Locks | BYU CocoaHeads

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When dealing with concurrency on the Mac and iPhone, there are some nice classes available, such as NSLock, NSConditionLock and NSRecursiveLock. However, if you need to handle a producer-consumer model with multiple consumers of the same resource, then these locks by themselves won't quite suffice.

Below is the code for a counting semaphore. It is compliant with the NSLocking protocol and is implemented with two mutexes (binary semaphores) and an NSInteger. It will work in a garbage-collected environment.

//CHCountingLock.h
#import <Foundation/Foundation.h>
#import <pthread.h>
 
@interface CHCountingLock : NSObject <NSLocking> {
	pthread_mutex_t s1;
	pthread_mutex_t s2;
	NSInteger count;
}
 
- (id) initWithCount:(NSInteger)initialCount;
- (BOOL) tryLock;
 
@end
 
//CHCountingLock.m
#import "CHCountingLock.h"
 
@implementation CHCountingLock
 
- (id) init {
	return [self initWithCount:0];
}
 
- (id) initWithCount:(NSInteger)initialCount {
	if (self = [super init]) {
		pthread_mutex_init(&s1, NULL);
		pthread_mutex_init(&s2, NULL);
		pthread_mutex_lock(&s2);
		count = initialCount;
	}
	return self;
}
 
- (void) dealloc {
	pthread_mutex_destroy(&s1);
	pthread_mutex_destroy(&s2);
	[super dealloc];
}
 
- (void) finalize {
	pthread_mutex_destroy(&s1);
	pthread_mutex_destroy(&s2);
	[super finalize];
}
 
- (void) lock {
	pthread_mutex_lock(&s1);
	count--;
	if (count < 0) {
		pthread_mutex_unlock(&s1);
		pthread_mutex_lock(&s2);
	}
	pthread_mutex_unlock(&s1);
}
 
- (void) unlock {
	pthread_mutex_lock(&s1);
	count++;
	if (count <= 0) {
		pthread_mutex_unlock(&s2);
	} else {
		pthread_mutex_unlock(&s1);
	}
}
 
- (BOOL) tryLock {
	if (pthread_mutex_trylock(&s1) == 0) {
		count--;
		if (count < 0) {
			pthread_mutex_unlock(&s1);
			pthread_mutex_lock(&s2);
		}
		pthread_mutex_unlock(&s1);
		return YES;
	}
	return NO;
}
 
@end

Below is the code for a read/write lock. It is compliant with the NSLocking protocol and is a simple wrapper around a pthread_rwlock_t. It will work in a garbage-collected environment.

//CHReadWriteLock.h
#import <Foundation/Foundation.h>
#import <pthread.h>
 
@interface CHReadWriteLock : NSObject <NSLocking> {
	pthread_rwlock_t lock;
}
 
- (void) lockForWriting;
- (BOOL) tryLock;
- (BOOL) tryLockForWriting;
 
@end
 
//CHReadWriteLock.m
#import "CHReadWriteLock.h"
 
 
@implementation CHReadWriteLock
 
- (id) init {
	if (self = [super init]) {
		pthread_rwlock_init(&lock, NULL);
	}
	return self;
}
 
- (void) dealloc {
	pthread_rwlock_destroy(&lock);
	[super dealloc];
}
 
- (void) finalize {
	pthread_rwlock_destroy(&lock);
	[super finalize];
}
 
- (void) lock {
	pthread_rwlock_rdlock(&lock);
}
 
- (void) unlock {
	pthread_rwlock_unlock(&lock);
}
 
- (void) lockForWriting {
	pthread_rwlock_wrlock(&lock);
}
 
- (BOOL) tryLock {
	return (pthread_rwlock_tryrdlock(&lock) == 0);
}
 
- (BOOL) tryLockForWriting {
	return (pthread_rwlock_trywrlock(&lock) == 0);
}
 
@end

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