递归锁 死锁。

1. synchronized
   static NSString* A = @"A";

   /** B锁 /
   static NSString B = @"B";
   dispatch_async(queue, ^{
   // NSLog(@"%@",[self sourceOut]) ;
   @synchronized(A){
   NSLog(@"锁A0");
   sleep(2);
   @synchronized(B){
   NSLog(@"锁B0");
   }
   }
   });

   dispatch_async(queue, ^{
   @synchronized(B){
   NSLog(@"锁B1");

        @synchronized(A){
            NSLog(@"锁A1");
        }
    }
   

   });
   打印：2018-04-06 15:35:56.206903+0800 COCOCOCO[13309:566143] 锁A0
   2018-04-06 15:35:56.206939+0800 COCOCOCO[13309:566145] 锁B1

2. NSLock
   [self.lock lock];
   [self.lock lock];//由于当前线程加锁，现在再次加同样的锁，需等待当前线程解锁，把当前线程挂起，不能解锁
   [_lock unlock];
   [_lock unlock];

3. 递归锁。 核心：允许同一个线程对一把锁进行重复加锁。 如果不是同一线程那么将死锁

• (void)__initMutexLock:(pthread_mutex_t *)mutex{
  // 递归锁：允许同一个线程对一把锁进行重复加锁

  // 初始化属性
  pthread_mutexattr_t attr;
  pthread_mutexattr_init(&attr);
  pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
  // 初始化锁
  pthread_mutex_init(mutex, &attr);
  // 销毁属性
  pthread_mutexattr_destroy(&attr);
  }

• (void)viewDidLoad {
  [super viewDidLoad];

  [self __initMutexLock:&_MutexLock];

  dispatch_async(dispatch_get_global_queue(0, 0), ^{
  pthread_mutex_lock(&_MutexLock);
  NSLog(@"加锁 %s =11111== %@",func, [NSThread currentThread]);

    dispatch_sync(dispatch_get_main_queue(), ^{
        pthread_mutex_lock(&_MutexLock);
        NSLog(@"加锁 %s   =22222== %@",__func__, [NSThread currentThread]);
        NSLog(@"解锁 %s   ==2222222= %@",__func__, [NSThread currentThread]);
        pthread_mutex_unlock(&_MutexLock);
    });
  
    NSLog(@"解锁 %s   ==11111111= %@",__func__, [NSThread currentThread]);
  
    pthread_mutex_unlock(&_MutexLock);
  

  });
  }

2020-12-30 19:34:27.331898+0800 iOS-LockDemo[19500:315177] 加锁 -[ViewController viewDidLoad]_block_invoke =11111== <NSThread: 0x600002d107c0>{number = 5, name = (null)}