NSRecursiveLock.md

NSRecursiveLock

A recursive mutex implementation that allows the same thread to acquire the lock multiple times without deadlocking.

Base Class

NSLock

Instance Variables

mulle_atomic_pointer_t   _thread;  // Current owning thread
mulle_atomic_pointer_t   _depth;   // Lock recursion depth

Methods

All methods are marked with MULLE_OBJC_THREADSAFE_METHOD:

Locking Operations

• -lock - Acquires or re-acquires the lock, incrementing depth counter
• -unlock - Decrements depth counter, releases lock when depth reaches 0
• -tryLock - Attempts to acquire lock without blocking, returns YES if successful

Usage Example

NSRecursiveLock *lock = [NSRecursiveLock new];

// Same thread can acquire multiple times
[lock lock];
[lock lock];  // Increments depth counter
// ... critical section ...
[lock unlock];  // Decrements depth counter
[lock unlock];  // Releases lock

// Try lock without blocking
if ([lock tryLock])
{
   if ([lock tryLock])  // Can acquire again
   {
      // ... nested critical section ...
      [lock unlock];
   }
   [lock unlock];
}

Important Notes

1. Thread Safety

   • All methods are thread-safe
   • Only the owning thread can recursively lock
   • Must balance lock/unlock calls

2. Implementation Details

   • Uses atomic operations for thread and depth tracking
   • Inherits base mutex functionality from NSLock
   • Validates thread ownership in unlock operations