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Why am i getting Segmentation fault (core dumped) when i try to print a struct using recursion?

December 29, 2022

I am new to C. I am trying to make a linked list with the following structure.

struct ListNode {
    int val;
    struct ListNode *next;
};

So i am using recursion to create the linked list from a given array.

struct ListNode makeNode(int val, struct ListNode* l){
    struct ListNode node;
    node.val = val;
    node.next = l;
    return node;
}

struct ListNode makeNodeFromList(int arr[], int length, int index){

    // Checks if the given index is the last element of the array
    // (Base Condition)
    if ((length-1) == index){
        return makeNode(arr[index], NULL);
    }

    struct ListNode next;
    next = makeNodeFromList(arr, length, index+1);
    return makeNode(arr[index],&next);

}

But when i try to print the result using the following code it keeps printing the second element and i get Segmentation fault (core dumped) error.

void printNode(struct ListNode *l){
    // Prints the each value of the struct, separated by tab.
    printf("%d\t",(l->val));

    if ((l->next) != NULL){
        printNode((l->next));
    }
    else{
        printf("%d\n",(l->val));
    }
}

int main() { 
    int arr[3] = {1,2,3};
    struct ListNode item;
    item = makeNodeFromList(arr,3,0);
    printNode(&item);
    }

*-Edit

The whole code instead of snippets.

#include <stdio.h>
#include <stdlib.h>
struct ListNode {
    int val;
    struct ListNode *next;
};

struct ListNode makeNode(int val, struct ListNode* l){
    struct ListNode node;
    node.val = val;
    node.next = l;
    return node;
}

struct ListNode makeNodeFromList(int arr[], int length, int index){
    printf("%d\t", arr[index]);

    // Checks if the given index is the last element of the array.
    if ((length-1) == index){
        return makeNode(arr[index], NULL);
    }

    struct ListNode next;
    next = makeNodeFromList(arr, length, index+1);
    return makeNode(arr[index],&next);

}

void printNode(struct ListNode *l){
    // Prints the each value of the struct, separated by tab.
    printf("%d\t",(l->val));

    if ((l->next) != NULL){
        printNode((l->next));
    }
    else{
        printf("%d\n",(l->val));
    }
}

int main() { 
    int arr[3] = {1,2,3};
    struct ListNode item;
    item = makeNodeFromList(arr,3,0);
    printNode(&item);

    }

I cant figure out the mistake i made.

I tried to find similar problems but couldn’t find a similar case.

>Solution :

Running you program under gdb, I find that your program segfaults in printf("%d\t",(l->val));. The variable l->next cannot be accessed. As @Someprogrammerdude told you above that it is because it’s a local variable next in makeNodeFromList() is out of scope when it returns.

The minimal fix is (and you need a matching free):

#include <string.h>

struct ListNode makeNodeFromList(int arr[], int length, int index){
    if ((length-1) == index) {
        return makeNode(arr[index], NULL);
    }
    struct ListNode next = makeNodeFromList(arr, length, index+1);
    struct ListNode *p = malloc(sizeof(next));
    memcpy(p, &next, sizeof(next));
    return makeNode(arr[index], p);
}