Declaration or Definition in C

问题:

From External Variables Wiki:

If neither the extern keyword nor an initialization value are present, the statement can be either a declaration or a definition. It is up to the compiler to analyse the modules of the program and decide.

I was not able to fully grasp the meaning of this statement with respect to C. For example, does it imply that:

int i;

is not necessarily a declaration (as I have been assuming until now), but could be a definition as well (by definition of Definition & Declaration on the same webpage, no puns intended)?

In a nutshell, is the above statement: a. just a declaration, or b. declaration + definition?

Reference: Variable declaration and definition

Summary of answers received:

                         Declaration    Definition    Tentative Definition   Initialized 
int i;   (inside a block)    Yes           Yes                No                No
int i=5; (inside a block)    Yes           Yes                No               Yes(to 5)
int i;   (otherwise)         Yes            No               Yes               Yes(to 0)
extern int i;                Yes            No                No                No


All definitions are declarations but not vice-versa.

回答1:

Assuming it's at file scope it's a 'tentative definition'. From 6.9.2/2 "External object definitions":

A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static, constitutes a tentative definition. If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0.

This means that it would be valid to also have the following in the translation unit:

int i = 42;

since that declaration has an explicit initializer, it's the definition of the variable i.

As far as if the declaration is in a block scope, the standard says the following (6.2.2/2 "Linkages of identifiers"):

Each declaration of an identifier with no linkage denotes a unique entity.

...

(paragraph 6) The following identifiers have no linkage: ... a block scope identifier for an object declared without the storage-class specifier extern.

So in block scope, the declaration would be a definition as well.

回答2:

The C standard says that

A definition of an identifier is a declaration for that identifier that: for an object, causes storage to be reserved for that object (…)

Definitions encompass declarations, i.e., every definition is necessarily a declaration, so it doesn’t make sense to say that

int i;

is not a declaration. It is a declaration which also happens to be a definition. Or, it is a definition, hence a declaration.

回答3:

In the context of variables:

• A declaration of a variable is a statement which describes how this variable looks like. So:

  extern int x;
  

  in global scope translates to: "somewhere in the code, there's a variable called x which has type int and extern linkage. A declaration is necessary before you ever refer to x. (The same goes to function declarations.)

• A definition is a statement which creates an instance of this variable. So:

  int x;
  

  in global scope creates a single variable of type int with extern linkage. So if you'd put that line in a header, every translation unit including that header would try to create its own copy of x, which is undesirable - that's why we only have declarations in header files. The same goes to functions: if you provide the function body, it's a definition.

Also, formally, every definition is a kind of declaration, as it also has to specify how this variable/function looks like - so if a definition already exists in a given scope, you don't need any additional declarations to use it.

回答4:

From the C99 spec:

A declaration of an identifier for an object that has file scope without an initializer, and without a storage-class specifier or with the storage-class specifier static, constitutes a tentative definition. If a translation unit contains one or more tentative definitions for an identifier, and the translation unit contains no external definition for that identifier, then the behavior is exactly as if the translation unit contains a file scope declaration of that identifier, with the composite type as of the end of the translation unit, with an initializer equal to 0.

So this is one case in which a simple declaration without an initializer can be a declaration.

回答5:

As C uses the terms:

A "definition" creates something (which occupies some sort of memory). It also describes something. This means a "definition" is also a "declaration".

A "declaration" just describes something. The idea is that the compiler needs to know how to build the code that uses the thing defined elsewhere. Later, the linker then links the use to the something.

Declarations allow you to compile code and link it (later) as a separate step.