'Extern' Storage Class Specifier

You can use extern keyword along with function declaration as well, but that's a redundant operation since functions have external linkage by default. ie; They can be accessed globally from other places.

'Typedef' Qualifier

'typedef' is not a storage class specifier, but it allows us to give an alias (new name) to existing data types (built-in or user defined datatypes). Sytax of 'typedef' is as given below,

typedef data_type alias; 

Let's consider the following example

typedef int number;

Here, we've given an alias to data type 'int' which is 'number'. So now we can use 'number' as a type in variable declaration and definition to hold an integer values. By creating an alias, we can use it as given below

number first, second;

It means 'first' and 'second' are of the 'number' data type which in turn is an integer. So, 'first' and 'second' are also integers.

Summary of Storage Class Specifiers

Table given below summarizes the storage location, initial default value, scope and life time of different storage class specifiers

Storage Class	Space Allocated in	Default Initial Value	Scope	Life
auto	Memory	Garbage	Local Scope	Alive within the declared code block
register	CPU Register	Garbage	Local Scope	Alive within the declared code block
static	Memory	0	Local Scope	Value retained between invocations
extern	Memory	0	Global Scope	Alive till program exit