来回发送的条件变量

Question

我有一个程序，我希望能够在不同线程之间切换。在这个例子中，我只是尝试打印一个字符串中的内容，以及另一个字符串中的内容，只是为了看看我是在两个线程之间发送的。我无法使用我的代码执行此操作，因为它挂起。理想情况下，我希望能够处理多个信号并能够来回切换。我花了大约四个小时在这上面，但我无法让它工作，我认为仅使用全局变量会更简单，然后我只需要使用互斥体而不是条件变量，或者甚至根本没有互斥锁。

我就是无法让这些变化无常的互斥锁和条件变量按预期运行。

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

char word1[10] = "Hello";
char word2[10] = "world";

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;  // mutual exclusion lock for prod
//pthread_mutex_t mutexSigalProd = PTHREAD_MUTEX_INITIALIZER;
//pthread_cond_t signalToProd = PTHREAD_COND_INITIALIZER;
pthread_cond_t addToQ = PTHREAD_COND_INITIALIZER;


void * prod(void *arg) { printf("Entering prod \n");
    pthread_mutex_lock(&mutex);
    int i = 0;

    while (i < 10) {
        printf("PROD adding line: %c \t", word1[i]);
        i++;
        if (i % 2 == 0)
        {
            pthread_mutex_unlock(&mutex);
            pthread_cond_signal(&addToQ);
            pthread_cond_wait(&addToQ, &mutex);
        }
    }
    pthread_exit(NULL);
}


void * con(void *arg) 
{
    printf("Entering con \n");
    pthread_mutex_lock(&mutex);
    //pthread_mutex_unlock(&mutex);
    pthread_cond_wait(&addToQ, &mutex);
    int i = 0;

    while (i < 10) 
    {
        printf ("CON adding line: %c \t", word2[i]);
        i++;
        if (i % 2 != 0)
        {
            //pthread_mutex_unlock(&mutex);
            pthread_cond_signal(&addToQ);
            pthread_cond_wait(&addToQ, &mutex);
        }
    }

    pthread_exit(NULL);
}

int main()
{
    pthread_t threadp1;        // Thread objects
    pthread_t threadp2;
    pthread_t threadc1;

    pthread_create(&threadp1, NULL, prod, NULL);  // start first producer thread
    //pthread_create(&threadp2, NULL, con1, NULL); // start second producer thread
    pthread_create(&threadc1, NULL, con,  NULL); // start consumer thread
    pthread_exit(NULL);              // main thread quits
}

Answer 1

代码中的竞争条件是:

Prod 发送信号而不检查 Con 是否正在等待它。如果 Con 尚未等待信号，则信号无效(来自手册页 - “如果 cond 当前没有阻塞线程，则 pthread_cond_broadcast() 和 pthread_cond_signal() 函数应无效。”)。

然后 Con 和 Prod 就会互相等待。因此陷入了僵局。

看看下面的代码是否有效:

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

char word1[10] = "Hello";
char word2[10] = "world";
int con_started = 0;

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;  // mutual exclusion lock for prod
pthread_cond_t addToQ = PTHREAD_COND_INITIALIZER;

void * prod(void *arg) {
    printf("PROD: Entering prod \n");
    fflush(stdout);
    int i = 0;
    int j = 1;
    while (i < 5) {
        printf("PROD adding line: %c \n", word1[i]);
        fflush(stdout);
        i++;
        if (i % 2 == 0)
        {
            pthread_mutex_lock(&mutex);
            while(!con_started)
            {
                pthread_mutex_unlock(&mutex);
                sched_yield();
                printf("PROD: Waiting for CON to wait.\n");
                fflush(stdout);
                pthread_mutex_lock(&mutex);
            }
            printf("PROD: Sending signal %d\n\n", j);
            fflush(stdout);
            pthread_cond_signal(&addToQ);
            printf("PROD: Will wait now \n\n");
            fflush(stdout);
            pthread_cond_wait(&addToQ, &mutex);
            printf("PROD received signal %d\n\n", j);
            fflush(stdout);
            pthread_mutex_unlock(&mutex);
            j++;
        }
    }
    sched_yield();
    sleep(1);
    printf("PROD: Sending signal %d\n\n", j);
    fflush(stdout);
    pthread_cond_signal(&addToQ);
    pthread_exit(NULL);
}


void * con(void *arg)
{
    int i = 0;
    int j=1;
    printf("CON: Entering con \n");
    fflush(stdout);
    pthread_mutex_lock(&mutex);
    con_started = 1;
    printf("CON: Will wait now\n \n");
    fflush(stdout);
    pthread_cond_wait(&addToQ, &mutex);
    printf("CON received first signal\n\n");
    fflush(stdout);
    while (i < 5)
    {
        printf ("CON adding line: %c \n", word2[i]);
        fflush(stdout);
        if (i % 2 != 0)
        {
            //pthread_mutex_unlock(&mutex);
            printf("CON: Sending signal %d\n\n", j);
            fflush(stdout);
            pthread_cond_signal(&addToQ);
            j++;
            printf("CON: Will wait now \n\n");
            fflush(stdout);
            pthread_cond_wait(&addToQ, &mutex);
            printf("CON received signal %d\n\n", j);
            fflush(stdout);
            pthread_mutex_unlock(&mutex);
        }
        i++;
    }
    pthread_exit(NULL);
}

int main()
{
    pthread_t threadp1;        // Thread objects
    pthread_t threadc1;
    pthread_create(&threadp1, NULL, prod, NULL);  // start first producer thread
    pthread_create(&threadc1, NULL, con,  NULL); // start consumer thread
    pthread_join(threadp1, NULL);
    printf("MAIN: threadp1 completed.\n");
    fflush(stdout);
    pthread_join(threadc1, NULL);
    printf("MAIN: threadc1 completed.\n");
    fflush(stdout);
}