Dynamic stack in C [pop crash] [closed] - c

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Im making a program to convert a decimal int to binary using a dynamic stack.
It crashes on the last pop.
ex. num: 4 output: 10crash
#include <stdio.h>
struct stack {
struct stack *prev;
int val;
struct stack *next;
};
struct stack *first,*cur,*tmp;
struct stack *GETNODE(){
struct stack *pt = (struct stack *)malloc(sizeof(struct stack));
};
int counter=1;
void push(int val){
tmp=GETNODE();
tmp->prev=NULL;
tmp->val=val;
tmp->next=NULL;
if(first==NULL){
first=tmp;
cur=first;
}else{
tmp->prev=cur;
cur->next=tmp;
cur=tmp;
}
counter++;
};
int pop(){
int val=tmp->val;
cur=tmp->prev;
free(tmp);
tmp=cur;
tmp->next=NULL;
counter--;
return(val);
};
main(){
int num = 4;
while(num!=0){
push(num%2);
num/=2;
}
while(counter!=1){
printf("%d ",pop());
}
}

Answer 1

The problem is in your pop function. If you think about how it operates, in the final pass you will free(tmp), which is currently pointing to the very first item. After you free it, you then assign:
tmp->next=NULL;
You are trying to dereference an invalid pointer at this point.

Answer 2

I made a lot of changes to your code. Mainly, it was far too complicated - only a singly linked list was needed, and you don't need a counter - just track the list pointer. Your GETNODE() function did not return a value, causing undefined behaviour in the calling function. I simplified that too, there is no need for a separate function to allocate memory because malloc() already does that.
#include <stdio.h>
#include <stdlib.h>
struct stack {
struct stack *prev;
int val;
};
struct stack *first = NULL;
void push(int val){
struct stack *pt = malloc(sizeof(struct stack));
if (pt == NULL){
printf ("Bad call to malloc()\n");
exit (1);
}
pt->prev=first;
pt->val=val;
first = pt;
}
int pop(){
int val;
struct stack *pt = first;
if (first == NULL)
return -1;
val=first->val;
first = first->prev;
free(pt);
return(val);
}
void dec2bin(int num){
printf("%-5d", num);
while(num!=0){
push(num%2);
num/=2;
}
while(first){
printf("%d",pop());
}
printf("\n");
}
int main(void){
dec2bin (4);
dec2bin (129);
dec2bin (160);
return 0;
}
Program output:
4 100
129 10000001
160 10100000

Answer 3

I change some part of your code and your code working now.
#include <stdio.h>
#include <stdlib.h>
struct stack {
struct stack *prev;
int val;
struct stack *next;
};
struct stack *first, *cur, *tmp;
int counter = 0;
struct stack *GETNODE()
{
return malloc(sizeof(struct stack));
}
void push(int val)
{
tmp = GETNODE();
tmp->prev = NULL;
tmp->val = val;
tmp->next = NULL;
if (first == NULL) {
first = tmp;
cur = first;
} else {
tmp->prev = cur;
cur->next = tmp;
cur = tmp;
}
counter++;
}
int pop(void)
{
int val = cur->val;
tmp = cur;
cur = cur->prev;
free(tmp);
counter--;
return val;
}
int main(int argc, char *argv[])
{
int num;
scanf("%d", &num);
while (num != 0) {
push(num % 2);
num /= 2;
}
while (counter != 0)
printf("%d ", pop());
printf("\n");
}