读写锁优先级的问题解决了

这个问题就是:对一个同享的数据布局,读的频率远弘远于写,所以用了读写锁.但是发现写线程老是抢不到锁.

按The Open Group 的Single UNIX? Specification所说，"Thepthread_rwlock_rdlock() function applies a read lock to the read-write lock referenced by rwlock. The calling thread acquires the read lock if a writer does not hold the lock and there are no writers blocked on the lock. It is unspecified whether the calling thread acquires the lock when a writer does not hold the lock and there are writers waiting for the lock" 意思就是说，没有writer在等写锁的时辰，reader是可以拿到读锁的。然则没有划定，若是有writer在期待写锁，该若何？

还好，Linux有pthread_rwlockattr_setkind_np这个函数，惋惜我用man查不到这个函数。我是在pthread.h头文件中发现的。

enum

{

PTHREAD_RWLOCK_PREFER_READER_NP,

PTHREAD_RWLOCK_PREFER_WRITER_NP,

PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP,

PTHREAD_RWLOCK_DEFAULT_NP = PTHREAD_RWLOCK_PREFER_READER_NP

};

?

extern int pthread_rwlockattr_setkind_np (pthread_rwlockattr_t *__attr,

int __pref) __THROW __nonnull ((1));

看到这些，我觉得这事不就这么简单的就解决了吗？默许是reader优先，改成writer优先不就好了吗？唰唰唰的写代码，同时在网上搜了一下，csdn上还有人很当真的教他人怎样用这个函数。啧啧！

pthread_rwlockattr_setkind_np(&attr,PTHREAD_RWLOCK_PREFER_WRITER_NP);

没用！为啥呢？连man页都没有，所以我思疑这个函数没实现，因而就用debuginfo-install glibc? 装glibc的调试符号，然后用gdb跟进去，发现pthread_rwlockattr_setkind_np确切是有实现的，代码很简单，更改了attr的一个成员变量。那是为啥呢？

再谷歌，终究找到了pthread_rwlockattr_setkind_np的man page，末尾有一段notes，让我年夜汗：

“Setting the value read-write lock kind to PTHREAD_RWLOCK_PREFER_WRITER_NP, results in the same behavior as setting the value to PTHREAD_RWLOCK_PREFER_READER_NP. As long as a reader thread holds the lock the thread holding a write lock will be starved. Setting the kind value to PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP, allows the writer to run. However, the writer may not be recursive as is implied by the name. “

意思就是说，PTHREAD_RWLOCK_PREFER_WRITER_NP和PTHREAD_RWLOCK_PREFER_WRITER_NP是一样滴！应当设置成PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP才对！可是PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP也是名存实亡滴，它才不会recursive 呢。