Where's the segmentation fault being generated in this threaded code?

Question

In this homework, two matrices should be added using a different thread for each quadrant. This is my attempt so far, it generates a seg fault that I haven't been able to identify.

#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#define N 4  //matrix dimension

int c[4][4];


int a[4][4]={1 ,3, 4, 5, 3, 4, 5, 6,7, 8, 9, 0, 2, 10, 3, 5};//test matrices

int b[4][4]={3, 4, 5, 6, 11, 7, 9, 3, 1, 1, 4, 5,1, 0, 0, 6};



void *cuadrante(void *argv[1]){

    int t,i,j,k;

    int opcion;

    opcion = atoi(argv[1]);      //meant to cast the quadrant number 


    switch (opcion){

    case 1:
        k=((N/2)-1); //A11 position 

        for (i=k;i=0;i--){          //1st quadrant
                for (j=k;j=0;j--){

                    c[i][j]=a[i][j]+b[i][j];
                }
            }
    break;

    case 2:

        k=((N/2)-1); //next to A11

        for (i=k-1;i=0;i--){            //2nd quadrant
            for (j=k;j<N;j++){

                c[i][j]=a[i][j]+b[i][j];
                }
            }

    break;

    case 3:

        t=N/2; //

        for (i=t;i<N;i++){          //3rd quadrant
                for (j=(t-1);j=0;j--){

                    c[i][j]=a[i][j]+b[i][j];
                }
        }

    break;

    case 4:

        t=N/2;      //position A22 
        for (i=t;i<N;i++){          //4th quadrant
                for (j=t;j<N;j++){

                c[i][j]=a[i][j]+b[i][j];
                 }
            }

    break;


    default: 
        printf("\n Opcion erronea. Intente de nuevo");

    }//fsw




}//end cuadrante






int main(int argc,char * argv[])
{
    int rc, t;

    pthread_t threads[4];

    for(t=0;t<4;t++){

        rc = pthread_create(&threads[t], NULL, cuadrante, (void *)argv[1]);
          if (rc){
             printf("ERROR; return code from pthread_create() is %d\n",rc);
             return -1;
          }//end if

        }//end for

   printf("Created %d threads.\n", t);
   pthread_exit(NULL);
   return 0;


}//end main

Answer 1

You're passing argv[1] as the parameter to all 4 threads; so won't they all end up taking the same route through the switch statement?

Also, your function declares its argument as a void **, but you're passing in a char * cast to a void *. This is bound to cause issues. You probably want this:

void *cuadrante(void *arg){

    int t,i,j,k;

    int opcion;

    opcion = atoi((char *)arg);      //meant to cast the quadrant number 

    ...

Answer 2

If all else fails sprinkle the code with printf. You'll find the problem in 15 minutes.