C program to add 2 polynomials and then displaying it - c

Question

I am writing a C program to add 2 polynomials given by user then displaying it in a separate function but it is not adding anything. In fact, it is not giving any type of error message so I am very confused. I think the mistake can be in 'addpoly' function or display function. I don't know what I am doing wrong. Any type of help will be appreciated.
#include<stdio.h>
#include<stdlib.h>
#define MAX 3
struct polynomial
{
int coeff;
int exp;
struct polynomial *next;
};
typedef struct polynomial polynomial;
polynomial *p1=NULL,*p2=NULL,*p3=NULL, *head_address=NULL;
polynomial* create()
{
polynomial *head_address=NULL, *prev_address=NULL, *new_address=NULL;
int i;
for(i=0;i<MAX;i++)
{
new_address=(polynomial*)malloc(sizeof(polynomial));
printf("Enter coeff:\n");
scanf("%d",&new_address->coeff);
printf("Enter exp:\n");
scanf("%d",&new_address->exp );
new_address->next=0;
if(head_address==NULL){
head_address=prev_address=new_address;
}
else{
prev_address->next=new_address;
prev_address=new_address;
}}
return head_address; }
void polyadd(polynomial *p1, polynomial *p2,polynomial *p3)
{
//polynomial* p3=NULL;
while(p1->next!=NULL && p2->next!=NULL){
if(p1->exp > p2->exp){
p3->coeff=p1->coeff;
p3->exp=p1->exp;
p1=p1->next;
}
else if(p1->exp < p2->exp){
p3->coeff=p2->coeff;
p3->exp=p2->exp;
p2=p2->next;
}
else if(p1->exp==p2->exp){
p3->coeff=p1->coeff + p2->coeff;
p3->exp=p1->exp;
p1=p1->next;
p2=p2->next;
}
p3->next=(polynomial*)malloc(sizeof(polynomial));
p3=p3->next;
p3->next=NULL; }
while(p1->next || p2->next){
if(p1->next){
p3->exp=p1->exp;
p3->coeff=p1->coeff;
p1=p1->next;
}
else if(p2->next){
p3->exp=p2->exp;
p3->coeff=p2->coeff;
p2=p2->next;
}
p3->next=(polynomial*)malloc(sizeof(polynomial));
p3=p3->next;
p3->next=NULL;
}
}
void display(polynomial *temp){
polynomial *p3;
p3=temp;
temp=head_address;
while(temp->next!=NULL)
printf("%dx^%d",temp->coeff,temp->exp);
temp=temp->next;
if(temp->next!=NULL){
printf(" + ");
}
}
int main(){
polynomial *p1,*p2,*p3;
p1=create();
printf("Next:\n");
p2=create();
polyadd(p1,p2,p3);
printf("Result:\n");
display(p3);
return 0;
}

Answer 1

Your issue looks like a memory reference error. In your polyadd() function you're not mallocing as much as you need to. Your base p3 pointer for your linked list is still unallocated when you start referencing off it in your first while loop. To fix your memory issue you'll want to malloc p3 before using it and then make sure the subsequent ->next instances all get malloced before being used.
I don't think this is causing you issues currently but you also have global variables defined at the top and then defined again in main(). The local variables will be the only ones used as long as they're present but I would delete your globals and make sure you don't have unused duplicates to avoid confusion.