Whenever using Eina we must include it: 
For this example we are going to define two classes, person and pilot, and since every pilot is a person we use inheritance. To be type safe we are going to add EINA_MAGIC to our classes: 
#define BASETYPE_MAGIC 0x12345
struct _person {
   char *name;
};
typedef struct _person person;
#define SUBTYPE_MAGIC 0x3333
struct _pilot {
   person base;
   char *callsign;
};
typedef struct _pilot pilot;
- Note
- The values of BASETYPE_MAGIC and SUBTYPE_MAGIC have no meaning, the only important thing about them is that they are unique.
Here we have a function to create a person given a name, nothing too fancy: 
person *
person_new(const char *name)
{
   person *ptr = malloc(sizeof(person));
   ptr->name = strdup(name);
   return ptr;
}
And now the counterpart, a function to free a person. 
void
person_free(person *ptr) {
 Before we start releasing resources we check that the pointer we are given actually points to a person, and if not we print an error message and quit: 
 - Note
- EINA_MAGIC_FAIL is a macro that makes it easy to print an appropriate (and consistent) error message. Now knowing that ptr is indeed of type person we proceed to set EINA_MAGIC to EINA_MAGIC_NONE and free the allocated memory:    free(ptr->name);    free(ptr); } 
- 
Setting EINA_MAGIC to EINA_MAGIC_NONE is important to prevent the struct from being used after it is freed.
Now we have our function to create a pilot, this one is a little more complex because we need to set EINA_MAGIC for the pilot and pilot->base, this is very important so that checking the EINA_MAGIC of (person*)my_pilot works: 
pilot *
pilot_new(const char *name, const char *callsign)
{
   pilot *ptr = malloc(sizeof(pilot));
   ptr->base.name = strdup(name);
   ptr->callsign = strdup(callsign);
   return ptr;
}
The function to free a pilot is not too different from the one that frees a person: 
void
pilot_free(pilot *ptr) {
   {
      return;
   }
   free(ptr->base.name);
   free(ptr->callsign);
   free(ptr);
}
We also create functions to print a person or a pilot that check the type of the pointers they receive: 
void
print_person(person *ptr)
{
      return;
   }
   printf("name: %s\n", ptr->name);
}
And for our main function where we declare some variables and initialize Eina: 
void
print_pilot(pilot *ptr)
{
      return;
   }
   print_person(&ptr->base);
   printf("callsign: %s\n", ptr->callsign);
}
int
{
   person *base;
   pilot *sub;
For Eina to be able to provide more informative error messages we are going to give names to our EINA_MAGIC types: 
Since our types won't live longer than the scope of the current function we can set the name without eina making a copy of the string: 
Now we create a person, a pilot, and print both as persons: 
   base = person_new("Tyrol");
   sub = pilot_new("thrace", "starbuck");
   print_person(base);
   print_person((person *)sub);
Now we try to print both as pilots, which obviously does not work since base is not a pilot: 
   print_pilot((pilot *)base); 
   print_pilot(sub);
That's all folks: 
See full source here.