synchronized->objc_sync_enter/objc_sync_exit
objc_sync_enter：递归锁，与objc_sync_exit配合使用，注意死锁
- Begin synchronizing on 'obj'. Allocates recursive pthread_mutex associated with 'obj' if needed.
- Parameters
obj
The object to begin synchronizing on. - Returns
OBJC_SYNC_SUCCESS once lock is acquired.
objc_sync_exit
- End synchronizing on 'obj'.
- Parameters
obj
The object to end synchronizing on. - Returns
OBJC_SYNC_SUCCESS or OBJC_SYNC_NOT_OWNING_THREAD_ERROR
示例如下：
let lockValue = 0
objc_sync_enter(lockValue)
//执行代码块
objc_sync_exit(lockValue)
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