callback.c

#include <stdint.h>
#include <stdio.h>
#include <callback.h>

static void fn(void *x, va_alist alist)
{   
    va_start_void(alist);                                                                                                                                                                                                                                                      
    printf("%i %i\n", (int)(uintptr_t)x, va_arg_int(alist));
    va_return_void(alist);
}

int main(void)
{
    int (*f)(int) = alloc_callback(fn, 42);

    f(23);

    return 0;
}

$ gcc callback.c
$ ./a.out
42 23