Question

Is accessing a non-const object through a const declaration allowed by the C standard?
E.g. is the following code guaranteed to compile and output 23 and 42 on a standard-conforming platform?

translation unit A:

int a = 23;
void foo(void) { a = 42; }    

translation unit B:

#include <stdio.h>

extern volatile const int a;
void foo(void);

int main(void) {
    printf("%i\n", a);
    foo();
    printf("%i\n", a);
    return 0;
}

In the ISO/IEC 9899:1999, I just found (6.7.3, paragraph 5):

If an attempt is made to modify an object defined with a const-qualified type through use
of an lvalue with non-const-qualified type, the behavior is undefined.

But in the case above, the object is not defined as const (but just declared).

UPDATE

I finally found it in ISO/IEC 9899:1999.

6.2.7, 2

All declarations that refer to the same object or function shall have compatible type;
otherwise, the behavior is undefined.

6.7.3, 9

For two qualified types to be compatible, both shall have the identically qualified
version of a compatible type; […]

So, it is undefined behaviour.

Answer 1

TU A contains the (only) definition of a. So a really is a non-const object, and it can be accessed as such from a function in A with no problems.

I’m pretty sure that TU B invokes undefined behavior, since its declaration of a doesn’t agree with the definition. Best quote I’ve found so far to support that this is UB is 6.7.5/2:

Each declarator declares one identifier, and asserts that when an
operand of the same form as the declarator appears in an expression,
it designates a function or object with the scope, storage duration,
and type indicated by the declaration specifiers.

[Edit: the questioner has since found the proper reference in the standard, see the question.]

Here, the declaration in B asserts that a has type volatile const int. In fact the object does not have (qualified) type volatile const int, it has (qualified) type int. Violation of semantics is UB.

In practice what will happen is that TU A will be compiled as if a is non-const. TU B will be compiled as if a were a volatile const int, which means it won’t cache the value of a at all. Thus, I’d expect it to work provided the linker doesn’t notice and object to the mismatched types, because I don’t immediately see how TU B could possibly emit code that goes wrong. However, my lack of imagination is not the same as guaranteed behavior.

AFAIK, there’s nothing in the standard to say that volatile objects at file scope can’t be stored in a completely different memory bank from other objects, that provides different instructions to read them. The implementation would still have to be capable of reading a normal object through, say, a volatile pointer, so suppose for example that the “normal” load instruction works on “special” objects, and it uses that when reading through a pointer to a volatile-qualified type. But if (as an optimization) the implementation emitted the special instruction for special objects, and the special instruction didn’t work on normal objects, then boom. And I think that’s the programmer’s fault, although I confess I only invented this implementation 2 minutes ago so I can’t be entirely confident that it conforms.