pop() method of stack linked list using C has an error

Question

the problem is printf() in pop() method display the weird address and don't run anymore. the print result is below.

push (10)
push (20)
push (30)
push (40)
40
-842150451

Here's the entire code.

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

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

node* head = NULL;

void init(){
    head = (node*)malloc(sizeof(node));
    head->data = 0;
    head->next = NULL;
}

void push(int data){

    node* temp = (node*)malloc(sizeof(node));
    if(temp == NULL){
        printf("Out Of Memory");
    }else{
        head = (node*)malloc(sizeof(node));
        temp->data = data;
        temp->next = head;
        head = temp;
        printf("push (%d)\n", data);
    }
}

void pop(){
    node* temp;
    if(head == NULL) return;
    temp = head;
    printf("%d\n", head->data);
    head = head->next;
    free(temp);
}

void main(){

    push(10);
    push(20);
    push(30);
    push(40);

    pop();
    pop();
    pop();
    pop();
}

and this pop method doesn't work. It display 40 at first time.

and then print -842150451. I don't get it why I receive this weird number.

void pop(){
    node* temp;
    if(head == NULL) return;
    temp = head;
    printf("%d\n", head->data);
    head = head->next;
    free(temp);
}

Stephen Docy · Accepted Answer · 2018-06-17 08:40:30Z

2

You have a weird, extra malloc in push(), I got rid of it and things looked much better:

void push(int data) {

    node* temp = (node*)malloc(sizeof(node));
    if (temp == NULL) {
        printf("Out Of Memory");
    } else {
        //head = (node*)malloc(sizeof(node));     <---- this is your problem
        temp->data = data;
        temp->next = head;
        head = temp;
        printf("push (%d)\n", data);
    }
}


push (10)
push (20)
push (30)
push (40)
40
30
20
10

answered Jun 17, 2018 at 8:40

Stephen Docy

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2 Comments

Noder Over a year ago

Oh, gosh! So thank you! When I noticed my problem by your answer. I felt being stupid. I think I didn't understand the logic well. but now clear. so thank you!

Stephen Docy Over a year ago

Don't sweat it...dynamic memory can be a tricky concept at first, but it looks like you are getting the hang of it.

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