Implements stack using linked list - is all this code needed? - linked-list

Question

I code what implementing stack using linked list as I coded I look some of code that somebody coded I have something awkward.
The point that I ask is what is different when I use
if (top == NULL){
top=temp;
top->next=NULL;
}
this part with when I don't use this. Is it needed? In my opinion that part is needless.
This is my full code
#include <stdio.h>
#include <stdlib.h>
#define Max 5
struct Node{
int value;
struct Node *next;
};
struct Node *top;
int count;
int main(){
int pop();
void push(int);
int stack_full();
int stack_empty();
void printStack();
void init();
void freeAllRes();
int opt;
int value;
init();
printf("initialization is performed!\n\n");
printf("1. Push to stack");
printf("\n2. Pop from Stack");
printf("\n3. values in Stack");
printf("\n4. Exit\n");
while(1){
printf("\nChoose Option : ");
scanf("%d",&opt);
switch(opt){
case 1:
if(stack_full()==-1){
printf("Error : Stack is full!!");
break;
}
else{
printf("\nEnter a value to push into Stack : ");
scanf("%d",&value);
push(value);
break;
}
case 2:
if(stack_empty()==-1){
printf("Error : Stack is empty!!");
break;
}
else{
printf("Popped num : %d ",pop());
break;
}
case 3:
if(stack_empty()==-1){
printf("Error : Stack is empty!!");
break;
}
else{
printStack();
break;
}
case 4:
freeAllRes();
printf("All resources are free!");
exit(0);
default:
printf("Error : wrong choice!!");
break;
}
}
}
int stack_full(){
if(count==Max)
return -1;
else
return 0;
}
int stack_empty(){
if(top==NULL)
return -1;
else
return 0;
}
void init(){
top=NULL;
count=0;
}
void freeAllRes(){
struct Node *temp;
while(top!=NULL){
temp = top->next;
free(top);
top=temp;
}
}
int pop(){
int popNum;
struct Node *temp=top;
popNum = temp->value;
top = top->next;
free(temp);
count--;
return popNum;
}
void push(int num){
struct Node *temp;
temp=(struct Node *)malloc(sizeof(struct Node));
if(temp==NULL){
printf("Error : malloc fail!!");
exit(0);
}
temp->value=num;
/*
if (top == NULL){
top=temp;
top->next=NULL;
}
*/
//else{
temp->next=top;
top=temp;
//}
count++;
}
void printStack(){
struct Node *temp=top;
printf("\nElements are as:\n");
while(temp!=NULL){
printf(" %d ",temp->value);
temp=temp->next;
}
}
And how to I code this code fancy and efficient and nice?