page 201, extern ?

[siavashApp]

The text states that: " when dealing with external variables, you can declare a variable as extern in many places, but you can define it only once."
I don't understand this statement completely. Does it mean that we can not have a method like this:

- (void) setGlobVar:(int)var
{
   extern int gGlobVar;
   gGlobVar=var;
   gGlobVar=102;
}

?

[afterDark]

as I understood it: yes, you can have a method as you described. That method would change the value of the global variable, changing it for 'everyone' accessing that variable. Also in other methods, or in other files within the program.

The word 'extern' in front of the variable definition means that this variable is already defined in another file in this program. And that we would like to access that same variable (and not create a local variable for just this file). Meaning: any changes we make to it, will be made to the global variable. Or: if any other method in any file changes it, we will see that here too.
So 'extern' is like saying "we are defining a variable here because it is unknown yet in our file. But it is already defined in some other file in this program. And we mean to access that very same variable"

[Tpdef]

it's mean you can define the variable only once,
 int gGlobalVar=5;//this is the definition of the global /external variable,

 and if you choose to treat it as external variable, then this is your only place that you can define it,meaning in others files you can only use the extern directive to let the compiler know that you intend to access to a global and external variabele and assign it value ,but you can't define this variable in any other files again
like in this way
 int gGlobalVar=5;
i hope you understand me :-)