Question

Here’s my code:

#include <stdio.h>

typedef struct node_struct {
    int data;
    struct node_struct *next;
} node;

void push(node *top, int data) {
    node *new_node = (node*) malloc(sizeof(node));
    new_node->data = data;
    new_node->next = top;
    top = new_node;
}

int main() {
    node *top = (node*) malloc(sizeof(node));
    top->data = 1;
    printf("Set data of top node to: %d\n", top->data);

    push(top, 2);
    printf("Pushed 2 to top, top->next->data = %d\n", top->next->data);
}

The program segfaults at the 3rd last line (push(top, 2);) and I think on the line top = new_node;

I’m just learning C (pointers right now).

What did I do wrong?

Answer 1

The problem here is that you pass the pointer to the top element by-value, and then you try to set the pointer inside the function but there it’s just a local variable and changes to it will not be visible outside of the function.

Pass the top pointer by reference instead, by using a pointer to the pointer:

void push(node **top, int data) {
    node *new_node = malloc(sizeof(node));
    new_node->data = data;
    new_node->next = *top;
    *top = new_node;
}

...

push(&top, 2);

---

An alternative is to return the new top from the function instead:

node *push(node *top, int data) {
    node *new_node = malloc(sizeof(node));
    new_node->data = data;
    new_node->next = top;
    return new_node;
}

...

top = push(top, 2);