C++多线程的错误和如何避免（7）

要以相同顺序获取多个锁

多线程在加锁解锁时，可能会出现死锁问题，比如，

线程 1 在加锁 mutex A 后，继续尝试获取 mutex B，而 mutex B 已经被线程 2 获取，而线程 2 在等待获取 mutex A，mutex B 只有线程 2 获取 mutex  A 后才能解锁，

这就导致线程 1 和线程 2 互相等待锁，而这一操作就是死锁情况，容易导致程序挂起。

 使用代码模拟死锁场景：

scoped_lock is a mutex wrapper that provides a convenient RAII-style mechanism for owning one or more mutexes for the duration of a scoped block.

When a scoped_lock object is created, it attempts to take ownership of the mutexes it is given. When control leaves the scope in which the scoped_lock object was created, the scoped_lock is destructed and the mutexes are released. If several mutexes are given, deadlock avoidance algorithm is used as if by std::lock.

或者我们也可以用 std::timed_mutex 来解决这个问题，当超过一定时间后自动释放锁

The timed_mutex class is a synchronization primitive that can be used to protect shared data from being simultaneously accessed by multiple threads.