Question

I am writing an Objective-C class that I want to be thread safe. To do this I am using pthreads and a pthread_rwlock (using @synchronized is overkill and I want to learn a bit more about pthreads). The lock is inited in the objects designated init method and destroyed in dealloc. I have three methods for manipulating the lock; readLock, writeLock, unlock. These three methods simply invoke the related pthread functions and currently nothing else.

Here are two of the objects methods, both of which require a writeLock:

-(void)addValue:(const void *)buffer
{
    [self writeLock];

    NSUInteger lastIndex = self.lastIndex;
    [self setValue:buffer atIndex:(lastIndex == NSNotFound) ? 0 : lastIndex+1];

    [self unlock];
}


-(void)setValue:(const void *)buffer atIndex:(NSUInteger)index
{
    [self writeLock];
    //do work here
    [self unlock];
}

Invoking setAddValue: will first obtain a write lock and then invoke setValue:atIndex: which will also attempt to obtain a write lock. The documentation states that the behaviour is undefined when this occurs. Therefore, how do I check if a thread has a lock before attempting to obtain a lock?

(I could ensure that critical section make no invocation that trigger another lock request, but that would mean code repetition and I want to keep my code DRY).

Answer 1

Not entirely clear what kind of lock you’re using. You indicate you’re using pthreads, and read/write lock, so I’m concluding that you’re using a pthread_rwlock.

If that’s true, then you should be able to use pthread_rwlock_trywrlock on the lock. From the man page,

If successful, the pthread_rwlock_wrlock() and pthread_rwlock_trywrlock()
functions will return zero. Otherwise, an error number will be returned
to indicate the error.

And, one of the errors is:

[EDEADLK] The calling thread already owns the read/write lock
(for reading or writing).

Therefore, I believe you should be able to call pthread_rwlock_trywrlock() and you will either be successful, it will return EBUSY if another thread has the lock, or you will get EDEADLK if the current thread has the lock.