Linux多线程锁属性设置

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：

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.

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。