Scope and Visibility
An identifier's "visibility" determines the portions of the program in which it can be referenced — its "scope." An identifier is visible (i.e., can be used) only in portions of a program encompassed by its "scope," which may be limited (in order of increasing restrictiveness) to the file, function, block, or function prototype in which it appears. The scope of an identifier is the part of the program in which the name can be used. This is sometimes called "lexical scope." There are four kinds of scope: function, file, block, and function prototype.
All identifiers except labels have their scope determined by the level at which the declaration occurs. The following rules for each kind of scope govern the visibility of identifiers within a program:
File scope
The declarator or type specifier for an identifier with file scope appears outside any block or list of parameters and is accessible from any place in the translation unit after its declaration. Identifier names with file scope are often called "global" or "external." The scope of a global identifier begins at the point of its definition or declaration and terminates at the end of the translation unit.
Function scope
A label is the only kind of identifier that has function scope. A label is declared implicitly by its use in a statement. Label names must be unique within a function. (For more information about labels and label names, see The goto and Labeled Statements.)
Block scope
The declarator or type specifier for an identifier with block scope appears inside a block or within the list of formal parameter declarations in a function definition. It is visible only from the point of its declaration or definition to the end of the block containing its declaration or definition. Its scope is limited to that block and to any blocks nested in that block and ends at the curly brace that closes the associated block. Such identifiers are sometimes called "local variables."
Function-prototype scope
The declarator or type specifier for an identifier with function-prototype scope appears within the list of parameter declarations in a function prototype (not part of the function declaration). Its scope terminates at the end of the function declarator.
The appropriate declarations for making variables visible in other source files are described in Storage Classes. However, variables and functions declared at the external level with the static storage-class specifier are visible only within the source file in which they are defined. All other functions are globally visible.