iOS - 锁

OSSpinLock
自旋锁(过期了)，等待锁的线程会处于忙等（busy-wait）状态，一直占用着CPU资源
目前应不再安全，可能出现优先级反转问题
优先级低的线程1先进入，锁住。CPU把大量时间给优先级高的线程2，优先级高的线程2等线程1解锁，优先级低的线程获得不到CPU的时间，无法解锁。出现了假死锁。
#import
OSSpinLock lock = OS_SPINLOCK_INIT; // 需要全局 初始化一次
// 锁
OSSpinLockLock(&lock);
// 解锁
OSSpinLockUnLock(&lock);

// 尝试加锁 
if (OSSpinLockTry(&lock)) {
    OSSpinLockUnLock(&lock);
}

os_unfair_lock
iOS10以后
优化了OSSpinLock，等待os_unfair_lock锁的线程
#import
typedef struct os_unfair_lock_s {
uint32_t _os_unfair_lock_opaque;
} os_unfair_lock, *os_unfair_lock_t;

@property(assign, nonatomic) os_Unfair_lock lock;

os_unfair_lock_lock(&lock);
os_unfair_lock_trylock(&lock);
os_unfair_lock_unlock(&lock);

pthread_mutex
pthread开头都是跨平台的
mutex:互斥
互斥锁，等待锁的线程会处于休眠状态
#import
// pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
// 初始化属性
pthread_mutexattr_t attr;
pthread_mutextattr_init(&attr);
// PTHREAD_MUTEX_DEFAULT 默认锁
// 检测错误锁
// PTHREAD_MUTEX_RECURSIVE 递归锁
允许同一个线程对一把锁进行重复加锁
允许同一个线程先多次加锁，然后再解锁，加锁多少次，解锁多少次。
a线程锁了，b线程来了要等待
pthread_mutextattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
// 初始化锁
pthread_mutex_init(&mutex, &attr);
// 销毁属性
pthread_mutexattr_destroy(&attr);

pthread_mutex_lock(&mutex);
pthread_mutex_unlock(&mutex);

// 销毁锁 （dealloc）
phread_mutex_destroy(&mutex);

- (void)test {
    pthread_mutex_lock(&mutex);    
    NSLog(@“—0”);
    [self test1];

    pthread_mutex_unlock(&mutex);
}
- (void)test1 {
    pthread_mutex_lock(&mutex);    

    NSLog(@“—1”);

    pthread_mutex_unlock(&mutex);
}
死锁，解决办法：两把锁，test1里再用一把锁mutex2即可

---

- (void)test {
    pthread_mutex_lock(&mutex);    
    // 递归 使用PTHREAD_MUTEX_RECURSIVE
    static int count = 0;
    if (count < 5) {
        count++;
        [self test];
    }

    pthread_mutex_unlock(&mutex);
}
允许先多次加锁，然后再解锁，加锁多少次，解锁多少次。

---

// 条件。
@property(assign, nonatomic)pthread_cond_t cond;

// 初始化
 pthread_cond_init(&cond, NULL);

// 两条线程 先执行remove方法
[[[NSTread alloc] initWithTarget:self selector:@selector(remove) objcet: nil] start];
[[[NSTread alloc] initWithTarget:self selector:@selector(add) objcet: nil] start];

- (void)remove {
    pthread_mutex_lock(&mutex);
    if (self.data.count == 0) {
        // 等待继续执行的条件cond 把mutex锁放开
        // 收到可以继续执行的条件 mutex加锁
        pthread_cond_wait(&cond, &mutex);
    }
    
    [self.data removeLastObject];
    NSLog(@“删除了元素”);
    
    pthread_mutex_unlock(&mutex);
}

- (void)add {
    pthread_mutex_lock(&mutex);
    sleep(1);

    [self.data addObject:@“te”];
    NSLog(@“添加了元素”);
    
    // 通知等待的cond 可以继续执行了
    pthread_cond_signal(&cond)
    
    pthread_mutex_unlock(&mutex);
}

NSLock
对pthread_mutex普通锁的封装

遵守NSLocking两个协议
    lock和unlock两个方法

-(BOOL)tryLock; // 尝试加锁
-(BOOL)lockBeforeDate: (NSDate *)limit; // 相比tryLock有个等待的时间 会阻塞
    在这个时间之前，能等到锁，就给这个锁加锁 加锁成功 继续往下走
    在时间内，没等到锁，加锁失败，代码继续往前走
@property(strong, nonatomic) NSLock *lock;
self.lock = [[NSLock allock] init];

[self.lock lock];
[self.lock unlock];

NSRecursiveLock
对pthread_mutex递归锁锁的封装
NSRecursiveLock方法同NSLock

NSCondition
对pthread_cond_t的封装 有lock和unlock方法
-(void)wait;
-(BOOL)waitUntilDate:(NSDate *)limit;
-(void)signal;
-(void)broadcast;

NSConditionLock
条件锁
内部存储的条件之是1（初始化是多少）时候才能加锁
对pthread_cond_t和pthread_mutex_t的封装

@property(strong, nonatomic) NSConditionLock *conditionLock;

self.conditionLock = [[NSConditonLock allok] initWithCondition:1];

- (void)one {
    [self.conditionLock lockWhenCondition: 1];// 条件之为1才能加锁
    NSLog(@“11”);
    [self.conditionLock lockWhenCondition: 2];// 设置内部条件值为2 并解锁
}

- (void)two {
    [self.conditionLock lockWhenCondition: 3];// 条件之为2才能加锁
    NSLog(@“22”);
    [self.conditionLock unlock];    
}
- (void)three {
    [self.conditionLock lockWhenCondition: 3];// 条件之为2才能加锁
    NSLog(@“22”);
    [self.conditionLock unlock];    
}
效果：一定先执行one 再执行two 最后执行three

dispatch_semaphore
信号量 信号量的初始值，可以用来控制线程并发访问的最大数量
最大并发量
线程同步 信号量设置为1 DISPATCH_TIME_FOREVER 即可 保证只有一条线程执行
@property (strong, nonatomic) dispatch_semaphore_t semaphore;

self.semaphore = dispatch_semaphore_create(5); // 最大并发5

- (void)test {
    for (i = 0; i< 20; i++) {
        [[[NSThread alloc] initWithTarget: self selector@selector(run) object:nil] start];
    }
}

- (void)run {
    // 信号量的值 > 0，就让信号量的值减1，然后继续执行代码
    // 信号量的值 <= 0 就会休眠等待 具体等多久 看dispatch_semaphore_wait第二个参数
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    
    sleep(2);
    NSLog(@“%@“, [NSTread crrentThread]);
    
    // 刚执行到这里 信号量为0 
    // 信号量+1
    dispatch_semaphore_signal(self.semaphore);
}

---

// 每个方法都是独立一把锁
-(void)test {
    static dispatch_semaphore_t semaphore;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^ {
        semaphore = dispatch_semaphore_create(1);
    })

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    // 要加锁的代码        

    dispatch_semahore_signal(semaphore);
}

// 宏
#define SemaphoreBegin \
static dispatch_semaphore_t semaphore; \
static dispatch_once_t onceToken; \
dispatch_once(&onceToken, ^ { \
    semaphore = dispatch_semaphore_create(1); \
})\ 
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

#define SemaphoreEnd \
    dispatch_semahore_signal(semaphore);

-(void)test {
    SemaphoreBegin;

    // 要加锁的代码        

    SemaphoreEnd;
}

dispatch_queue(DISPATCH_QUEUE_SERIAL)
串行队列
@property(strong, nonatomic)dispatch_queue_t queue;

self.queue = dispatch_queue_create(“q”, DISPATCH_QUEUE_SERIAL);

- (void)saleTicket {
    dispatch_sync({self.queue, ^{
        NSLog(@“sale“);
    });
}

@synchronized
对prhead_mutex递归锁的封装
不推荐使用
{
@synchronized(self) {
[self run];
}
}

---

读写安全
1、dispatch_semaphore_t 信号量
读和写都给锁住了 读没必要锁住
2、

import

@property (assign, nonatomic)pthread_rwlock lock;

{

pthread_rwlock_init(&lock, NULL)    

[[[NSTread alloc] initWithTarget:self selector:@selector(read) objcet: nil] start];
[[[NSTread alloc] initWithTarget:self selector:@selector(write) objcet: nil] start];

}

-(void)read {
pthread_rwlock_rdlock(&lock);
NSLog(@“du”);
pthread_rwlock_unlock(&lock);
}

-(void)write {
pthread_rwlock_wrlock(&lock);
NSLog(@“xie”);
pthread_rwlock_unlock(&lock);
}

-(void)dealloc {
pthread_rwlock_dispose(&lock);
}

---

-(void)read {
dispatch_async(q, ^{
NSLog(@“du”);
})
}

-(void)write {
dispatch_barrier_async(q, ^ {
NSLog(@“xie”);
})
}