pthread_join() not working

Question

I have got troubles with my code. The following code starts n threads that compete to find the max value of each diagonal of n different matrices.

#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <time.h>
#include <semaphore.h>

void crea_matrix(int **);
void trova_max(void *);

struct bin_sem
{
    pthread_mutex_t mutex;
    pthread_cond_t  cond;
    int cnt;
}    shared= {PTHREAD_MUTEX_INITIALIZER,PTHREAD_COND_INITIALIZER };

int n,**matrix,max;

int main ()
{   

    int i;
    srand(time(NULL));
    printf("N of matrices: \n");
    scanf("%d", &n);
    int **matrix = (int **)malloc(3 * sizeof(int *));
     for (i=0; i<3; i++)
         matrix[i] = (int *)malloc(3 * sizeof(int));

    pthread_t *tids;
    tids=malloc(n*sizeof(pthread_t));
    for(i=0;i<n;i++)
        pthread_create(tids+i,NULL,trova_max,matrix);
    for(i=0;i<n;i++)
    {
        pthread_mutex_lock(&shared.mutex);
        max=0;
        crea_matrix(matrix);
        shared.cnt=i;
        pthread_cond_signal(&shared.cond);
        pthread_mutex_unlock(&shared.mutex);
        sleep(1);
    }
    for(i=0;i<n;i++)
        pthread_join(tids[i],NULL);
}

void crea_matrix(int **matrix)
    {
    int i,j;
    for(i=0;i<3;i++)
        for(j=0;j<3;j++)
    matrix[i][j]=rand()%101;
    for(i=0;i<3;i++)
    {
        for(j=0;j<3;j++)
            printf("%d ",matrix[i][j]);
        printf("\n");
    }


}


void trova_max(void* arg)
{
    int i=0, j=0;
    int **arr2d;
    arr2d=(int**)arg;
    do
    {
        pthread_mutex_lock(&shared.mutex);
        pthread_cond_wait(&shared.cond,&shared.mutex);
        printf("\nThread: %ld took control of the mutex...\n",(long int)     pthread_self());
        for(i=0;i<3;i++)
        {

            if(arr2d[i][i]>max)
                max=arr2d[i][i];
                printf("\nFirst diag max: %d\n",max);
        }
    i=0;

        for (j=2;j>=0;j--)
        {
            printf("\nSecond diag max:  %d\n",max);
                if (arr2d[i][j]>max)
                    max=arr2d[i][j];
        i++;
        }
    printf("Max found: %d,matrix n° %d", max,shared.cnt);
    pthread_mutex_unlock(&shared.mutex);
    } while(shared.cnt !=n-1);


    pthread_exit(NULL);

}

The thing is, most of my code works, but when the max evaluations are done, only 1 thread gets access to the pthread_exit(NULL); line. I am struggling to find a solution, Hope you can help me. Thanks.

Answer 1

Referring to this question: Is it guaranteed that pthread_cond_signal will wake up a waiting thread?

It is possible that the remaining threads are still blocked on pthread_cond_wait. Doing a final pthread_cond_broadcast before the join will release all the blocked threads.

However, this solution will bring its own set of problems as all the threads will eventually execute the body of the loop. You may have to add an extra check for shared.cnt != n-1. That has to be done atomically though as noticed in one of the comments.