there is some issue with memory allocation, segmentation fault

Question

I wrote this code to implement sparse matrix using linked list but I don't why I am getting error(segmentation fault).....can somebody help me with that.

I created an array of linked list, the index of array represents row of matrix and in linked-list I am storing col and data value.

There is a link to a image below that may help you understand the code better.

https://i.sstatic.net/9rwH9.png

#include <stdio.h>
#include<stdlib.h>

struct Node {
  int col;
  int data;
  struct Node *next;
}*first;

void Create(struct Node **A, int m, int n) {
  struct Node  *t, *last;
  printf("Enter the elements of sparse matrix:\n");
  for(int i=0; i<m; i++){
    first = NULL;
    last = NULL;
    for(int j=0; j<n; j++){
      int val;
      scanf("%d", &val);
      if(val != 0){
        t= (struct Node *)malloc(sizeof(struct Node));
        t->col = j;
        t->data = val;
        t->next = NULL;

        if(first) 
          first = t;
        if(last)
          last->next = t;
        last = t;
      }
    }
    A[i] = first;
  }
  return;
}

void Display(struct Node **A, int m, int n){
  printf("\nSparse matrix is:\n");
  for(int i=0; i<m; i++){
    struct Node *p = A[i];
    for(int j=0; j<n; j++){
      if(p->col == j){
        printf(" %d ", p->data);
        if(p)
          p = p->next;
      }
      else printf(" 0 ");
    }
    printf("\n");
  }
}

int main() {
  int m=5, n=6;
  struct Node **A = (struct Node **)malloc(m * sizeof(struct Node));
  Create(A, m, n); 
  Display(A, m, n);
  return 0;
}

Answer 1

There are three things to fix your code:

• When you allocate, you should allocate space for the data type the handle to the memory points to, in your case a struct Node *:

  struct Node **A = malloc(m * sizeof(struct Node *));
  

  You can write this as:

  struct Node **A = malloc(m * sizeof(*A));
  

  You should also free all allocated data after using it. (The mis-allocation doesn't hurt here, because you allocate more than you need, but if you get the types wrong and allocate too little, you're in for surprises.)

• When you insert the first node, head is null, so that's the condition to check:

  if (first == NULL) 
    first = t;
  

  Your original code never adds any nodes to the list.

• When you print, you access p, but p might be null. That's the very first thing you have to check before any access to p:

  if (p && p->col == j){
    printf(" %d ", p->data);
    p = p->next;
  }