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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.