synchronized到objc_sync_enter/obj

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)