C - threads - condition variables - mutex (customer-producer)

See more:

C

Ubuntu

threads

Hello everyone.I have a problem.

(In main): I create 5 threads that put 1 item (at a time) in a rack and 5 threads that get 3 items (at a time) from the rack.
So,the threads that get items ,have to wait the rack to fill and when the rack has >= 3 items send a signal to the other threads in order to wake up and continue the job.This happens 10 times (for loop inside put_on_the_rack() ).
So,when I run the code,this happens:

nikos@nick-ubuntu:~/Desktop$ gcc cond_var__.c -o cond_var__ -lpthread
nikos@nick-ubuntu:~/Desktop$ ./cond_var__
Putting items in the rack: 1
Putting items in the rack: 2
Putting items in the rack: 3
Putting items in the rack: 4
Putting items in the rack: 5
Getting items from the rack. Items left:  2
Waiting the rack to full...
Waiting the rack to full...
Waiting the rack to full...
Waiting the rack to full...
Putting items in the rack: 3
Putting items in the rack: 4
Getting items from the rack. Items left:  1
Putting items in the rack: 2
Waiting the rack to full...
Putting items in the rack: 3
Getting items from the rack. Items left:  0
Putting items in the rack: 1
Waiting the rack to full...
Waiting the rack to full...
Putting items in the rack: 2
Putting items in the rack: 3
Getting items from the rack. Items left:  0
Putting items in the rack: 1
Putting items in the rack: 2
Waiting the rack to full...
Putting items in the rack: 3
Getting items from the rack. Items left:  0
Putting items in the rack: 1
Putting items in the rack: 2
Putting items in the rack: 3
Putting items in the rack: 4
Putting items in the rack: 5
Putting items in the rack: 6
Putting items in the rack: 7
Putting items in the rack: 8
Putting items in the rack: 9
Putting items in the rack: 10
Putting items in the rack: 11
Putting items in the rack: 12
Putting items in the rack: 13
Putting items in the rack: 14
Putting items in the rack: 15
Putting items in the rack: 16
Putting items in the rack: 17
Putting items in the rack: 18
Putting items in the rack: 19
Putting items in the rack: 20
Putting items in the rack: 21
Putting items in the rack: 22
Putting items in the rack: 23
Putting items in the rack: 24
Putting items in the rack: 25
Putting items in the rack: 26
Putting items in the rack: 27
Putting items in the rack: 28
Putting items in the rack: 29
Putting items in the rack: 30
Putting items in the rack: 31
Putting items in the rack: 32
Putting items in the rack: 33
Putting items in the rack: 34
Putting items in the rack: 35
nikos@nick-ubuntu:~/Desktop$ 

So, my question is ,why threads that get the items don't wake up in order to do the job?
I don't understand what is happening at the end.Provided that I have 35 items ,why threads don't do their job (get the items from the rack).
I miss something here and I want your help to understand and fix it.

Thanks a lot.



Code:

What I have tried:

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <pthread.h>

int num_of_items = 0;
pthread_mutex_t mutex_item__;
pthread_cond_t cond_item;

void *put_on_the_rack(void *arg)
{
    for(int i = 0; i < 10; i++) // put 10 times
    {
        pthread_mutex_lock(&mutex_item__);
        num_of_items++; 
        printf("Putting items in the rack: %d\n" , num_of_items);
        pthread_mutex_unlock(&mutex_item__);
        pthread_cond_broadcast(&cond_item);
        sleep(1);
    }
}


void *get_from_the_rack(void *arg)
{
    pthread_mutex_lock(&mutex_item__);
    while(num_of_items < 3)
    {
        printf("Waiting the rack to full...\n");
        pthread_cond_wait(&cond_item , &mutex_item__);
    }
    
    num_of_items -= 3;
    printf("Getting items from the rack. Items left:  %d\n" , num_of_items);
    pthread_mutex_unlock(&mutex_item__);
}


int main()
{
    pthread_t thread_put[5] , thread_get[5];
    pthread_cond_init(&cond_item , NULL);
    pthread_mutex_init(&mutex_item__ , NULL);

    for(int i = 0; i < 5; i++)
    {
        pthread_create(&thread_put[i] , NULL , &put_on_the_rack , NULL);
    }
    for(int i = 0; i < 5; i++)
    {
        pthread_create(&thread_get[i] , NULL , &get_from_the_rack , NULL);
    }
    
    
    for(int i = 0; i < 5; i++)
    {
        pthread_join(thread_put[i] , NULL);
    }
    for(int i = 0; i < 5; i++)
    {
        pthread_join(thread_get[i] , NULL);
    }

    return 0;
}