CODEWITHSUNDEEP
crecursionlinked-listsegmentation-fault
renga_in_stack
renga_in_stack

Reputation: 196

Getting segmentation fault when trying to print linked list data reversely using recursion

I have created linked list of nodes. And wanted to print the data of each node reversely using recursion technique. I got the working as well as segfaulted code. I am trying to understand whats actually problem in segfaulted code.

Actually I tried debugging using GDB but I dont know exactly how to troubleshoot this. Any clues will be more helpful towards the clear understanding of recursion.

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

typedef struct node {
        char c;
        struct node *next;
} node_t;

node_t *head = NULL;

void insert_list(node_t *node)
{
    static node_t *temp = NULL;
    if (!head) {
        head = node;
        //temp = node;
    }
    else {
        temp->next = node;
    }
    temp = node;

}

void create_list(char c)
{
    node_t *temp = NULL;
    temp = malloc(sizeof(node_t));
    if (temp) {
        temp->c = c;
        temp->next = NULL;
        insert_list(temp);
    }
    else
        return;
}

void print_list_reversely(node_t *temp)
{
    if (!temp)
        return;
    //print_list_reversely(temp->next); /* Working piece */
    temp = temp->next; /* This and next line together*/
    print_list_reversely(temp); /* Causing SEGFAULT */
    printf("data is %c\n", temp->c);
    return;
}

int main()
{
    create_list('a');   
    create_list('b');   
    create_list('c');
    print_list_reversely(head);
    return 0;
}

After some GDB debugging got the below info:

A) print_list_reversely(temp->next);

Breakpoint 4, print_list_reversely (temp=0x0) at create.c:40
40      if (!temp)
(gdb) p temp
$5 = (node_t *) 0x0
(gdb) n
41          return;
(gdb) n
47  }
(gdb) n
print_list_reversely (temp=0x602050) at create.c:45
45      printf("data is %c\n", temp->c);

=======

B) temp = temp->next; print_list_reversely(temp);

Breakpoint 4, print_list_reversely (temp=0x0) at create.c:40
40      if (!temp)
(gdb) p temp
$3 = (node_t *) 0x0
(gdb) n
41          return;
(gdb) n
47  }
(gdb) 
print_list_reversely (temp=0x0) at create.c:45
45      printf("data is %c\n", temp->c);

Upvotes: 0

Views: 159

Answers (3)

kiran Biradar
kiran Biradar

Reputation: 12732

Consider you are at the last node.

//temp = temp->next; /* This and next line together*/
//print_list_reversely(temp); /* Causing SEGFAULT */
printf("data is %c\n", temp->c);

You assign tmp to NULL and you try to print it causing NULL pointer dereference.

Consider you have list as below

1->2->NULL

And your recurssion calls are

print_list_reversely(1)
           tmp = [2]
                 --->      print_list_reversely(2)
                           tmp = [NULL]
                                          --->      print_list_reversely(NULL)
                                                    return;
                           print(null->c) //Seg fault

Upvotes: 4

Eovl.Han
Eovl.Han

Reputation: 1

The last pointer temp passed in the recursion is NULL, so accessing NULL must result in a segment error

Program received signal SIGSEGV, Segmentation fault.
0x00000000004005c1 in print_list_reversely (temp=0x0) at linked_list.c:40
40      printf("data is %c\n", temp->c);
(gdb) bt
#0  0x00000000004005c1 in print_list_reversely (temp=0x0) at linked_list.c:40
#1  0x00000000004005bd in print_list_reversely (temp=0x601050) at linked_list.c:39
#2  0x00000000004005bd in print_list_reversely (temp=0x601030) at linked_list.c:39

Upvotes: 0

Maxime B.
Maxime B.

Reputation: 1236

Your method print_list_reversely() is called recursively from the first element to the last, and that's the intended behaviour.

Seen how you defined your lists, the next element of the last will be NULL.

if you uncomment your two faulty lines (EDIT: you uncommented them now), when the temp = temp->next; is executed on the last element, you have null. And you dereference this pointer with printf("data is %c\n", temp->c);

Therefore, this code is incorrect and segfault.

You have to check that your pointer is not null before calling the function back (or dereferencing it!)

Upvotes: 1

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