To understand how external variables relate to the extern keyword, it is necessary to know the difference between defining and declaring a variable. When a variable is defined, the compiler allocates memory for that variable and possibly also initializes its contents to some value. When a variable is declared, the compiler requires that the variable be defined elsewhere. The declaration informs the compiler that a variable by that name and type exists, but the compiler does not need to allocate memory for it since it is allocated elsewhere. The extern keyword means "declare without defining". In other words, it is a way to explicitly declare a variable, or to force a declaration without a definition. It is also possible to explicitly define a variable, i.e. to force a definition. It is done by assigning an initialization value to a variable. If neither the extern keyword nor an initialization value are present, the statement can be either a declaration or a definition. It is up to the compiler to analyse the modules of the program and decide. A variable must be defined exactly once in one of the modules of the program. If there is no definition or more than one, an error is produced, possibly in the linking stage. A variable may be declared many times, as long as the declarations are consistent with each other and with the definition. It may be declared in many modules, including the module where it was defined, and even many times in the same module. But it is usually pointless to declare it more than once in a module. An external variable may also be declared inside a function. In this case the extern keyword must be used, otherwise the compiler will consider it a definition of a local variable, which has a different scope, lifetime and initial value. This declaration will only be visible inside the function instead of throughout the function's module. The extern keyword applied to a function prototype does absolutely nothing. A function prototype is always a declaration and never a definition. Also, in standard C, a function is always external, but some compiler extensions allow a function to be defined inside a function.
An external variable can be accessed by all the functions in all the modules of a program. It is a global variable. For a function to be able to use the variable, a declaration or the definition of the external variable must lie before the function definition in the source code. Or there must be a declaration of the variable, with the keyword extern, inside the function. The static keyword, applied to the definition of an external variable, changes this a bit: the variable can only be accessed by the functions in the same module where it was defined. But it is possible for a function in the same module to pass a reference of the variable to another function in another module. In this case, even though the function is in another module, it can read and modify the contents of the variable—it just cannot refer to it by name. It is also possible to use the static keyword on the definition of a local variable. Without the static keyword, the variable is automatically allocated when the function is called and released when the function exits. Its value is not retained between function calls. With the static keyword, the variable is allocated when the program starts and released when the program ends. Its value is not lost between function calls. The variable is still local, since it can only be accessed by name inside the function that defined it. But a reference to it can be passed to another function, allowing it to read and modify the contents of the variable. External variables are allocated and initialized when the program starts, and the memory is only released when the program ends. Their lifetime is the same as the program's. If the initialization is not done explicitly, external and local static variables are initialized to zero. Local automatic variables are uninitialized, i.e. contain "trash" values. The static keyword applied to a function definition changes the linkage of the function so that it is only visible from the translation unit where its definition is located. This prevents the function from being called by name from outside its module. Declaring a function using the static keyword is also a good way to keep its name short while avoiding name clashes.
Example (C programming language)
File 1: // Explicit definition, this actually allocates // as well as describing int Global_Variable; // Function prototype, assumes // defined elsewhere, normally from include file. void SomeFunction; int main
File 2: // Implicit declaration, this only describes and // assumes allocated elsewhere, normally from include extern int Global_Variable; // Function header void SomeFunction
In this example, the variable Global_Variable is defined in File 1. In order to utilize the same variable in File 2, it must be declared. Regardless of the number of files, a global variable is only defined once; however, it must be declared in any file outside of the one containing the definition. The normal methodology is for allocation and actual definitions to go into.c files, but mere declarations and prototypes do not allocate and just describe the types and parameters so that the compiler can work correctly, and that information belongs in a.h header file that others can safely include without any possible conflict.
