Linux多线程锁属性设置方法
互斥锁是Linux下多线程资源保护的常用手段，但是在时序复杂的情况下，很容易会出现死锁的情况。
可以通过设置锁的属性，避免同一条线程重复上锁导致死锁的问题。
通过int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int type)接口设置
一般是以下四种属性：
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PTHREAD_MUTEX_NORMAL
This type of mutex does not detect deadlock. A thread attempting to relock this mutex without first unlocking it will deadlock. Attempting to unlock a mutex locked by a different thread results in undefined behaviour. Attempting to unlock an unlocked mutex results in undefined behaviour.
PTHREAD_MUTEX_ERRORCHECK
This type of mutex provides error checking. A thread attempting to relock this mutex without first unlocking it will return with an error. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.
PTHREAD_MUTEX_RECURSIVE
A thread attempting to relock this mutex without first unlocking it will succeed in locking the mutex. The relocking deadlock which can occur with mutexes of type PTHREAD_MUTEX_NORMAL cannot occur with this type of mutex. Multiple locks of this mutex require the same number of unlocks to release the mutex before another thread can acquire the mutex. A thread attempting to unlock a mutex which another thread has locked will return with an error. A thread attempting to unlock an unlocked mutex will return with an error.
PTHREAD_MUTEX_DEFAULT
Attempting to recursively lock a mutex of this type results in undefined behaviour. Attempting to unlock a mutex of this type which was not locked by the calling thread results in undefined behaviour. Attempting to unlock a mutex of this type which is not locked results in undefined behaviour. An implementation is allowed to map this mutex to one of the other mutex types.
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这里主要指同一条线程重复上锁，不同线程上锁，无论设置什么属性，当锁已经被锁定后都会互斥阻塞。
使用PTHREAD_MUTEX_RECURSIVE属性，当同一条线程在没有解锁的情况下尝试再次锁定会返回成功。
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