Iam working on a program to perform addition,subtraction,multiplication and differentiation operations on a polynomial using linked list in c. The other operations are working fine except multiplication one. here is the code

#include
#include
#include
struct link{
   int coeff;
   int pow;
   struct link *next;
   };
struct link *poly1=NULL,*poly2=NULL,*poly=NULL;
void create(struct link *node)
{
 char ch;
 do
 {
  printf("\n\nenter coeff:");
  scanf("%d",&node->coeff);
  printf("\nenter power:");
  scanf("%d",&node->pow);
  node->next=(struct link*)malloc(sizeof(struct link));
  node=node->next;
  node->next=NULL;
printf("\ncontinue(y/n):");
ch=getch();
}
while(ch=='y' || ch=='Y');
}
void show(struct link *node)
{
while(node->next!=NULL)
{

printf("%dx^%d",node->coeff,node->pow);
node=node->next;
if(node->next!=NULL)
printf(" + ");


}
}
void polyadd(struct link *poly1,struct link *poly2,struct link *poly)
{
 while(poly1->next &&  poly2->next)
 {
  if(poly1->pow>poly2->pow)
  {
   poly->pow=poly1->pow;
   poly->coeff=poly1->coeff;
   poly1=poly1->next;
   }
  else if(poly1->powpow)
  {
   poly->pow=poly2->pow;
   poly->coeff=poly2->coeff;
   poly2=poly2->next;
   }
  else
  {
   poly->pow=poly1->pow;
   poly->coeff=poly1->coeff+poly2->coeff;
   poly1=poly1->next;
   poly2=poly2->next;
   }
  poly->next=(struct link *)malloc(sizeof(struct link));
  poly=poly->next;
  poly->next=NULL;
 }
 while(poly1->next || poly2->next)
 {
  if(poly1->next)
  {
   poly->pow=poly1->pow;
   poly->coeff=poly1->coeff;
   poly1=poly1->next;
   }
  if(poly2->next)
  {
   poly->pow=poly2->pow;
   poly->coeff=poly2->coeff;
   poly2=poly2->next;
   }
   poly->next=(struct link *)malloc(sizeof(struct link));
   poly=poly->next;
   poly->next=NULL;
   }

}

void polysub(struct link *poly1,struct link *poly2,struct link *poly)
{
 while(poly1->next &&  poly2->next)
 {
  if(poly1->pow>poly2->pow)
  {
   poly->pow=poly1->pow;
   poly->coeff=poly1->coeff;
   poly1=poly1->next;
   }
  else if(poly1->powpow)
  {
   poly->pow=poly2->pow;
   poly->coeff=poly2->coeff;
   poly2=poly2->next;
   }
  else
  {
   poly->pow=poly1->pow;
   poly->coeff=poly1->coeff-poly2->coeff;
   poly1=poly1->next;
   poly2=poly2->next;
   }
  poly->next=(struct link *)malloc(sizeof(struct link));
  poly=poly->next;
  poly->next=NULL;
 }
 while(poly1->next || poly2->next)
 {
  if(poly1->next)
  {
   poly->pow=poly1->pow;
   poly->coeff=poly1->coeff;
   poly1=poly1->next;
   }
  if(poly2->next)
  {
   poly->pow=poly2->pow;
   poly->coeff=poly2->coeff;
   poly2=poly2->next;
   }
   poly->next=(struct link *)malloc(sizeof(struct link));
   poly=poly->next;
   poly->next=NULL;
   }

}

void polymul(struct link *n1, struct link *n2, struct link *n)
{
    struct link * n2beg=n2;

            while (n1) 
            {
                    struct link * temp=(struct link *)malloc(sizeof(struct link));
                    temp->next=NULL;    
                    n2=n2beg;
                    while (n2) 
                    {
                            temp->coeff = n1->coeff * n2->coeff;

                            temp->pow = n1->pow + n2->pow;

                            n2 = n2->next;
                            temp->next=(struct link *)malloc(sizeof(struct link));
                            temp=temp->next;
                            temp->next=NULL;

                    }

                    polyadd(temp,n,n);
                    n1 = n1->next;
                    free(temp);
            }

}

void diff(struct link* p1,struct link* p2)
{

while(p1->next!=NULL)
{
    p2->coeff=p1->coeff*p1->pow;
    p2->pow=p1->pow-1;
    p2->next=NULL;
    p2->next=(struct link *)malloc(sizeof(struct link));
   p2=p2->next;
   p2->next=NULL;
   p1=p1->next;
}

}
  main()
  {
  int op;
  char ch;
  do{
  poly1=(struct link *)malloc(sizeof(struct link));
  poly2=(struct link *)malloc(sizeof(struct link));
  poly=(struct link *)malloc(sizeof(struct link));
  printf("\n\nWhat do you want to do?\n1.Addition\n2.Subtraction
          \n3.Multiplication\n4.Differentiation\n0.Exit
          \nEnter your choice:");
  scanf("%d",&op);
  switch(op)
  {
        case 1:
            printf("\n\nenter 1st polynomial:");
            create(poly1);
            printf("\n\nenter 2nd polynomial:");
            create(poly2);
            printf("\n1st Polynomial:\t");
            show(poly1);
            printf("\n2nd Polynomial:\t");
            show(poly2);
            polyadd(poly1,poly2,poly);
            printf("\nAdded polynomial:\t");
            show(poly);
            break;
        case 2:
            printf("\n\nenter 1st polynomial:\t");
            create(poly1);
            printf("\n\nenter 2nd polynomial:\t");
            create(poly2);
            printf("\n\n1st Polynomial:\t");
            show(poly1);
            printf("\n\n2nd Polynomial:\t");
            show(poly2);
            polysub(poly1,poly2,poly);
            printf("\n\nSubtracted polynomial:\t");
            show(poly);
            break;  
        case 3:
            printf("\n\nenter 1st polynomial:");
            create(poly1);
            printf("\n\nenter 2nd polynomial:");
            create(poly2);
            printf("\n\n1st Polynomial:\t");
            show(poly1);
            printf("\n\n2nd Polynomial:\t");
            show(poly2);
            polymul(poly1,poly2,poly);
            printf("\n\nMultiplied polynomial:\t");
            show(poly);
            break;
        case 4:
            printf("\n\nenter polynomial:");
            create(poly1);
            printf("\n\nPolynomial:\t");
            show(poly1);
            diff(poly1,poly2);
            printf("\n\nDifferentiated Polynomial:\t");
            show(poly2);
            break;
        }

 /* printf("\n Want to continue? Y/N:");
  ch=getch();*/
  }
  while(op);

}