多线程随机性死锁问题

Question

代码如下，期望效果是四个线程按顺序循环打印0123，但是实际总是死锁，一个循环0123都打不完，实在是搞不懂为什么会死锁

```c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>

#define NUM 4
pthread_mutex_t mutex;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

static int num = 0;

static int next(int i)
{
    if(i == 3)
        return 0;
    return i+1;
}

static void *handler(void *p)
{
    int i = *(int*)p;
    while(1)
    {
        while(num != i)
            pthread_cond_wait(&cond, &mutex);
        //printf("thread %d\n", i);
        pthread_mutex_lock(&mutex);
        printf("%d\n", num);
        //fflush(NULL);
        num = next(num);
        pthread_cond_broadcast(&cond);
        pthread_mutex_unlock(&mutex);
    }
    pthread_exit(p);
}

int main()
{

    int i = 0;
    pthread_t tid[NUM];
    void *ptr = NULL;
    
    pthread_mutex_init(&mutex, NULL);
    pthread_mutex_lock(&mutex);
    for(; i < NUM; i++)
    {
        int *num = malloc(sizeof(int));
        *num = i;
        if(pthread_create(tid+i, NULL, handler, (void*)num))
        {
            perror("pthread_create()");

            exit(-1);
        }
    }
    pthread_cond_broadcast(&cond);
    pthread_mutex_unlock(&mutex);

    //alarm(1);

    for(i = 0; i < NUM; i++)
    {
        pthread_join(tid[i], ptr);
        free(ptr);
    }
    
    exit(0);
}

```

Answer 1

明白原因了，应该在判断num之前加锁，而不应该在wai之后加锁，没有搞清楚pthread_cond_wait的使用方法