main.c

#include <stdio.h>

int main(int argc, char *argv[])
{
    Person *eric = createPerson("Eric", 65, 12, 165);
    Person *ibo  = createPerson("Ibrahim", 18, 100, 55);

    personEats(eric);
    personEats(ibo);
}

person.c

// other library file called person.c
typedef struct Person {

    char *name;
    int  age;
    int height;
    double weight;

} Person;

Person *createPerson(char *name, int age, int height, double weight)
{
    Person *person = malloc(sizeof(Person));
    person->name   = name;
    person->age    = age;
    person->height = height;
    person->weight = weight;

    return person;
}

void destroyPerson(Person *person)
{
    free(person->name);
    free(person);
}

bool personEats(Person *person)
{
    person->weight += 0.45;
}