Declaration vs definition in C

Question

Consider the code:

int main(void)
{
    int a;
}

As far as I know, int a; is a definition, as it causes storage to be reserved. Citing the C standard (N1570 Committee Draft — April 12, 2011):

6.7/5 Semantics A declaration specifies the interpretation and attributes of a set of identifiers. A definition of an identifier is a declaration for that identifier that:

— for an object, causes storage to be reserved for that object;

...

Here comes the question: the compiler may optimize away the storage, since we are not using the variable. Is then int a; a declaration then? And what if we do a printf("%p", &a) in main(void) - certainly now the compiler has to allocate storage, so is the concept of declaration/definition dependent on whether you later use the identifier or not?

Answer 1

The text you quoted from 6.7/5 is actually meant to be interpreted the other way around than what you have done: the text is saying that definitions cause storage to be allocated.

The text which specifies that int a; is a definition is elsewhere.

C is defined in terms of an abstract machine. There is storage allocated in the abstract machine. Whether or not any memory is allocated on your PC is unrelated.

Answer 2

Is then int a; a declaration then?

Yes.

In fact, every definition is also a declaration. A variable can have only one definition, but could have multiple declarations.