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Synchronization and Thread Safety

The presence of multiple threads in an application opens up potential issues regarding safe access to resources from multiple threads of execution. Two threads modifying the same resource might interfere with each other in unintended ways. For example, one thread might overwrite another’s changes or put the application into an unknown and potentially invalid state. If you are lucky, the corrupted resource might cause obvious performance problems or crashes that are relatively easy to track down and fix. If you are unlucky, however, the corruption may cause subtle errors that do not manifest themselves until much later, or the errors might require a significant overhaul of your underlying coding assumptions.

When it comes to thread safety, a good design is the best protection you have. Avoiding shared resources and minimizing the interactions between your threads makes it less likely for those threads to interfere with each other. A completely interference-free design is not always possible, however. In cases where your threads must interact, you need to use synchronization tools to ensure that when they interact, they do so safely.

Mac OS X provides numerous synchronization tools for you to use, ranging from tools that provide mutually exclusive access to those that sequence events correctly in your application. The following sections describe these tools and how you use them in your code to affect safe access to your program’s resources.

Contents:

Synchronization Tools
Synchronization and Performance
Thread Safety and Signals
Tips for Thread-Safe Designs
Using Atomic Operations
Using Locks
Using Conditions

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