简单的生产-消费者程序

#include <stdlib.h>
#include <pthread.h>
#include <stdio.h>
#include <sys/unistd.h>

#define PRINT_LINE  printf("FILE: %s,  LINE: %d
", __FILE__, __LINE__);
#define CYCLE_TIME  (5)
typedef struct msg {
    struct msg *prev;
    struct msg *next;
    int num;
} *pMsg;

pMsg head;
pMsg tail;
pthread_cond_t has_product = PTHREAD_COND_INITIALIZER;
pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
void printList();
void *consumer(void *p)
{
    struct msg *mp = NULL;
    for (;;) {
    pthread_mutex_lock(&lock);
    while (head == NULL && tail == NULL) {
        printf("head is: %p, tail is: %p
", head, tail);
        pthread_cond_wait(&has_product, &lock);
    }
    //printList();
    mp = tail;
    //if tail point the first Node, tail->next is NULL, and tail->prev is also NULL!
    //So, in this condition, tail point to NULL after statement "tail = tail->prev;"
    //this happened before second node is produced, because consumer get the lock
    tail = tail->prev;
    if (tail) {//tail not point to first
        tail->next = NULL;
    }
    else //tail point to first, so after consume node, List is EMPTY!
        head = NULL;//init List to NULL, so producer will add the next node to 
                    //List and arise tail to it, add consumer will wait until List is Not EMPTY.
    pthread_mutex_unlock(&lock);
    printf("Consume	%d
", mp->num);
    free(mp);        //PRINT_LINE
    sleep(rand() % CYCLE_TIME);
    }
}

void *producer(void *p)
{
    struct msg *mp;
    for (;;) {
    mp = malloc(sizeof(struct msg));
    mp->num = rand() % 1000 + 1;
    printf("Produce	%d
", mp->num);
    pthread_mutex_lock(&lock);
    mp->prev = NULL;
    mp->next = head;
    if (tail == NULL) {    //if the first Node is consumed
        //before the second node is produced,
        //this statement arise tail to second node.
        tail = mp;
    }
    if (head)
        head->prev = mp;
    head = mp;
    //printList();
    pthread_mutex_unlock(&lock);
    pthread_cond_signal(&has_product);

    sleep(rand() % CYCLE_TIME);
    }
}

int main(int argc, char *argv[])
{
    head = tail = NULL;
    pthread_t pid, cid;
    srand(time(NULL));
    pthread_create(&pid, NULL, producer, NULL);
    pthread_create(&cid, NULL, consumer, NULL);
    pthread_join(pid, NULL);
    pthread_join(cid, NULL);
    return 0;
}

void printList()
{
    pMsg mp=NULL;
    printf("------------List Start ----------
");
    mp = head;
    while (mp) {
    printf
        ("element: mp - %p, mp->prev - %p, mp->num - %d, mp->next - %p;
",
         mp, mp->prev, mp->num, mp->next);
    printf("head: %p, tail: %p
", head, tail);
    mp = mp->next;
    }
    printf("------------List End ----------
");
}

参考的<<Linux C 一站式编程>> "第35章 线程 第3节 线程同步"的代码和习题