多线程同步之读者写者问题

问题定义：

    现有一块共享内存，多个读进程和多个写进程。多个读进程可以同时读，但是当有一个写进程正在写时，其他任何读进程或写进程都不能执行。

该问题有3种变种。第一种称为“读者优先”(readers-preference)。在此情况下，只要有进程在读，写进程就得等待。

实现如下：

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

#define M 6        //number of readers
#define N 2        //number of writers
int readCount = 0; //current number of readers
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;    //用于互斥地修改readCount变量
pthread_mutex_t rw = PTHREAD_MUTEX_INITIALIZER;      //用于读、写以及写、写之间的互斥

void* write(void *arg)
{
    pthread_mutex_lock(&rw);
    printf("writer %d is writing
", arg);
    sleep(1);
    printf("writer %d is leaving
", arg);
    pthread_mutex_unlock(&rw);
}

void* read(void *arg)
{
    pthread_mutex_lock(&mutex);
    if (readCount == 0)
        pthread_mutex_lock(&rw);
    ++readCount;
    pthread_mutex_unlock(&mutex);

    printf("reader %d is reading
", arg);
    sleep(1);
    printf("reader %d is leaving
", arg);

    pthread_mutex_lock(&mutex);
    --readCount;
    if (readCount == 0)
        pthread_mutex_unlock(&rw);
    pthread_mutex_unlock(&mutex);
}

int main()
{
    pthread_t readers[M], writers[N];
    int i;
    for (i = 0; i < M; ++i)
        pthread_create(&readers[i], NULL, read, (void*)i);
    for (i = 0; i < N; ++i)
        pthread_create(&writers[i], NULL, write, (void*)i);
    sleep(10);
    return 0;
}

这种情况下，容易造成写者饿死现象。