%p format specifier needs explicit cast to void* for all types but char* in printf

Question

I've read a lot of answers about the %p format specifier usage in C language here in Stack Overflow, but none seems to give an explanation as to why explicit cast to void* is needed for all types but char*.
I'm of course aware about the fact that this requirement to cast to or from void* is tied with the use of variadic functions (see first comment of this answer) while non-mandatory otherwise.

Here's an example :

int i;    
printf ("%p", &i);

Yields a warning about type incompatibility and that &i shall be casted to void* (as required by the standard, see again here).

Whereas this chunk of code compiles smoothly with no complaint about type casting whatsoever:

char * m = "Hello";    
printf ("%p", m);

How does that come that char* is "relieved" from this imperative?

PS: It's maybe worth adding that I work on x86_64 architecture, as pointer type size depends on it, and using gcc as compiler on linux with -W -Wall -std=c11 -pedantic compiling options.

Answer 1

There is no explicit cast needed for arguments of type char*, as char * has the same representation and alignment requirement as void *.

Quoting C11, chapter §6.2.5

A pointer to void shall have the same representation and alignment requirements as a pointer to a character type. ⁽⁴⁸⁾ [...]

and the footnote 48)

The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions.

Answer 2

The C11 standard 6.2.5/28 says:

A pointer to void shall have the same representation and alignment requirements as a pointer to a character type. 48)

with footnote 48 being:

The same representation and alignment requirements are meant to imply interchangeability as arguments to functions, return values from functions, and members of unions.

However 7.21.6.1 ("The fprintf function") says about %p:

The argument shall be a pointer to void.

---

This is apparently a contradiction. In my opinion, a sensible interpretation is to say that the intent of 6.2.5/28 is that void * and char * are in fact interchangeable as the types for function arguments which do not correspond to a prototype. (i.e. arguments to non-prototyped functions, or matching the ellipsis of a prototype of variadic function).

Apparently the compiler you're using takes a similar view.

To back this up, the specification of argument types in 7.21.6.1, if taken literally without regard to intent, has a lot of other inconsistencies that have to be disregarded in practice (e.g. it says that printf("%lx", -1); is well-defined, but printf("%u", 1); is undefined behaviour).