问题
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?
回答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.
回答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.
回答3:
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.
回答4:
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;}
来源：https://stackoverflow.com/questions/33163028/declaration-vs-definition-in-c