iOS 多线程与线程安全（二）

2024-03-24 16:20:22

iOS 中实现线程安全

　　实现线程安全可以通过加锁、单线程访问等方式。

　　iOS 中常用的几种锁：

　　　　　　os_unfair_lock

　　　　　　pthread_mutex

　　　　　 dispatch_semaphore

　　　　　　NSLock

　　　　　　NSRecursiveLock

　　　　　　NSCondition

NSConditionLock

　　　　　　@synchorized

单线程访问：

　　　　　　dispatch_queue(DISPATCH_QUEUE_SERIAL)，串行队列

　　　　　　dispatch_barrier_async

一、os_unfair_lock

#import  <os/lock.h>

static os_unfair_lock _unfailLock;
static int _osCount;
- (void)unfailrLock {
    _unfailLock = OS_UNFAIR_LOCK_INIT; // 初始化锁
    _osCount = 20;
    for (int i = 0; i < 20; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            [self unfailrLockBlock];
        });
    }
}

- (void)unfailrLockBlock {
    os_unfair_lock_lock(&_unfailLock); // 加锁
    sleep(1);
    _osCount--;
    NSLog(@"os_unfair_lock %ld", (long)_osCount);
    os_unfair_lock_unlock(&_unfailLock); // 解锁
}

二、pthread_mutex

static pthread_mutex_t _mutexLock;

static int _mutexCount;

- (void)pthreadMutex {

pthread_mutexattr_t attr;

pthread_mutexattr_init(&attr);

pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT); // PTHREAD_MUTEX_RECURSIVE 递归锁

pthread_mutex_init(&_mutexLock, &attr); // pthread_mutex_init(&_mutexLock, NULL);

_mutexCount = 20;

for (int i = 0; i < 20; i++) {

dispatch_async(dispatch_get_global_queue(0, 0), ^{

[self pthreadMutexBlock];

});

}

- (void)pthreadMutexBlock {

pthread_mutex_lock(&_mutexLock);

sleep(1);

_mutexCount--;

NSLog(@"pthread_mutex %ld", (long)_mutexCount);

pthread_mutex_unlock(&_mutexLock);

}

三、dispatch_semaphore，当信号量小于1的时候，线程进入休眠等待。

static dispatch_semaphore_t _semaphore;
static int _semaphoreCount;
- (void)semaphore {
    _semaphore = dispatch_semaphore_create(1);
    _semaphoreCount = 20;
    for (int i = 0; i < 20; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            [self semaphoreBlock];
        });
    }
}

- (void)semaphoreBlock {
    dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_FOREVER);
    sleep(1);
    _semaphoreCount--;
    NSLog(@"semaphore %ld", (long)_semaphoreCount);
    dispatch_semaphore_signal(_semaphore);
}

四、NSLock 对pthread_mutex 普通类型锁的封装，NSRecursiveLock pthread_mutex 递归类型锁的封装。递归锁，同一个线程可以对一个锁多次加锁。

static NSLock *_nslock;
static int _lockCount;
- (void)nslock {
    _nslock = [[NSLock alloc] init];
    _lockCount = 20;
    for (int i = 0; i < 20; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            [self nslockBlock];
        });
    }
}

- (void)nslockBlock {
    [_nslock lock];
    sleep(1);
    _lockCount--;
    NSLog(@"nslock %ld", (long)_lockCount);
    [_nslock unlock];
}

五、NSCondition、NSConditionLock 是对 pthread_mutex 的 condition 属性封装。

static NSCondition *_cond;
static int _condCount;
- (void)condition {
    _cond = [[NSCondition alloc] init];
    _condCount = 10;
    for (int i = 0; i < 15; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            [self reduce];
        });
    }

    for (int i = 0; i < 10; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            [self add];
        });
    }
}

- (void)reduce {
    [_cond lock];
    if (_condCount == 0) {
        [_cond wait];
    }
    sleep(1);
    _condCount--;
    NSLog(@"reduce %ld", (long)_condCount);
    [_cond unlock];
}

- (void)add {
    [_cond lock];
    if (_condCount >= 0) {
        [_cond signal];   // 一次singnal只能唤醒一个在等待condition的线程，如果想唤醒多个可使用[_cond broadcast];
    }
    sleep(1);
    _condCount++;
    NSLog(@"add %ld", (long)_condCount);
    [_cond unlock];
}

六、@synchorized，runtime 会给 synchorized 调用的每一个对象分配一个递归锁并存储在哈希表中，因此效率比较低不建议使用。

　　@synchorized(nil) 会加锁失败。

七、pthread_rwlock 用于文件读写操作的锁，pthread_rwlock_rdlock(&lock) 读操作加锁，临界区（被加锁的代码）并发执行，pthread_rwlock_wrlock(&lock)）写操作加锁，临界区只会串行执行

static pthread_rwlock_t _rwlock = PTHREAD_RWLOCK_INITIALIZER;
static int _rwlockCount;
- (void)pthreadRWLock {
    _rwlockCount = 10;
    for (int i = 0; i < 15; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            [self rwRead];
        });
    }

    for (int i = 0; i < 2; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            [self rwWrite];
        });
    }
}

- (void)rwRead {
    pthread_rwlock_rdlock(&_rwlock);
    sleep(1);
    NSLog(@"pthread_rw read %ld", (long)_rwlockCount);
    pthread_rwlock_unlock(&_rwlock);
}

- (void)rwWrite {
    pthread_rwlock_wrlock(&_rwlock);
    sleep(1);
    _rwlockCount++;
    NSLog(@"pthread_rw write %ld", (long)_rwlockCount);
    pthread_rwlock_unlock(&_rwlock);
}

输出
2020-06-08 22:46:01.137977+0800 多线程[7019:829191] pthread_rw read 10
    2020-06-08 22:46:01.137977+0800 多线程[7019:829189] pthread_rw read 10
    2020-06-08 22:46:01.137978+0800 多线程[7019:829194] pthread_rw read 10
    2020-06-08 22:46:01.137978+0800 多线程[7019:829190] pthread_rw read 10
    2020-06-08 22:46:01.137989+0800 多线程[7019:829195] pthread_rw read 10
    2020-06-08 22:46:01.137977+0800 多线程[7019:829188] pthread_rw read 10
    2020-06-08 22:46:01.138022+0800 多线程[7019:829256] pthread_rw read 10
    2020-06-08 22:46:01.138021+0800 多线程[7019:829252] pthread_rw read 10
    2020-06-08 22:46:01.138022+0800 多线程[7019:829257] pthread_rw read 10
    2020-06-08 22:46:01.138024+0800 多线程[7019:829255] pthread_rw read 10
    2020-06-08 22:46:01.138022+0800 多线程[7019:829254] pthread_rw read 10
    2020-06-08 22:46:01.138028+0800 多线程[7019:829253] pthread_rw read 10
    2020-06-08 22:46:01.138047+0800 多线程[7019:829258] pthread_rw read 10
    2020-06-08 22:46:01.138048+0800 多线程[7019:829259] pthread_rw read 10
    2020-06-08 22:46:01.138053+0800 多线程[7019:829260] pthread_rw read 10
    2020-06-08 22:46:02.143706+0800 多线程[7019:829261] pthread_rw write 11
    2020-06-08 22:46:03.144493+0800 多线程[7019:829262] pthread_rw write 12

七、atomic、自旋锁与互斥锁

atomic：原子性，当一个属性被标记为atomic的时候，get和set方法通过加锁的方法实现原子性，iOS手机上一般不使用atomic属性。

自旋锁：当线程获取不到锁的时候，线程处于忙等状态（类似while循环），cpu处于忙等状态。

互斥锁：当线程获取不到锁的时候，线程进入休眠状态，cpu 资源得到释放。目前 iOS 的锁基本都是互斥锁。

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