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Threading

Objective-C provides support for thread synchronization and exception handling, which are explained in this article and “Exception Handling.” To turn on support for these features, use the -fobjc-exceptions switch of the GNU Compiler Collection (GCC) version 3.3 and later.

Synchronizing Thread Execution

Objective-C supports multithreading in applications. This means that two threads can try to modify the same object at the same time, a situation that can cause serious problems in a program. To protect sections of code from being executed by more than one thread at a time, Objective-C provides the @synchronized() directive.

The @synchronized()directive locks a section of code for use by a single thread. Other threads are blocked until the thread exits the protected code; that is, when execution continues past the last statement in the @synchronized() block.

The @synchronized() directive takes as its only argument any Objective-C object, including self. This object is known as a mutual exclusion semaphore or mutex. It allows a thread to lock a section of code to prevent its use by other threads. You should use separate semaphores to protect different critical sections of a program. It’s safest to create all the mutual exclusion objects before the application becomes multithreaded to avoid race conditions.

Listing 11-1 shows an example of code that uses self as the mutex to synchronize access to the instance methods of the current object. You can take a similar approach to synchronize the class methods of the associated class, using the Class object instead of self. In the latter case, of course, only one thread at a time is allowed to execute a class method because there is only one class object that is shared by all callers.

Listing 11-1  Locking a method using self

- (void)criticalMethod

{

    @synchronized(self) {

        // Critical code.

        ...

    }

}

Listing 11-2 shows a general approach. Before executing a critical process, the code obtains a semaphore from the Account class and uses it to lock the critical section. The Account class could create the semaphore in its initialize method.

Listing 11-2  Locking a method using a custom semaphore

Account *account = [Account accountFromString:[accountField stringValue]];

// Get the semaphore.

id accountSemaphore = [Account semaphore];

@synchronized(accountSemaphore) {

    // Critical code.

    ...

}

The Objective-C synchronization feature supports recursive and reentrant code. A thread can use a single semaphore several times in a recursive manner; other threads are blocked from using it until the thread releases all the locks obtained with it; that is, every @synchronized() block is exited normally or through an exception.

When code in an @synchronized() block throws an exception, the Objective-C runtime catches the exception, releases the semaphore (so that the protected code can be executed by other threads), and re-throws the exception to the next exception handler.

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