocoa programming I remember from my brief interlude with an iMac is so dimly remembered that it's near useless), just use the concept, translating it to whatever language or framework you have:

• lock the mutex before using or changing a protected resource.
• use or change the resource.
• unlock the mutex.
• bonus advice 1: lock the mutex as late as possible and unlock it as soon as possible.
• bonus advice 2: only lock what you need so you avoid unnecessary delays.

Explaining that last point some more: if you synchronise on self for two totally unrelated things (say a session ID and a user ID), they will block each other despite the fact that it's not necessary to do so. I would prefer two separate mutexes to keep the granularity low.

Of course, if you only have a mutex on the session ID alone (but see below for caveat), feel free to use synchronized(self) but I'd prefer to do it my way so I wouldn't get caught out adding another protected resource later.

In any case (this is the caveat mentioned), you will probably find that synchronising on self would not adequately protect a static variable, which would be shared across multiple objects. The mutex should belong to the data rather than whatever is using it.

Answer 2

I think you're better off using atomic operations to modify session_id. A previous question talks about atomic increment/decrement operations for OS X, and this page talks about the OSAtomic header file. Atomic operations on integers, something that's readily hardware-supported, will probably be substantially faster than using locking constructs.