producer consumer problem c no output for consumer

Question

here im trying to, use a circular queish thing for my insert and remove methods, as well as use the insert and remove functions for synchronization im compiling with gcc file.c -lpthread -lrt and running with ./a.out 50 1 1.

upon running my producer produces and outputs correctly

producer thread #0
consumer thread #0
producer produced 7

but for whatever reason i cannot get my consumer to output,

i have tried putting my return 0; for each functions outside the locks and semaphores and it does indeed output consumer, but too many times when it should only run once per thread,

its also using a standard counting semaphore 2 of them as well as a single mutex lock

this is what happens when i move the return below the locks

producer thread #0
consumer thread #0
producer produced 7
consumer consumed 7
producer produced 5
producer produced 5
consumer consumed 5
consumer consumed 2
producer produced 7
producer produced 4
consumer consumed 7
consumer consumed 4
producer produced 6
consumer consumed 6
producer produced 1
consumer consumed 1
producer produced 9
consumer consumed 9
producer produced 2
consumer consumed 2
producer produced 6
consumer consumed 6

when in reality there should if i run ./a.out 50 1 1 there should only be one producer and one consumer

update: working on a new version on this so taking back my previous code, will post new version

@EmanuelP yup - OP is sleeping in main() instead. The process ends after the sleep. Who needs join? :) — Martin James, Apr 18 '21 at 15:03
yea the join was there just in case i decided to implement it — Bell, Apr 18 '21 at 17:03

score 2 · Accepted Answer · answered Apr 18 '21 at 04:00

2

In the functions insert_item() and remove_item(), returning (end of execution of the functions) happens before executing pthread_mutex_unlock(). This will prevent them from unlocking and prevent 2nd or later execution of the functions.

Instead of this, you should store the value to return in a variable and return that after unlocking. This can be done like this:

int insert_item(buffer_item item)
{
    int ret;
    sem_wait(&empty);
    pthread_mutex_lock(&lock);
 
    if (size_check < BUFFER_SIZE)
    {
        buffer[insertBounds_check++] = item;
        size_check++;
        ret = 0;
    }
    else
    {
        ret = -1;
    }
    pthread_mutex_unlock(&lock);
    sem_post(&full);
    return ret;
}

answered Apr 18 '21 at 04:00

MikeCAT

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yea ive tried that and i get a endlless amount of products and consumers, when i only want to produce 1 and consume 1 if i pass in 50 1 1 – Bell Apr 18 '21 at 04:50
1

@Bell But your threads run loops `while(true)`. Even with one thread, it'll just output until you exit. – Emanuel P Apr 18 '21 at 06:07
yea i need it to terminate, after unlocking the consumer thread, but not running for enternity, so this solution causes another problem – Bell Apr 18 '21 at 17:11
@Bell does the entire process not stop once the main() sleep runs out? – Martin James Apr 18 '21 at 23:27
really late on a update on this, but it turns out, that the output i was getting, was the result i needed, i just didnt identify that, it was in fact what i was looking for, so the "error" was never a error in the first place, i just misunderstood the specifications required – Bell Apr 30 '21 at 19:55

producer consumer problem c no output for consumer

1 Answers1