读写锁写锁饿死

    今天一程序由于读写锁的问题，导致程序一线程死掉。查完了所有的代码，发现加锁之后，均有释放的地方。后来，推测出最有可能的情况：多个线程去读，在写线程加锁时，至少有一个线程没有释放，而后续的读线程继续加锁，导致该写线程饿死。
    于是写了以下代码进行测试：

#include <stdio.h>
#include <pthread.h>
#include <time.h>
#include <errno.h>
#include <string.h>
#include <strings.h>

pthread_rwlock_t link;

void * t_hand()
{


        int i = 0;
        while(1)
        {
                printf("++++++++++++++++++++++++++\n");
                pthread_rwlock_rdlock(&link);
                printf("---------------------------\n");
                pthread_rwlock_unlock(&link);
                printf("=========================== i = %d \n",i++);
                sleep(1);
                if( i == 7)
                    break;

        }

        pthread_rwlock_unlock(&link);

}




int main(int argc, char ** argv)
{
    int ret = 0;
    printf(" EBUSY = %d, EINVAL = %d, EAGAIN = %d, EDEADLK = %d\n", EBUSY, EINVAL, EAGAIN, EDEADLK);

    pthread_rwlock_init(&link,NULL);


    pthread_t pid ;

    pthread_rwlock_rdlock(&link);

    pthread_create(&pid,NULL,t_hand,NULL);

    sleep(1);

    printf("before wrlock\n");
    pthread_rwlock_wrlock(&link);
    printf("after wrlock\n");

    sleep(100);

}

最后发现输出是以下内容：

EBUSY = 16, EINVAL = 22, EAGAIN = 11, EDEADLK = 35
++++++++++++++++++++++++++
---------------------------
=========================== i = 0
before wrlock
++++++++++++++++++++++++++
---------------------------
=========================== i = 1
++++++++++++++++++++++++++
---------------------------
=========================== i = 2
++++++++++++++++++++++++++
---------------------------
=========================== i = 3
++++++++++++++++++++++++++
---------------------------
=========================== i = 4
++++++++++++++++++++++++++
---------------------------
=========================== i = 5
++++++++++++++++++++++++++
---------------------------
=========================== i = 6
after wrlock

从输出上不难看出，在有一把读锁加上之后，再加上写锁，而后续再有读锁加上时，写锁将一直处于阻塞状态，直到所有的读锁都释放掉。而写锁要加上的条件就是：当前没有读锁。

在将代码修改后：

#include <stdio.h>
#include <pthread.h>
#include <time.h>
#include <errno.h>
#include <string.h>
#include <strings.h>



pthread_rwlock_t link;


void * t_hand()
{


        int i = 0;
        while(1)
        {
                printf("++++++++++++++++++++++++++\n");
                pthread_rwlock_rdlock(&link);
                printf("---------------------------\n");
                pthread_rwlock_unlock(&link);
                printf("=========================== i = %d \n",i++);
                sleep(1);
                if( i == 7)
                    break;

        }

        pthread_rwlock_unlock(&link);

}




int main(int argc, char ** argv)
{
    int ret = 0;
    printf(" EBUSY = %d, EINVAL = %d, EAGAIN = %d, EDEADLK = %d\n", EBUSY, EINVAL, EAGAIN, EDEADLK);

    //pthread_rwlock_init(&link,NULL);

    pthread_rwlockattr_t attr;
    pthread_rwlockattr_setkind_np(&attr, PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
    pthread_rwlock_init(&link,&attr);

    pthread_t pid ;

    pthread_rwlock_rdlock(&link);

    pthread_create(&pid,NULL,t_hand,NULL);

    sleep(1);

    printf("before wrlock\n");
    pthread_rwlock_wrlock(&link);
    printf("after wrlock\n");

    sleep(100);

}

输出以下内容：

EBUSY = 16, EINVAL = 22, EAGAIN = 11, EDEADLK = 35
++++++++++++++++++++++++++
---------------------------
=========================== i = 0
before wrlock
++++++++++++++++++++++++++

然后一直处于等待状态（此时，已经死锁）。

从上不难看出，一直使用的默认属性对于读写锁的处理方式是：有写不能能，有读不能写但是可读。该策略容易对一些读频繁，写较少的程序造成写线程饿死。