How are extern variables defined?

2018-06-14 06:35:17

This question already has an answer here:

What is the difference between a definition and a declaration? 25 answers

How do I use extern to share variables between source files? 15 answers

extern int i

...is a variable declaration, since it only tells the compiler "there exists a variable called i , but it's defined somewhere else".

int i

...is a variable definition, since it tells the compiler to create the actual variable.

The keyword extern is used to declare external variables, so the book is right.

There is one exception, if an initializer is added, then it's a definition:

extern int i = 42;

declaration : variable just declared

Ex: int i;

definition : variable declaration+its initialization with value.

Ex: int i=10;

In your case

extern int i;// is a declaration    
//because you did not initialize value to i here.

Assuming j is Global variable. then it is initialized with ZERO at the time of declaration.
Here declaration+initialization =definition

int j; //definition

if j is local then it is declaration only.

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