Input Validation Using Function in C
===

I m validating user Input through functions.
Here is my code snippet:

    #include <stdio.h>
    void get_income (double *pschool, double *pincome);
    double get_double(void);
    
    main ()
    {
    
    get_income (&pschool, pincome);
    
    }
    
    void get_income (double *pschool, double *pincome) {
    int tuition, supplies;
    
    printf("enter tuition:");
    scanf("%d", &tuition);
    printf("Enter supplies")
    scanf("%d", &supplies);
    
    *pSchool = tuition + supplies ;
     return ;
    }
    
    double get_double(void) {
    // validation code should go in here 
    }

Now i need to validate tuition and supplies to check user only put 0 or more number and it not empty. no negative value and nothing other than digits.

Validation code should be in get_double function.. and I cant seems to figure out how I validate something that resides in separate function.

Please guide.