1.10 External Variables and Scope

• Variables (automatic) in a function are local or private to the particular function
• Each local variable in a function comes into existence only when the function is called, and disappeared when the function exited
• Automatic variables do not retain their values from one call to the next, and must be explicitly set upon each entry. If they are not set, they will contain garbage
• External variables are globally accessible, exist permanently and retain values after function returned, oppose to automatic variables
• An external variable must be defined, exactly once, outside of any function; to set aside storage for it
• The external variable must also be declared in each function that want to access it; this states the type of the variable. The declaration may be an explicit "extern" statement or may be implicit from context
• If the definition of an external variable occurs in the source file before its use in a particular function, then there is no need for an extern declaration in the function
• Common practice is to place definition of all external variables at the beginning of source files and then omit all extern declarations
• If a program is in several source files, and a variable is defined in file1 and used in file2 and file3, then the extern declarations are needed in file2 and file3 to connect the occurrences of the variable
• The usual practice is to collect extern declaration of variables and functions in a separate file (header)