互斥锁与条件变量实现读写锁----pthread_rwlock_rdlock

#include "../unipc.h"
#include "pthread_rwlock.h"

void rwlock_cancelrdwait(void *arg)
{
    my_pthread_rwlock_t *rw;

    rw = (my_pthread_rwlock_t*)arg;
    rw->rw_nwaitreaders--;
    pthread_mutex_unlock(&rw->rw_mutex);
}

int my_pthread_rwlock_rdlock(my_pthread_rwlock_t *rw)
{
    int rc;

    if(rw->rw_magic != RW_MAGIC)
        return EINVAL;

    rc = pthread_mutex_lock(&rw->rw_mutex);
    if (rc != 0)
        return rc;

    while((rw->rw_refcount == -1) || (rw->rw_nwaitwriters > 0)) {
        rw->rw_nwaitreaders++;
        pthread_cleanup_push(rwlock_cancelrdwait,(void*)rw);
        rc = pthread_cond_wait(&rw->rw_condreaders,&rw->rw_mutex);
        pthread_cleanup_pop(0);
        rw->rw_nwaitreaders--;
    }

    rw->rw_refcount++;
    pthread_mutex_unlock(&rw->rw_mutex);
    return rc;
    //pthread_cond_signal(&rw->rw_condwriters);
}

#include "../unipc.h"
#include "pthread_rwlock.h"

void rwlock_cancelrdwait(void *arg)
{
    my_pthread_rwlock_t *rw;

    rw = (my_pthread_rwlock_t*)arg;
    rw->rw_nwaitreaders--;
    pthread_mutex_unlock(&rw->rw_mutex);
}

int my_pthread_rwlock_rdlock(my_pthread_rwlock_t *rw)
{
    int rc;

    if(rw->rw_magic != RW_MAGIC)
        return EINVAL;

    rc = pthread_mutex_lock(&rw->rw_mutex);
    if (rc != 0)
        return rc;

    while((rw->rw_refcount == -1) || (rw->rw_nwaitwriters > 0)) {
        rw->rw_nwaitreaders++;
        pthread_cleanup_push(rwlock_cancelrdwait,(void*)rw);
        rc = pthread_cond_wait(&rw->rw_condreaders,&rw->rw_mutex);
        pthread_cleanup_pop(0);
        rw->rw_nwaitreaders--;
    }

    rw->rw_refcount++;
    pthread_mutex_unlock(&rw->rw_mutex);
    return rc;
    //pthread_cond_signal(&rw->rw_condwriters);
}

无论何时操作pthread_rwlock_t类型的结构，都必须给mutex上锁。
rw_refcount 等于-1表示写线程持有该读写锁，rw_nwaitwriters表示有线程正等着获取该读写锁的的一个写入锁，此时无法获取读写锁的一个读出锁。