Linked List implementation of Stack - c

Question

I am trying to implement stack as a linked list. Here is my current code:
#include <stdio.h>
#include <stdlib.h>
#include <stddef.h>
typedef struct node {
int data;
struct node* link;
} Node;
typedef Node* list;
list head;
void push(list top, int item) {
if (head == NULL) {
head = (list) malloc(sizeof(Node));
head->data = item;
head->link = NULL;
top = head;
} else{
list temp = (list) malloc(sizeof(Node));
temp->data = item;
temp->link = top;
top = temp;
}
}
int pop(list top) {
if (top == NULL) {
printf("stack is empty");
/*int tdata=top->data;
top=top->link;
return tdata;*/
} else {
int tdata = top->data;
top = top->link;
return tdata;
}
}
void display(list top){
while (top != NULL) {
printf("%d", top->data);
top = top->link;
}
}
int main() {
int ch, item;
list top;
for (;;) {
printf("1.push\t2.pop\t3.display\t4.exit");
scanf("%d", &ch);
switch (ch) {
case 1:
printf("enter the element to be pushed");
scanf("%d",&item);
push(top,item);
break;
case 2:
item=pop(top);
printf("element popped is: %d",item);
break;
case 3:
display(top);
break;
case 4:
exit(0);
break;
default:
printf("enter valid choice");
}
}
}
When I press '2' the pop method is called, but irrespective of whatever item is on the top it prints the message "element popped is: 11". When I press '3' for the display method, I get "segmentation fault(core dumped)". Why is this happening? What modifications are needed to get this code working?

Answer 1

I have made several alterations to your program. The most important is to pass a pointer to the list head to functions, which itself is a pointer, so that it can be altered by the function.
I also removed the global head and initialised the local top. I have commented in the code about other matters.
#include <stdio.h>
#include <stdlib.h>
typedef struct node {
int data;
struct node *link;
} Node; // removed pointer type to Node
void push(Node **top, int item) { // takes address of the pointer
Node *temp= malloc(sizeof(Node)); // do not cast malloc
if(temp == NULL) // check that malloc did work
exit(42);
temp->data = item; // no need for separate clause at first push
temp->link = *top; // link is previous top
*top = temp; // top is new struct, pass it back
}
int pop(Node **top) { // takes address of the pointer
int tdata;
Node *temp;
if (*top == NULL) {
printf("stack is empty\n"); // added newline here and other places
return 0;
}
tdata = (*top)->data; // collect the data
temp = (*top)->link; // remember the next list item
free(*top); // give memory back
*top = temp; // pass new top back to caller
return tdata;
}
void display(Node *top){
while (top != NULL) {
printf("%d ", top->data); // added spacing
top = top->link;
}
printf("\n"); // added newline
}
int main(void) {
int ch, item;
Node *top = NULL; // initialise the list !!
do {
printf("1.push 2.pop 3.display 4.exit ");
scanf("%d", &ch);
switch (ch) {
case 1:
printf("enter the element to be pushed ");
scanf("%d",&item);
push(&top,item); // pass address so pointer can be updated
break;
case 2:
item=pop(&top); // pass address so pointer can be updated
printf("element popped is: %d\n",item);
break;
case 3:
display(top);
break;
case 4:
break;
default:
printf("enter valid choice\n");
}
} while(ch != 4); // changed the loop style
}

Answer 2

What is happening is that you simply do not initialize or set any value to your list pointer top.Take a good look at your insertion function push. It receives a pointer. If you want to send your member top as an [out] paramter to this function you should send a pointer to a pointer (**) or a reference to a pointer (*&). Sending top like this does not modify its value and leaves it with junk as this variable was also never initialized...