Pair object just like that in lisp.

pairObject.c

#include <stdio.h>
#include <stdlib.h>

typedef enum {
    OBJ_INT,
    OBJ_PAIR
} ObjectType;

typedef struct sObject {
    ObjectType type;
    union {
        int value;
	struct {
	    struct sObject* head;
	    struct sObject* tail;
	};
    };
} Object;

void objectPrint(Object* object) {
    switch (object->type) {
    case OBJ_INT:
	printf("%d", object->value);
	break;
    case OBJ_PAIR:
	printf("(");
	objectPrint(object->head);
        printf(" . ");
        objectPrint(object->tail);
        printf(")");
        break;
    }
}

Object* newIntObject(int value) {
    Object* object = malloc(sizeof(Object));
    object->type = OBJ_INT;
    object->value = value;
    return object;
}

Object* newPairObject(Object* head, Object* tail) {
    Object* object = malloc(sizeof(Object));
    object->type = OBJ_PAIR;
    object->head = head;
    object->tail = tail;
    return object;
}

Object* cons(Object* head, Object* tail) {
    return newPairObject(head, tail);
}

Object* car(Object* object) {
     switch (object->type) {
     case OBJ_INT:
         return NULL;
     case OBJ_PAIR:
         return object->head;
     }
}

Object* cdr(Object* object) {
     switch (object->type) {
     case OBJ_INT:
         return NULL;
     case OBJ_PAIR:
         return object->tail;
     }
}

int main(int argc, const char * argv[]) {
    Object* intObject1 = newIntObject(1);
    Object* intObject2 = newIntObject(2);

    Object* pairObject1 = newPairObject(intObject1, intObject2);
    objectPrint(pairObject1);
    printf("\n");

    Object* pairObject2 = cons(intObject1, pairObject1);
    objectPrint(pairObject2);
    printf("\n");

    Object* object = car(cdr(pairObject2));
    objectPrint(object);
    printf("\n");

    free(intObject1);
    free(intObject2);
    free(pairObject1);
    free(pairObject2);
    free(object);
}