[C언어/포인터] 컴파일러 경고 메세지 원인을 잘 모르겠습니다

#include <stdio.h>
#include <stdlib.h>
 
typedef struct linkedList {
  int *dataPtr;
  struct linkedNode *link;
} *t_node;
 
void createNode(t_node *ptr, int value);
 
int main(void)
{
  t_node node = NULL;
  createNode(&node, 7);
  createNode(&node, 75);
 
  t_node firstNode = node;
  t_node secondNode = firstNode->link; // <--------------------------------- 이 부분 경고!
  printf("첫번째 노드의 값: %d\n", *(firstNode->dataPtr));
  printf("두번째 노드의 값: %d\n", *(secondNode->dataPtr));
 
}
 
void createNode(t_node *ptr, int value)
{
  t_node newNode = NULL;
  int *data = NULL;
 
  newNode = malloc(sizeof(struct linkedList));
  data = malloc(sizeof(int));
 
  newNode->dataPtr = data;
  *(newNode->dataPtr) = value;
  newNode->link = NULL;
 
  if(*ptr == NULL)
    *ptr = newNode;
  else {
    t_node nextPtr = *ptr;
    while (nextPtr->link != NULL)
      nextPtr = nextPtr->link;  // <---------------------------------- 이 부분 경고!
    nextPtr->link = newNode;    // <---------------------------------- 이 부분 경고!
  }
 
}

$ gcc linkedListSample.c
linkedListSample.c: In function ‘main’:
linkedListSample.c:20:23: warning: initialization from incompatible pointer type [-Wincompatible-pointer-types]
   t_node secondNode = firstNode->link;
                       ^
linkedListSample.c: In function ‘createNode’:
linkedListSample.c:44:15: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
       nextPtr = nextPtr->link;
               ^
linkedListSample.c:45:19: warning: assignment from incompatible pointer type [-Wincompatible-pointer-types]
     nextPtr->link = newNode;
                   ^