This is an example of how to program with OOP paradigm in C (why would you ever do this)

OOP_in_C.c

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

typedef int(*int_ptr)(test);
typedef struct {
	int a;
	int_ptr next;
} test;

int next_r(test* self)
{
	return self->a + 1;
}

test* test_maker()
{
	test* a = malloc(sizeof(test));
	a->next = &next_r;
	return a;
}

int main()
{
	test* ss = test_maker();
	ss->a = 1;
	printf("%d\n", ss->next(ss));  // Prints 2
	free(ss);
}