C Declaraction vs Definition

February 3, 2014

In C, there is a difference between declaration and definition.

Declaration (C99, 6.7)

A declaration specifies the interpretation and attributes of a set of identifiers.

Definition (C99, 6.7)

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
for an enumeration constant or typedef name, is the (only) declaration of the identifier.

Tentative definition (C99, 6.9.2)

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

In a nutshell

Code	Scope (C99, 6.2.1)	Declaration	Definition	Tentative Definition
`int x;`	File	Yes	No	Yes
`static int x;`	File	Yes	No	Yes
`int x = 10;`	File	Yes	Yes	No
`extern int i;`	File	Yes	No	No
`int x = 10;`	Block	Yes	Yes	No
`void foo( void );`	File	Yes	No	No
`void foo( void ) {}`	File	Yes	Yes	No
`void foo( void ); void foo() {}`	File	Yes `void foo( void );`	Yes `void foo( void ) {}`	No
`void foo( void ) { int x;}`	File	Yes	Yes	No

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