6.16 Declarations - 2

6.16 Declarations - 2

• Variables and functions need to be defined once. When a variable is defined storage space is set aside for it. A variable can only be initialized when it is defined. When a function is defined code is generated.
• Variables need to be declared in each function that wants to access it. This may be done explicitly, with a storage class specifiers of extern or implicitly by context.
• If the definition of an external variable occours in the same source file before it is used, no declaration is needed. Kernighan p 33.
• Common practice is to place all definitions for all external variables at the start of the file and omit all declarations Kernighan p 33.
• extern is used to declared a variable not to define it.
• If a external variable is to referenced before it is defined, or if it is defined in a different source file from the one where it is being used, then an extern declaration is required.
• A static storage class object has a lifetime of the entire execution of the program and is initilized only once. ANSI "C" Standard 9899 p32 6.2.4 paragraph 3. Internal static variables provide private permanent storage within a single function. Kernighan p83.
• The scope of a function can be limited to a single source file (or compilation unit) by using the static storage class.

6.16 Declarations - 2