Valgrind: Conditional jump or move depends on uninitialised value. Check is null. C

Question

I have this code:

// list.h

typedef struct Node {
  struct Node *next;
  int *data;
} Node;

typedef struct List {
  Node *head;
} List;

// main.c

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

int main()
{
  // creating list
  List *list = (List*)malloc(sizeof(List));
  Node *node = (Node*)malloc(sizeof(Node));
  list->head = node;

  // creating 5 empty nodes
  for (int i = 1; i < 5; i++)
  {
    node->next = (Node*)malloc(sizeof(Node));
    node = node->next;
  }

  // trying to fill it if it is empty
  node = list->head;

  for (int i = 1; i < 5; i++)
  {
    if (node->data == NULL)
    {
      node->data = (int*)malloc(sizeof(int));
      *(node->data) = 1;
    }
  }

  // freeing
  node = list->head;
  Node *tmp;
  while (node != NULL)
  {
    free(node->data);
    tmp = node;
    node = node->next;
    free(tmp);
  }

  free(list);
  return 0;
}

I am trying to use valgrind and this is what he tells:

==340== Conditional jump or move depends on uninitialised value(s)
==340==    at 0x10870A: main (main1.c:24)

I don't strictly need to have something inside node, so my list can be like this [1,1, null, 1, null]. But then in the code I want to use if statement to check if node->data is already exists (this is a pointer, so I have to check if memory allocated and data points to some value).

To be honest my program is working somehow (it is more complicated than this example code I showed, but the valgrind error is similar), but I think something goes wrong anyway.

Answer 1

you must terminate your linked list with a NULL, so after creation the last node should lastnode->next == NULL, therefore when done creating you must initialise it with NULL lastnode->next = NULL;