Standard C requires that pointer types used with
va_arg in functions with variable argument lists either must be compatible with that of the actual argument, or that one type must be a pointer to
void and the other a pointer to a character type. GNU C implements the POSIX XSI extension that additionally permits the use of
va_arg with a pointer type to receive arguments of any other pointer type.
In particular, in GNU C ‘va_arg (ap, void *)’ can safely be used to consume an argument of any pointer type.