是extern是写一个不是定义的声明的唯一方法吗？

Question

Is that sane to conclude that the only type of variable declaration is "extern declaration" ?

It's known that the following code define (not declare ) x , because it allocate memory for variable x (KR chapter 4). Typically though, we would normally just say that we are declaring x (but not initializing it).

int x; // <-- definition NOT declaration

The only declaration example I can find is to use with the keyword extern :

extern int x;
extern int a[];

Is there any other variable declaration situation apart from extern declaration ?

Answer 1

A definition is a declaration with storage. From standard:

6.7 Declarations 3/ 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;

— for a function, includes the function body;119)

— for an enumeration constant, is the (only) declaration of the identifier;

— for a typedef name, is the first (or only) declaration of the identifier.

Then a function prototype is a declaration, which may not be qualified extern if used for a forward declaration. A

For more details, read the standard, section 6.7.

Answer 2

extern is used , mostly for functions when the function exists in another file , either sourcecode or an object library.

auto is used for stack variables, but since it is the default few people wite it explicitly.

static is for static variables, kept even when a function exits. auto variables are lost when a function exits, so static variables are often used for total counts, user settings, settings of state, which may change, but are used throughout the whole time the program is running.