互斥锁挂起或不起作用

Question

我正在使用 pthreads 在父进程旁边创建一个子进程。我正在尝试使用互斥锁在子进程中的第一个打印语句后停止，并在父进程中的第二个打印语句后恢复：

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

pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

void *print(void *attr)
{
printf("I am the child process\n");
pthread_mutex_lock(&lock);
pthread_cond_wait(&cond, &lock);
printf("The child process is done\n");
pthread_mutex_unlock(&lock);
return 0;
}

int main()
{

pthread_t child;
pthread_mutex_lock(&lock);
printf("I am the parent process\n");
pthread_create(&child, NULL, print, NULL);
pthread_join(child, NULL);
printf("The parent process is done\n");
pthread_mutex_unlock(&lock);
pthread_cond_signal(&cond);
return 0;
}

原输出如下：

I am the parent process
I am the child process
The child process is done
The parent process is done

我要找的输出如下：

I am the parent process
I am the child process
The parent process is done
The child process is done

我一生都无法弄清楚如何使用互斥锁来实现这一点，我最接近的方法是让它在无限期挂起之前达到前两个语句。我也有原始输出，好像互斥语句什么都不做。

谁能帮我解决这个问题？提前致谢。

Answer 1

您只需要在父线程和子线程之间发出更好的信号。应该这样做：

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

pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t  cond = PTHREAD_COND_INITIALIZER;
int             child_done = 0;
int             parent_done = 0;

void *print(void *attr)
{
    printf("I am the child process\n");
    pthread_mutex_lock(&lock);
    child_done = 1;
    pthread_cond_broadcast(&cond); // Because this thread also waits
    while (!parent_done)
        pthread_cond_wait(&cond, &lock);
    printf("The child process is done\n");
    pthread_mutex_unlock(&lock);
    return 0;
}

int main()
{
    pthread_t child;
    pthread_mutex_lock(&lock);
    printf("I am the parent process\n");
    pthread_create(&child, NULL, print, NULL);
    while (!child_done)
        pthread_cond_wait(&cond, &lock);
    printf("The parent process is done\n");
    parent_done = 1;
    pthread_cond_signal(&cond);
    pthread_mutex_unlock(&lock);
    pthread_join(child, NULL);
    return 0;
}