20.8 extern Declarations

An extern declaration is used to refer to a global variable whose principal declaration comes elsewhere—in the same module, or in another compilation module. It looks like this:

extern basetype decorated-variable;

Its meaning is that, in the current scope, the variable name refers to the file-scope variable of that name—which needs to be declared in a non-extern, non-static way somewhere else.

For instance, if one compilation module has this global variable declaration

int error_count = 0;

then other compilation modules can specify this

extern int error_count;

to allow reference to the same variable.

The usual place to write an extern declaration is at top level in a source file, but you can write an extern declaration inside a block to make a global or static file-scope variable accessible in that block.

Since an extern declaration does not allocate space for the variable, it can omit the size of an array:

extern int array[];

You can use array normally in all contexts where it is converted automatically to a pointer. However, to use it as the operand of sizeof is an error, since the size is unknown.

It is valid to have multiple extern declarations for the same variable, even in the same scope, if they give the same type. They do not conflict—they agree. For an array, it is legitimate for some extern declarations can specify the size while others omit it. However, if two declarations give different sizes, that is an error.

Likewise, you can use extern declarations at file scope (see File-Scope Variables) followed by an ordinary global (non-static) declaration of the same variable. They do not conflict, because they say compatible things about the same meaning of the variable.