CODEWITHSUNDEEP
c
GhostRavenstorm
GhostRavenstorm

Reputation: 754

Need Help Understanding Segmentation Faults

I get what segfaults are and what causes them, my question is why is my pointer causing them? I'm trying to write a simple linked list that adds on nodes containing 5 and I get the segfault at temp->x = 5;. I thought malloc() should have allowed me access to the memory it needs?

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

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


void append(struct node *root){

   struct node *temp, *right;
   temp = (struct node *)malloc(sizeof(struct node));
   temp->x = 5;
   right = (struct node *)root;

   while(right->next != NULL){
       right = right->next;
   }

   right->next = temp;
   right = temp;
}

int main(){

   struct node *root;

   root = NULL;


   int userInput;

   printf("Pick Operation: ");
   scanf("%d", &userInput);

   if(userInput == 1){
       append(root);
   }


}

Upvotes: 0

Views: 87

Answers (4)

John Coleman
John Coleman

Reputation: 51998

Insert the line

printf("%d is okay\n",temp->x);

right after temp->x = 5;. Your program still crashes -- but only after it prints. Others have already pointed out the source of the error -- but it still doesn't hurt to know the basic trick of using judicious print statements to test your assumptions. Why did you think that line caused the error?

Upvotes: 1

user007
user007

Reputation: 2172

Your program does not cause a segfault at temp->x = 5;. It causes a segfault when you enter the while loop. while(right->next != NULL)

Since you initialize your root to null, so on the first call to append at the entrance of your while loop, you check right->next which is on a null object, and causes a segfault!!

Inserting an if condition at the beginning would solve your purpose, something like this ::

if(root == NULL) {
root = temp;
} else {
    while(right->next != NULL) {
       /*your loop*/
    }
}

Upvotes: 1

Dmitriy Kalugin-Balashov
Dmitriy Kalugin-Balashov

Reputation: 343

Your mistake is here:

while(right->next != NULL){
   right = right->next;
}

You are trying to check "right->next" where "right" is NULL.

Upvotes: 1

Soren
Soren

Reputation: 14688

You are accessing a NULL pointer. The root is NULL when you are calling append, and in the code

   ...
   right = (struct node *)root;
   while(right->next != NULL){
   ...

then in you are trying to dereference the null pointer when you are accessing right-next.

You need to step trough your code with a debugger.

Upvotes: 0

Related Questions