What does the extern keyword mean?

Question

What does the extern keyword mean? I\'ve seen that in front of an function declaration like

extern void DoFoo ...

Answer 1

For Beginners,

Initially I was confused to learn that, "the extern keyword declares a variable or function and specifies that it has external linkage " by @Romain Hippeau.

Now I understood that, we will be able to share our variables with other classes through extern keyword.

For Example: Notification.h

 #import 
 extern const NSString* notificationConstant;

Notification.m

 #import "Notification.h"
 const NSString* notificationConstant = @"NotificationConstant";

By importing notification.h in any of my other classes, I can read the value of string NotificationConstant.

Without extern keyword For Notification constant will create following error.