Standard C requires that pointer types used with
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
and the other a pointer to a character type. GNU C implements the POSIX XSI extension that additionally permits the use of
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.