c - C structure and malloc function

Question

I am new to programming.The other day I was playing around with structures and pointers...I was getting errors.I tried to rectify them.It got rectified...But I cant justify why there was an error at the first place.Please help me...

struct node{
   int data;
   struct node*next;    
};  

int main(){
   struct node *newnode=NULL;
   struct node *start=NULL;
   newnode=start=(struct node*)malloc(sizeof(struct node));
   newnode->data=1;

   //code snippet 
   newnode->next=NULL;
   newnode=newnode->next;
   newnode=(struct node*)malloc(sizeof(struct node));
   newnode->data=2;
   start=start->next;//error probably as start->next is perceived as NULL Address
   printf("%d",start->data);
   return 0;    
}

when replacing code snippet with this code

newnode->next=(struct node*)malloc(sizeof(struct node));
newnode=newnode->next;
newnode->data=2;
start=start->next;
printf("%d",start->data);

error dissapers..How does one justify this ?

标签： cpointersdata-structuresstructmalloc

Answer 1

You are overwritting the address of newnode here

newnode = newnode->next;

You probably want:

start = malloc(sizeof(struct node));
start->data = 1;

newnode = malloc(sizeof(struct node));
newnode->data = 2;
newnode->next = NULL;

start->next = newnode;

printf("%d", start->data);