CODEWITHSUNDEEP
cmultithreadinglockingmutexunlock
Tim Dirks
Tim Dirks

Reputation: 449

Can't execute first thread with mutex

I want to finish my threads with a mutex. The first thread won't execute, thread 2 & 3 execute.

Does anyone know what this problem could be? Sometimes thread 1 is executed but then 2 or 3 is not executed. I don't know what the problem is here. 

Thread created successfully
    Thread created successfully
    Thread created successfully
    ----------------------------------------------------
    J:0
    NUM_REQUESTS (before function): 0
    J:0
    ----------------------------------------------------
    ----------------------------------------------------
    J:1
    Third thread processing done
    WRITE DATA TO LIST!
    NUM_REQUESTS(function): 1
    NUM_REQUESTS (before function): 0
    J:1
    ----------------------------------------------------
    ----------------------------------------------------
    J:2
    Second thread processing done
    WRITE DATA TO LIST!
    NUM_REQUESTS(function): 0
    NUM_REQUESTS (before function): 0
    J:2
    ----------------------------------------------------

Program:

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

pthread_mutex_t request_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t got_request = PTHREAD_COND_INITIALIZER;

pthread_t tid[3];
int thr_id[3];
int ret1,ret2,ret3;
int i = 0;
int err;
int *ptr[2];
int num_requests = 0;
int rc = 0;

This is the function of the threads, the first thread is not executed!

void* doSomeThing(void *arg)
{
    unsigned long i = 0;
    pthread_t id = pthread_self();

    for(i=0; i<1000000;i++);

    if(pthread_equal(id,tid[0]))
    {
         printf("First thread processing done\n");
         printf("WRITE DATA TO LIST!\n");
         num_requests--;
         printf("NUM_REQUESTS(function): %d\n",num_requests);
         rc = pthread_mutex_unlock(&request_mutex);
         pthread_exit(&tid[0]);
    }else if(pthread_equal(id,tid[1])){

        printf("Second thread processing done\n");
        num_requests--;
        printf("WRITE DATA TO LIST!\n");
        printf("NUM_REQUESTS(function): %d\n",num_requests);
        rc = pthread_mutex_unlock(&request_mutex);
        pthread_exit(&tid[1]);
    }else if(pthread_equal(id,tid[2])){

        printf("Third thread processing done\n");
        printf("WRITE DATA TO LIST!\n");
        printf("NUM_REQUESTS(function): %d\n",num_requests);
        num_requests--;
        rc = pthread_mutex_unlock(&request_mutex);
        pthread_exit(&tid[2]);
    }
    return NULL;
}

This is where i create the output of the threads

void add_request(int j,pthread_mutex_t* p_mutex,pthread_cond_t* p_cond_var)
{
    printf("----------------------------------------------------\n");
    printf("J:%d\n",j);
    if(num_requests > 3){
        printf("WAIT TILL THREADS ARE FREE!\n");
    }else{

    rc = pthread_mutex_lock(&request_mutex);
    printf("NUM_REQUESTS (before function): %d\n",num_requests);    

    num_requests++;
    rc = pthread_mutex_unlock(&request_mutex);

    rc = pthread_mutex_lock(&request_mutex);
    rc = pthread_cond_signal(p_cond_var);
    printf("J:%d\n",j);
    printf("----------------------------------------------------\n");
    }
}

In the main i only create the threads and use the add_request function to execute the threads

int main(void)
{
    //create 3 threads
    while(i < 3)
    {
    thr_id[i] = i;
        err = pthread_create(&(tid[i]), NULL, &doSomeThing, (void*)&thr_id[i]);
        if (err != 0)
            printf("can't create thread :[%s]", strerror(err));
        else
            printf("Thread created successfully\n");

        i++;
    }    

    int j;
    for(j=0;j<3;j++){
    add_request(j, &request_mutex, &got_request);
    }


    return 0;
}

Upvotes: 0

Views: 175

Answers (1)

Guy L
Guy L

Reputation: 2954

what are you trying to do?

At least one problem I see it that you unlock the mutex without locking it...

When you create a thread it starts spontaneously (more or less) so you can't assume add_request will be called after doSomething().

You can lock and then unlock. Also consider using ptheard_join if you want to wait until the threads finish.

EDIT this is what you want to do https://computing.llnl.gov/tutorials/pthreads/samples/condvar.c -taken from https://computing.llnl.gov/tutorials/pthreads/

Good luck

Upvotes: 1

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