How to subtract integer value from a structure with user input in C linked list?

struct node                                                   // main structure
{
        int itemcode;
        int itemnum;                             //itemnum
        char name[30];
        struct node *next;
}
struct node *create,*end,*start,*temp,*prev,*temp1,*start1,*end1;
int x,y,x1,num,total;


void orderitem( int x,int x1)         // I am passing item code and itemnum 
{


    if(isempty())                        // function to check head pointer is !null
    {

        printf("Empty, can't order now");
    }
    else
    {

        temp=start;
        while(temp->next!=NULL && temp->itemcode!= x)
        {
            prev=temp;
            temp = temp-> next;
        }

        if(temp->next==NULL&&temp->itemcode!=x)  // searching with x i.e item code

        {
            printf("Element %d is not present in the list\n",x);
            return;
        }

        else if(start->itemcode==x)              //if item code is at first 
        {
            start1->itemnum=x1;

            start->itemnum-=start1->itemnum;

            if(start->itemnum<=0)         //deletes start if itemnumber less than 0
            {         
                start=start->next;
            }
        }
        else if(end->itemcode==x)              //checks the item code with end  pos
        {                                             

            end1->itemnum=x1;
            end->itemnum-=end1->itemnum;
        }
        if(end->itemnum<=0)                       //deletes end 
        {   end=prev;
            end->next=NULL;

        }


        else
        {
                temp1->itemnum=x1;                 //storing user's item number in  
                                                      temp1

                temp->itemnum-=temp1->itemnum;       

                if(temp->itemnum<=0)
                    prev->next=temp->next;

           
        }

    }
}