c - How to input pointer of a structure -

i writing program finding addition, multiplication , division of 2 rational numbers using structure , pointers. having problem inputting numbers pointers. how should code corrected? thanks!

#include <stdio.h> struct rational {     int nu;     int de; }*p1,*p2,*p3; struct rational add() {     p1->nu = p1->nu*p2->de + p1->de*p2->nu;     p3->de = p1->de * p2->de;     printf("%d\n--\n%d\n",p3->nu,p3->de); } struct rational multiply() {     p3->nu = p1->nu * p2->nu;     p3->de = p1->de * p2->de;     printf("%d\n--\n%d\n",p3->nu,p3->de); } struct rational divide() {     p3->nu = p1->nu * p2->de;     p3->de = p1->de * p2->nu;     printf("%d\n--\n%d\n",p3->nu,p3->de); } int main() {     int a,b,choice;     printf("enter first rational number.\n");     scanf("%d%d",&p1->nu,&p1->de);     printf("enter second rational number.\n");     scanf("%d%d",&p2->nu,&p2->de);     scanf("%d",&choice);     switch (choice)     {         case 1: add();                 break;         case 2: multiply();                 break;         case 3: divide();                 break;     }     return 0; } 

1. you have declared pointers struct rational did not assign them point such struct. example:

   struct rational rat_a; p1 = & rat_a; 

2. you declare functions returning struct rational (i.e. struct rational add()) don't seem return anything. if function not return should declared void - void add()

3. why use pointers structs , not structs themselves? (if declared global in code)