Checking if pointer is NULL gives segmentation fault

Question

The problem statement: We have to delete every alternate node of a Linked List. Eg: Original list: 1->2->3->4->5 to: 1->3->5

complete problem statement: https://practice.geeksforgeeks.org/problems/delete-alternate-nodes/1/?ref=self

As you can see, this is a function problem so I'm not actually writing the complete code(just have to complete the function). Here is the code that I'm writing:

void deleteAlt(struct Node *head){
    // Code here
    struct Node *traverse=head,*alternate=head->next;

    if(alternate->next==NULL)
    {
        head->next=NULL;
        return;
    }

    while(traverse->next!=NULL && alternate->next!=NULL)
    {
        traverse->next = alternate->next;
        traverse = traverse->next;
        alternate = traverse->next;
        if((alternate->next)==NULL) //presence of this if statement causes segmentation fault
        {
            traverse->next=NULL;
        }
    }
}

I have gone through similar problems on stackoverflow but their code and objective were different, like, not initializing the pointer and comparing it. However, my problem is different.

In the case when number of nodes is even, alternate is always going to be NULL and hence there should be no initialization problem.

Answer 1

you do

while(traverse->next!=NULL && alternate->next!=NULL)
  traverse->next = alternate->next;
  traverse = traverse->next;
  alternate = traverse->next;
  if((alternate->next)==NULL) //presence of this if statement causes segmentation fault

so in fact

while(traverse->next!=NULL && alternate->next!=NULL)
  traverse = alternate->next;
  alternate = traverse->next;
  if((alternate->next)==NULL) //presence of this if statement causes segmentation fault

so in fact

while(traverse->next!=NULL && alternate->next!=NULL)
  alternate = alternate->next->next;
  if((alternate->next)==NULL) //presence of this if statement causes segmentation fault

so in fact

while(traverse->next!=NULL && alternate->next!=NULL)
  if((alternate->next->next->next)==NULL) //presence of this if statement causes segmentation fault

when alternate->next->next is NULL (not checked by the while ) alternate->next->next->next causes your segmentation fault

---

A solution is :

void deleteAlt(struct Node * head)
{
  if (head != NULL) {
    while (head->next != NULL) {
      Node * d = head->next;

      head->next = head->next->next;
      free(d);
      head = head->next;
    }
  }
}

A complete program to prove :

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

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

Node * make(int v, Node * n)
{
  Node * r = malloc(sizeof(Node));

  r->v = v;
  r->next = n;

  return r;
}

void pr(Node * l)
{
  while (l != NULL) {
    printf("%d ", l->v);
    l = l->next;
  }
  putchar('\n');
}

void deleteAlt(struct Node * head)
{
  if (head != NULL) {
    while (head->next != NULL) {
      Node * d = head->next;

      head->next = head->next->next;
      free(d);
      head = head->next;
    }
  }
}

int main()
{
  Node * l = make(1, make(2, make(3, make(4, make(5, NULL)))));

  pr(l);
  deleteAlt(l);
  pr(l);

  /* free rest of list */
  while (l != NULL) {
    Node * n = l->next;

    free(l);
    l = n;
  }
}

Compilation and execution:

pi@raspberrypi:/tmp $ gcc -pedantc -Wextra l.c
pi@raspberrypi:/tmp $ ./a.out
1 2 3 4 5 
1 3 5 
pi@raspberrypi:/tmp $ 

Execution under valgrind to check memory accesses/leaks

pi@raspberrypi:/tmp $ valgrind ./a.out
==2479== Memcheck, a memory error detector
==2479== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==2479== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==2479== Command: ./a.out
==2479== 
1 2 3 4 5 
1 3 5 
==2479== 
==2479== HEAP SUMMARY:
==2479==     in use at exit: 0 bytes in 0 blocks
==2479==   total heap usage: 6 allocs, 6 frees, 1,064 bytes allocated
==2479== 
==2479== All heap blocks were freed -- no leaks are possible
==2479== 
==2479== For counts of detected and suppressed errors, rerun with: -v
==2479== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)

---

(edit) In case the length of the list can be even the definition must be changed to :

void deleteAlt(struct Node * head)
{
  while ((head != NULL) && (head->next != NULL)) {
    Node * d = head->next;

    head->next = head->next->next;
    free(d);
    head = head->next;
  }
}

modifying main to check :

int main()
{
  {
    Node * l = make(1, make(2, make(3, make(4, make(5, NULL)))));

    pr(l);
    deleteAlt(l);
    pr(l);

    /* free rest of list */
    while (l != NULL) {
      Node * n = l->next;

      free(l);
      l = n;
    }
  }
  {
    Node * l = make(1, make(2, make(3, make(4, NULL))));

    pr(l);
    deleteAlt(l);
    pr(l);

    /* free rest of list */
    while (l != NULL) {
      Node * n = l->next;

      free(l);
      l = n;
    }
  }
}

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -pedantic -Wextra l.c
pi@raspberrypi:/tmp $ ./a.out
1 2 3 4 5 
1 3 5 
1 2 3 4 
1 3 

and under valgrind :

pi@raspberrypi:/tmp $ valgrind ./a.out
==3450== Memcheck, a memory error detector
==3450== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==3450== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==3450== Command: ./a.out
==3450== 
1 2 3 4 5 
1 3 5 
1 2 3 4 
1 3 
==3450== 
==3450== HEAP SUMMARY:
==3450==     in use at exit: 0 bytes in 0 blocks
==3450==   total heap usage: 10 allocs, 10 frees, 1,096 bytes allocated
==3450== 
==3450== All heap blocks were freed -- no leaks are possible
==3450== 
==3450== For counts of detected and suppressed errors, rerun with: -v
==3450== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)

Answer 2

try this:

void deleteAlt(struct Node *head){

    struct Node * head_tmp=head;
    struct Node * tmp=NULL;

    while(head_tmp->next!=NULL){

        tmp=head_tmp->next;

        head_tmp->next=tmp->next;

        head_tmp=tmp->next;

        //do something for freeing tmp node 

    }


}

Answer 3

I can see a code smell in

if(alternate->next==NULL)
    {
        head->next=NULL;
        return;
    }

What if i have a single node? In that case alternate points to null.