Linux 线程 生产者消费者问题 《Unix网络编程》

[men_wen]

这个代码是《Unix网络编程卷2》第129页的代码：

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

#define MAXNITEMS       1000000
#define MAXNTHREADS     5

int nitems;
struct {
        pthread_mutex_t mutex;
        int buff[MAXNTHREADS];
        int nput;
        int nval;
} shared = {
        PTHREAD_MUTEX_INITIALIZER
};

void *producer(void *arg)
{
        while(1) {
                pthread_mutex_lock(&shared.mutex);
                if(shared.nput >= nitems) {
                        pthread_mutex_unlock(&shared.mutex);
                        return NULL;
                }
                shared.buff[shared.nput] = shared.nval;
                shared.nput++;
                shared.nval++;
                pthread_mutex_unlock(&shared.mutex);
                *((int *)arg) += 1;
        }
}

void *consumer(void *arg)
{
        int i;
        for(i = 0; i < nitems; i++) {
                if(shared.buff[i] != i) {
                        printf("buff[%d] = %d\n", i, shared.buff[i]);
                }
        }
        return NULL;
}

int main(void)
{
        int i, nthreads, count[MAXNTHREADS];
        pthread_t tid_producer[MAXNTHREADS], tid_consumer;

        nitems = MAXNITEMS;
        nthreads = MAXNTHREADS;

        //pthread_setconcurrency(nthreads + 1);
        for(i = 0; i < nthreads; i++) {
                count[i] = 0;
                pthread_create(&tid_producer[i], NULL, producer, &count[i]);
        }

        for(i = 0; i < nthreads; i++) {
                pthread_join(tid_producer[i], NULL);
                printf("count[%d] = %d\n", i, count[i]);
        }

        pthread_create(&tid_consumer, NULL, consumer, NULL);
        pthread_join(tid_consumer, NULL);

        exit(0);
}

运行两次的结果：

# SYNCHRONIZATION ./a.out
count[0] = 0
count[1] = 0
count[2] = 0
count[3] = 0
count[4] = 9
buff[5] = 9
buff[6] = 9
# SYNCHRONIZATION ./a.out
count[0] = 9
count[1] = 0
count[2] = 0
count[3] = 0
count[4] = 0
buff[5] = 9
buff[6] = 9

创建的生产者线程应该在 if(shared.nput >= nitems) 成立返回，nitems值为1000000，但是运行结果却是shared.nput等于9的时候退出，这是为什么？