c - 如何在 C 语言中使用互斥锁

Question

我对在 C 中使用多个互斥锁感到困惑。

int main() {

        pthread_t thread1;
        char *message1 = "Thread 1";
        int  r;
        pthread_mutex_init(&mutex1, NULL);
        pthread_mutex_init(&mutex2, NULL);

        pthread_mutex_lock(&mutex1);

        r = pthread_create( &thread1, NULL, print_message_function, (void*) message1);

        printf("Parent 1\n");


        pthread_mutex_lock(&mutex2);
        printf("Parent 2\n");
        pthread_mutex_unlock(&mutex2);


        pthread_mutex_unlock(&mutex1);


        pthread_join( thread1, NULL);

        printf("Thread 1 returns: %d\n",r);
        return 0;
}

void *print_message_function( void *str ) {

        pthread_mutex_lock(&mutex1);
        char *message;
        message = (char *) str;
        printf("Child 1 received message: %s \n", message);


        pthread_mutex_lock(&mutex2);
        printf("child 2\n");
        pthread_mutex_unlock(&mutex2);




        pthread_mutex_unlock(&mutex1);

        return NULL;
}

输出是

Parent 1
Parent 2
Child 1 received message: Thread 1 
child 2
Thread 1 returns: 0

我想要的是

Parent 1
Child 1 received message: Thread 1 
Parent 2
child 2
Thread 1 returns: 0

Answer 1

当您调用 pthread_create 时，您已经锁定了 mutex1。这意味着调用 pthread_mutex_lock(&mutex1); 的所有其他线程都将等待互斥体解锁。这就是创建第二个线程时发生的情况:mutex1 已经被锁定，因此第二个线程无法进入临界区，需要等待互斥锁解锁。这发生在 main 函数的末尾。

您需要重新组织代码以获得您想要的输出。

但是，要获得这样的结果，您应该检查同步系统，例如 semaphores或 condition variables ;它们将提供更清晰、更简单的线程同步方式。
您也可以查看此教程:POSIX Threads Programming

---

使用信号量的简单解决方案(未测试，但应该有效):

#include <stdio.h>
#include <semaphore.h>

sem_t sem1, sem2;

void* f(void* str) {

    sem_wait(&sem1);
    printf("Child 1 received message: %s \n",(char*)str);

    sem_post(&sem2);
    sem_wait(&sem1);

    printf("Child 2\n");

    return NULL;
}



int main (int argc, const char * argv[]) {

    pthread_t thread;
    char* message = "Thread 1";

    int r;

    sem_init(&sem1,0,0);
    sem_init(&sem2,0,0);

    r = pthread_create(&thread, NULL, f, (void*)message);
    sem_post(&sem1);
    sem_wait(&sem2);

    printf("Parent 2\n");

    sem_post(&sem1); 

    pthread_join(thread1, NULL);
    printf("Thread 1 returns: %d\n",r);

    return 0;
}