From the latest C11 draft — §7.16 Variable arguments <stdarg.h>
:
...if type is not compatible with the type of the actual next argument
(as promoted according to the default argument promotions), the behavior
is undefined, except for the following cases:
— one type is a signed integer type, the other type is the corresponding
unsigned integer type, and the value is representable in both types;
— one type is pointer to void and the other is a pointer to a character type.