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

Answer 1

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.

Answer 2

A declaration introduces an identifier and describes its type, be it a type, object, or function. A declaration is what the compiler needs to accept references to that identifier. These are declarations:

extern int bar;
extern int g(int, int);

A definition actually instantiates/implements this identifier. It's what the linker needs in order to link references to those entities. These are definitions corresponding to the above declarations:

int bar;
int g(int lhs, int rhs) {return lhs*rhs;}

Answer 3

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 4

int a;

This is a definition There is a memory allocated for variable a

extern int a;

This is a declaration. Memory is not allocated because it is not defined.

Once a variable is defined you can use the address of it which is totally legal.