How does the c keyword "extern" work ?

Question

I don't fully understand the concept

I've been browsing the web here and there and found something similar to this:
* Let's say you declare a global variable like so:
int iGlobalVar;
* Then you find yourself inside a code-block and wish to access the global variable:
{
// this will apparently link to the globally declared variable
extern int iGlobalVar;
}
* If I declare the latter one without the "extern" keyword, it would temporarily hide the global version of the variable
* Why do I have to use the extern keyword ?
* Why can't I just use the scope resolution operator ?
{
::iGlobalVar; // this would do the same thing right?
}
Hope I didn't annoyed you with these questions

Answer 1

The extern keyword is used to declare a global variable without defining it, telling the compiler that it has already been defined in a different source file.



C does not have a scope resolution operator. C++ does, but you asked about C.

Answer 2

You can use just the scope resolution operator within the same file. But if it's in a different file, how would the compiler know what type the variable was without the extern declaration?

Answer 3

When you define a normal variable, such as int b = 0; the variable is defined and initialised at run time. That means that space in memory will be allocated at run time and then given the variable name and initial value. A static variable on the other hand static int a=11; is allocated and initialised at compile time. So when the program is loaded into memory at startup the static variables are already created. Have fun.