C Function Definitions

A function definition specifies the name of the function, the types and number of parameters it expects to receive, and its return type. A function definition also includes a function body with the declarations of its local variables, and the statements that determine what the function does.

Syntax

translation-unit:
 external-declaration
 translation-unit external-declaration

external-declaration: /* Allowed only at external (file) scope */
 function-definition
 declaration

function-definition:
 declaration-specifiers_opt attribute-seq_opt declarator declaration-list_opt compound-statement

/* attribute-seq is Microsoft-specific */

Prototype parameters are:

declaration-specifiers:
 storage-class-specifier declaration-specifiers_opt
 type-specifier declaration-specifiers_opt
 type-qualifier declaration-specifiers_opt

declaration-list:
 declaration
 declaration-list declaration

declarator:
 pointer_opt direct-declarator

direct-declarator: /* A function declarator */
 direct-declarator ( parameter-type-list ) /* New-style declarator */
 direct-declarator ( identifier-list_opt ) /* Obsolete-style declarator */

The parameter list in a definition uses this syntax:

parameter-type-list: /* The parameter list */
 parameter-list
 parameter-list , ...

parameter-list:
 parameter-declaration
 parameter-list , parameter-declaration

parameter-declaration:
 declaration-specifiers declarator
 declaration-specifiers abstract-declarator_opt

The parameter list in an old-style function definition uses this syntax:

identifier-list: /* Used in obsolete-style function definitions and declarations */
 identifier
 identifier-list , identifier

The syntax for the function body is:

compound-statement:
 { declaration-list_opt statement-list_opt }

The only storage-class specifiers that can modify a function declaration are extern and static. The extern specifier signifies that the function can be referenced from other files; that is, the function name is exported to the linker. The static specifier signifies that the function can't be referenced from other files; that is, the name isn't exported by the linker. If no storage class appears in a function definition, extern is assumed. In any case, the function is always visible from the definition point to the end of the file.

The optional declaration-specifiers and mandatory declarator together specify the function's return type and name. The declarator is a combination of the identifier that names the function and the parentheses following the function name. The optional attribute-seq nonterminal is a Microsoft-specific feature defined in Function Attributes.

The direct-declarator (in the declarator syntax) specifies the name of the function being defined and the identifiers of its parameters. If the direct-declarator includes a parameter-type-list, the list specifies the types of all the parameters. Such a declarator also serves as a function prototype for later calls to the function.

A declaration in the declaration-list in function definitions can't contain a storage-class-specifier other than register. The type-specifier in the declaration-specifiers syntax can be omitted only if the register storage class is specified for a value of int type.

The compound-statement is the function body containing local variable declarations, references to externally declared items, and statements.

The sections Function Attributes, Storage Class, Return Type, Parameters, and Function Body describe the components of the function definition in detail.

See also

Functions