ADT (using a header file, source file and main file) in C

Question

I am trying to create a simple ADT using a structure that takes 2 dates. Then returns an age. It must use a Header file, a source file for the Header file, and a main file.

This is what I have it runs and nothing happens. Can someone tell me what i am doing wrong?

age.h

#ifndef AGE_H_

#define AGE_H_

typedef struct getage * Age;

#define MAX  5

Age get_Age(int birthYear, int yearNow);

void age_Destroy(Age a);


#endif

age.c

#include <stdio.h>

#include "age.h"


struct getage {

       int birthYear;

       int yearNow;

};

Age a[1];

Age get_Age(int birthYear, int yearNow){

             int giveAge = 0;

             giveAge = a[0]->yearNow - a[0]->birthYear;

             printf("%d",giveAge);

             return 0;

            }

void age_Destroy(Age a){

                     free(a);

     }

main.c

#include <windows.h>

#include <stdio.h>

#include "age.h"


void age_print(Age a);

void age_print(Age a){

        printf("%d\n", &a);

}



int main() {

    Age a;

    get_Age(1986, 2020);
    
    age_print(a);

    printf("%d\n", &a);

    system("pause");

    //age_Destroy(a);
    
    
}

Answer 1

What are wrong:

• In the function get_Age:
  • Instead of allocating structures, a[0] (global variable, initialized to NULL) is dereferenced.
  • 0 (converted to NULL) is returned instead of returning an age.
• In the function age_Destroy:
  • free() is used without declaration nor including proper header.
• In the function age_print:
  • Data having wrong type is passed to printf(): %d requests int but Age* is passed.
• In the function main:
  • The return value of get_Age is dropped.
  • Data having wrong type is passed to printf(): %d requests int but Age* is passed.

Fixed code that won't cause Segmentation Fault nor undefined behavior:

age.h (not changed)

#ifndef AGE_H_

#define AGE_H_

typedef struct getage * Age;

#define MAX  5

Age get_Age(int birthYear, int yearNow);

void age_Destroy(Age a);

#endif

age.c

#include <stdio.h>

#include <stdlib.h> // for malloc() and free()

#include "age.h"


struct getage {

       int birthYear;

       int yearNow;

};

Age get_Age(int birthYear, int yearNow){

        Age a = malloc(sizeof(*a)); // allocate a structure

        if (a == NULL) { perror("malloc"); exit(1); }

        a->yearNow = yearNow; // assign data

        a->birthYear = birthYear;

        int giveAge = 0;

        giveAge = a->yearNow - a->birthYear;

        printf("%d",giveAge);

        return a; // return pointer to the allocated structure

}

void age_Destroy(Age a){

        free(a);

}

main.c

#include <stdlib.h> // more portable header for system()

#include <stdio.h>

#include "age.h"


void age_print(Age a);

void age_print(Age a){

        printf("%p\n", (void*)a); // use valid combination of format and data

}



int main() {

    Age a;

    a = get_Age(1986, 2020); // assign the return value
    
    age_print(a);

    printf("%p\n", (void*)a); // use valid combination of format and data

    system("pause");

    age_Destroy(a); // enable freeing
    
    
}

(Some behavior may look weird, but I believe this is valid because not desired behavior is described.)