我可以使用pthread_cond_wait(，)同时发信号通知多个线程吗? [英] Can I signal multiple threads simultaneously with pthread_cond_wait(,)?

问题描述

我正在编写各种代码段，看看会发生什么.下面的代码旨在延迟所有线程，直到所有线程都到达代码中的某个点为止，然后使每个线程打印一个唯一的数字.由于所有线程都执行此操作，因此编号应该以随机顺序出现.

I am writing various code snippets and see what happens. The code below was intended to delay all threads until all reached a certain point in the code and then make each print a distinctive number. Since the threads all do that, the numbers should occur in a random order.

我当前的问题是我让他们的线程忙于等待.如果线程数变大，程序将显着减慢速度.

My current problem is that I keep they threads busy waiting. If the number of threads gets big, the program slows down significantly.

我想通过使用信号来更改它，为此我找到了pthread_cond_wait()，但是我看不到如何使用它来向所有线程发出信号，请它们唤醒.

I would like to change that by using signals, I found pthread_cond_wait() for that, however I don't see how one would use that to signal all threads that they would please wake up.

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

#define threads 10

int counter=0; 
pthread_mutex_t lock; 

void handler(void *v) {

    pthread_mutex_lock(&lock);  
    counter++;
    printf("%d\n", counter); 
    pthread_mutex_unlock(&lock);   
    while(counter != threads); // busy waiting

    printf("I am first! %d\n", v); 

}


int main() {

    pthread_t t[threads]; 
    for(int i =0; i < threads; i++) {       
    pthread_create(&t[i], NULL, handler, (void*) i); 
    }
    for(int i =0; i < threads; i++) {       
    pthread_join(t[i], NULL); 
    }

    return 0; 
}

我将代码更改为以下代码，但是，它仍然不起作用:/

I changed the code to the following, however, it still does not work :/

pthread_mutex_t lock; 
pthread_cond_t cv; 

void handler(void *v) {
    pthread_mutex_lock(&lock);
    pthread_cond_wait(&cv, &lock);
    printf("I am first! %d\n", v); 
    pthread_mutex_unlock(&lock);
}


int main() {
    pthread_t t[threads]; 
    for(int i =0; i < threads; i++) 
        pthread_create(&t[i], NULL, handler, (void*) i); 
        sleep(2); 
    pthread_cond_signal(&cv); 
    for(int i =0; i < threads; i++) 
        pthread_join(t[i], NULL); 
    return 0; 
}

推荐答案

使用broadcast()?

http://pubs.opengroup.org/onlinepubs/009696699/functions /pthread_cond_broadcast.html

pthread_cond_broadcast()函数应取消阻止当前在指定条件变量cond上阻塞的所有线程.

The pthread_cond_broadcast() function shall unblock all threads currently blocked on the specified condition variable cond.

pthread_cond_signal()函数应解除对至少一个在指定条件变量cond上被阻塞的线程的阻塞(如果在cond上任何线程被阻塞).

The pthread_cond_signal() function shall unblock at least one of the threads that are blocked on the specified condition variable cond (if any threads are blocked on cond).

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