Sum of elements in a matrix using threads in c - c

Question

The problem statement goes like this: Calculate the sum of the elements in a bidimensional matrix, using a separate thread to calculate the sum of each row. The main thread adds up these sums printing the final result.
From what I have seen so far, the code runs correctly. The only issue is when I choose the number of rows to be smaller than the number of columns (e.g. rows = 2, columns = 3), because it only computes the sum for the first 2 columns, completely ignoring the third one.
This is the code I wrote in C and I would really appreciate any help understanding what I did wrong or missed. Thank you.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define M 10
#define N 10
int rows, columns, a[M][N], s[M];
// compute the sum of each row
void* f(void* p) {
int k = *((int*) p);
int i;
for (i = 0; i < columns; i++) {
s[i] += a[k][i];
}
return NULL;
}
int main() {
int i, j, *p, rc;
int sum = 0;
pthread_t th[M];
// matrix creation
printf("no. of rows = ");
scanf("%d", &rows);
printf("no. of columns = ");
scanf("%d", &columns);
for (i = 0; i < rows; i++) {
for (j = 0; j < columns; j++) {
printf("a[%d][%d] = \n", i, j);
scanf("%d", &a[i][j]);
}
}
printf("\nThe matrix is: \n");
for(i=0; i < rows; i++) {
for(j=0; j < columns; j++)
printf("%d ", a[i][j]);
printf("\n");
}
// thread creation
for (i=0; i < rows; i++) {
p = malloc(sizeof(int));
*p = i;
rc = pthread_create(&th[i], NULL, f, p);
if (rc != 0) {
printf("Thread creation failed");
exit(-1);
}
}
for (i=0; i < rows; i++) {
pthread_join(th[i], NULL);
}
// compute the final sum
for (i=0; i < rows; i++) {
sum += s[i];
}
printf("The sum is = %d\n", sum);
return 0;
}

Answer 1

You need
s[k] += a[k][i];
instead of
s[i] += a[k][i];
Sum of each rows should be added up in each index of each row as s is declared as s[M].