DOUBT ON IMPLEMENTATION OF STACK

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C

data

WHEN I RUN THE COMPILED FILE ITS SHOWING SEGMENTATION FAULT.
WHAT'S WRONG WITH THIS?
PLEASE HELP!!!

#include<stdio.h>
#include<malloc.h>
struct node{
	int data;
	struct node *link;
	}*top,*ptr,*start;
main()
{	
	top->link=NULL;
	int choice,info;
	printf("CHOOSE AN OPTION\n");
	scanf("%d",&choice);
	printf("1.PUSH AN ELEMENT\n");
	printf("2.POP AN ELEMENT\n");
	printf("3.DISPLAY\n");
	printf("4.EXIT\n");
	switch(choice)
	{
		case 1:
		printf("ENTER THE ELEMENT YOU WANT TO PUSH\n");
		scanf("%d",&info);
		push(info);
		break;
		/*case 2:
		pop(top->data);
		break;*/
		case 3:
		display();
		break;
	}
}
push(int info)
{	
	if(top->link!=NULL)
	{
		ptr->link=top;
		top=malloc(sizeof(struct node));
		top->data=info;
		top->link=NULL;
	}
	else
	{
	start->link=top;
	top->data=info;
	top->link=NULL;
	}
	
}

display()
{	ptr=start;
	while(ptr->link!=NULL)
	{	
		int info;
		info=(ptr->data);
		printf("%d\t",info);
		ptr=ptr->link;
	}
}

WHEN I RUN THE COMPILED FILE ITS SHOWING SEGMENTATION FAULT.
WHAT'S WRONG WITH THIS?
PLEASE HELP!!!