NSRecursiveLock

Inherits From:
NSObject

Conforms To:
NSLocking
NSObject (NSObject)

Declared In:
Foundation/NSLock.h

Class Description

NSRecursiveLock defines a lock that may be acquired multiple times by the same thread without causing a deadlock, a situation where a thread is permanently blocked waiting for itself to relinquish a lock. While the locking thread has one or more locks, all other threads are prevented from accessing the code protected by the lock. Here's an example where a recursive lock functions properly but other lock types would deadlock:

NSRecursiveLock  *theLock = [[NSRecursiveLock alloc] init];
...
[theLock lock];
/* lengthy operations involving global data */
[theLock lock];   /* possibly invoked in a subroutine */
...
[theLock unlock]; /* relinquishes most recent lock */
...
[theLock unlock]; /* relinquishes the first lock */

Unless theLock was an NSRecursiveLock, a deadlock condition would occur at the second lock message in the example above.

The NSConditionLock, NSLock, and NSRecursiveLock classes all implement the NSLocking protocol with various features and performance characteristics; see the other class descriptions for more information.

Adopted Protocols

NSLocking

- lock
- unlock

Method Types

Acquiring a lock

- lockBeforeDate:
- tryLock

Instance Methods

lockBeforeDate:

- (BOOL)lockBeforeDate:(NSDate *)limit

Attempts to acquire a lock before the date represented by limit. Returns YES if the lock is acquired within this time limit. Returns NO if the time limit expires before a lock can be acquired.

tryLock

- (BOOL)tryLock

Attempts to acquire a lock. Returns immediately with a value of YES if successful and NO otherwise.