for loop not executing after assigning pointer to NULL in main()

Question

I wrote this C code from a pointers practice video and the problem I am having is that after integrating head as a local main variable over a global variable declared before main(), the program is no longer entering the for loop and I can't seem to figure out why. The interesting part is that I delete the head=NULL, the loop seems to be executing. Does anyone know where the problem with assigning NULL to head is?

Here is my code:

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

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

struct Node* Insert(struct Node* head, int data){
    struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));    
    (*new_node).data = data;
    (*new_node).next = head;

    head = new_node;
}

void Print(struct Node* head){
    struct Node* temp = (struct Node*)malloc(sizeof(struct Node));
    temp = head;

    while(temp!=NULL){
        printf("%d", (*temp).data);
        temp = (*temp).next;
    }

    printf("\n");
}

int main(){
    struct Node* head = NULL;

    int n, i, x;
    printf("%s", "How large do you want your array to be?\n");
    scanf("%d", &n);

    for(i; i<n; i++){
        printf("Enter a number:\n");
        scanf("%d",&x);
        head = Insert(head, x);
        Print(head);    
    }
    return 0;
}

Not necessarily relevant, but why are you using `malloc`, storing the pointer in `temp`, then immediately overwriting `temp`? That's a memory leak. — Carcigenicate, Jun 01 '19 at 21:35
Turn on compiler warnings and read them. Your code gave me two warnings. — klutt, Jun 01 '19 at 21:39
@Carcigenicate, you are right, that was actually part of the problem — callmeGuy, Jun 01 '19 at 21:48
@xing, oh, right. that is so annoying because my compiler does not say anything about htis — callmeGuy, Jun 01 '19 at 21:48
@Carcigenicate, on a second thought, I don't think this is necessarily a memory leak, because it is a function variable, so as soon as the function terminates, variable temp is freed. — callmeGuy, Jun 01 '19 at 22:00
@callmeGuy It's not `temp` that's leaked, it's the memory allocated by the call to `malloc` that's leaked. The problem is not the pointer, it's the thing it points to. — David Schwartz, Jun 01 '19 at 22:09

score 2 · Accepted Answer · answered Jun 01 '19 at 21:48

I can see a couple of problems with your code:

struct Node* temp = (struct Node*)malloc(sizeof(struct Node));
temp = head;

should be

struct Node* temp = head;

Then we have

for(i; i<n; i++)

that should be

for(i=0; i<n; i++)

This can be the cause of your problem, since this is undefined behavior.

Another thing that can cause trouble is that you don't do return head as a final statement in Insert.

Two of these could be easily discovered by turning on compiler warnings.

Also, don't cast malloc

that is it indeed, thanks. My compiler did not detect me not assigning i=0 and I've overlooked that, and yes, that memory leak from temp variable. — callmeGuy, Jun 01 '19 at 21:51

for loop not executing after assigning pointer to NULL in main()

1 Answers1