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Significance of extern and static keywords in C

· Sai kiran

Revised on 30-07-2017 : Added figures

Consider a module in a C program. When I say a module ,it can be a piece of large project. Let us call it as a translation unit.

module

Modular programming helps the programmers working on a large project. We use static and extern keywords extensively. So, to understand these keywords, we need to understand linkage and storage classes in C. Most of the times description of the above keywords confuse.

modular programming

Storage Classes

There are two types of storage classes (describes the life time of the variable) in C.

Those are

  1. automatic storage class
  2. static storage class

A variable with automatic storage class will have its lifetime equal to the execution time of the block it is defined in.

A variable with static storage will have its lifetime equal the execution time of the program (The variable can retain its value between function calls) .

We can divide any translation unit into two parts.

  1. internal to a function
  2. external to all functions

You can find global definitions/declarations (not every external declaration/definition is global because of scope ) and preprocessor directives (most often) external to all function. Of course you can have preprocessor directives anywhere in the translation unit.

According to the standard all the variables defined external to all functions will have static storage class (It has nothing to do with static keyword).

But,

If you want to define a variable internal to any function with static storage class then you need to use static keyword.

static storage for internal variable

Linkage

Linkage determines whether the same name in another scope refers to the same object or function. You can have two types of linkages. Those are

  1. internal linkage
  2. external linkage
  3. no linkage

We use internal linkage only for translation unit. You may want to hide some of global variables or function definitions (global means external to all functions ) from other translation units, in that case you can specify internal linkage using static keyword. Hence, internally linked variables/functions can’t be accessed from other translation units.

We generally use external linkage for communication among translation units. We can allow all translation units to use the same definition of a function/variable. Unless you use static keyword with functions/variables defined external to all functions, they will have external linkage, hence can be called/accessed from any translation unit, provided that the translation unit knows the declarations of the functions/variables (may be using header file). What if you want to use the same definition of a variable in all translation units. We know that we can have any number of declarations of a function/variable but only one definition. Functions have prototypes to declare (no definition). How can you actually declare a variable?. Here is the way (only way) of declaring a variable in C, i.e, by using extern keyword. This tells the compiler that the identifier has been defined somewhere (and resolve its location during linking) but I will access here.

A block-scope variable defined without extern will have no linkage.

If the definition of the external variable occurs in the translation unit before its use in a particular function, then there is no need for an extern declaration in the function. Because of this reason we are able to access global variables in functions.

Linkage

Specifying storage class & linkage

Some keywords are used for syntactic convenience in C. That confuses a lot.

We use storage-class specifiers (some keywords) to specify the storage properties of a variable, those are auto,register,static,extern and typedef .

Don’t think all these keywords will specify storage class (either automatic or static).

  • auto - announces variable has automatic storage-class.

  • register - announces variable has automatic storage-class (if possible store it in CPU registers)

  • static - It has two roles.

    1. Specify static-storage class for internally defined variables.
    2. Announce internal linkage for an identifier (function/variable) defined external to all functions.

  • extern - It specifies that this identifier has been defined somewhere(external) hence no need to allocate memory

    1. If you want to use function defined in somewhere we need to use extern ret-type func(type arg,...); But it is same as ret-type func(type arg,...);. Hence it is optional to use extern with declaration of identifier for a function
    2. For variable identifiers : If you want to use variables defined in some other scope (like other translation units) you need to specify: extern type var-name

  • typedef - does not reserve storage and is called a storage-class specifier only for syntactic convenience.

Bookmarks

References

  • The C Programming Language 2nd Ed by Brian Kernighan and Dennis Ritchie

Code Examples

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