如何使用 pthreads 通知线程新数据可用？

Question

I have new data appearing over a bus. I want my main thread to "wake up" when the new data arrives. My original version of the code is this:

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

int data = 0;

void* thread_func(void* args)
{
    while(1)
    {
        sleep(2);
        
        data = random() % 5;
    }
    
    return NULL;
}


int main()
{
    int tid;

    pthread_create(&tid, NULL, &thread_func, NULL);
    
    while(1)
    {
        // Check data.
        printf("New data arrived: %d.\n", data);
        sleep(2);
    }
    
    return 0;
}

But clearly an infinite while loop in the main thread is overkill. So I thought how about this?

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

int data = 0;
pthread_mutex_t mtx;

void* thread_func(void* args)
{
    while(1)
    {
        sleep(2);
        
        // Data has appeared and can be read by main().
        data = random() % 5;
        
        pthread_mutex_unlock(&mtx);
    }
    
    return NULL;
}


int main()
{
    int tid;
       

    pthread_mutex_init(&mtx, NULL);
    
    pthread_create(&tid, NULL, &thread_func, NULL);
    
    while(1)
    {
        pthread_mutex_lock(&mtx);
        printf("New data has arrived: %d.\n", data);
    }
    
    return 0;
}

This works, but is it the best way?

In actual fact, I don't just have a main thread, but several threads that I would like to be asleep until new data for them arrived. This would involve using one mutex lock for each thread. Is this the best way to do things?

I hope it's clear. Thanks.

Answer 1

You can use pthread_cond_wait to wait for a change on the data you share between your threads. This function automatically blocks your mutex and you have to release it afterwards. To notify your threads that the data is ready use the pthread_cond_signal function.

But be careful, you must always lock and unlock your mutex in each of your threads, not as you do in your example.