C threads wait-signal

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Hello.I have a small program.I have to create 2 threads in order to increase a global variable until 20 (for example).At the same time I want my 3rd thread
(3 threads total) to wait until global variable reach the number 20.if number is 20,then I want my 3rd thread to print a message ("I'm awake").
I did some things ,but I'm stuck where the phtread_cond_wait must be(in code)...

What I have tried:

C++

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

pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

void *func(void *arg);
int g = 1;

int main()
{
    pthread_t thr[2];
    pthread_t thr_w;
    
    
    for(int i = 0; i < 2; i++)
    {
        pthread_create(&thr[i] , NULL , func , &g);
    }

    for(int i = 0; i < 2; i++)
    {
        pthread_join(thr[i] , NULL);
        pthread_cond_signal(&cond);
    }
    
    printf("\nCount finished...\n");
    return 0;
}



void *func(void *arg)
{
    int local = *(int *)arg;

    for(int i = local; i <= 10; i++)
    {
        pthread_mutex_lock(&my_mutex);
        printf("g = %d\n" , g);
        g++;
        pthread_mutex_unlock(&my_mutex);
    }
}

This code just increase the global variable until 20,but I'm stuck on how to continue.I need help.

Thanks in advance.

Posted 10-Mar-21 8:26am

Nick_is_asking

Updated 10-Mar-21 20:34pm

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KarstenK 11-Mar-21 1:24am

What is your plan?

1 solution

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CPallini · Answer 1 · 2021-03-10T20:34:00

Try, for instance (see Using Condition Variables (Multithreaded Programming Guide)[^])

C

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

pthread_mutex_t my_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

void * inc_func(void * arg);
void * wait_for_twenty_func(void * arg);
int g = 1;

int main()
{
    pthread_t thr[2];
    pthread_t thr_w;


    for(int i = 0; i < 2; i++)
    {
        pthread_create(&thr[i] , NULL , inc_func , &g);
    }

     pthread_create(&thr_w , NULL , wait_for_twenty_func , NULL);


    for(int i = 0; i < 2; i++)
    {
        pthread_join(thr[i] , NULL);
        //pthread_cond_signal(&cond);
    }
    pthread_join(thr_w , NULL);

    printf("\nCount finished...\n");
    return 0;
}

void * inc_func(void * arg)
{
    for(int i = 0; i < 10; i++)
    {
        pthread_mutex_lock(&my_mutex);
        printf("g = %d\n" , g);
        if ( g < 20)
        {
          g++;
          pthread_cond_signal(&cond); // signal the condition
        }
        pthread_mutex_unlock(&my_mutex);
    }
  return NULL;
}

void * wait_for_twenty_func(void * arg)
{
  int is_twenty = 0;
  while (! is_twenty )
  {
    pthread_mutex_lock(&my_mutex);
    pthread_cond_wait(&cond, &my_mutex); // wait for the condition
    printf("wait expired\n");
    is_twenty = (g==20);

    pthread_mutex_unlock(&my_mutex);
  }
  printf("twenty reached\n");
  return NULL;
}