Storage class specifier

Storage class specifier declares that a variable or function declred in a program where stored.There are following types of storage specifier in C :-
1)auto
2)register
3)extern
4)static
5)typedef
6)__declspec
7)mutable
8)thread_local

(1)auto:-auto is a keyword used to declare an automatic storage class.It is declared as following types :-

auto  a=2;              //auto

auto  b=3.8;            //float

auto c='h';            //char

 auto  b1=3.8f;          //float 
 auto  b2=3.8F;          //float
 auto  b3=3.8l;         //double
 auto  b4=3.8L;          //long double 

 printf("a=%d\n",a);    //int 

printf("b=%f\n",b);    // float

printf("c =%c\n",c);   //char

2)resister:-resister is also a storage class used to faster access of variable because the storage will be done in CPU registers .It is a hint to the compiler that the variable will be heavily used and that  recommend it will be kept in a processor register if possible.We can declare a datatype register in following manner:-

register int ri=98;

3)extern:-extern is also a storage class specifier .It is used for the use a variable which is initialized in another file in same project.When we use extern modifier with any variables it is only declaration i.e. memory is not allocated for these variable.We can declare register keyword in following manner:-

extern int ei=23;

4)Static:-static is also a storage class specifier used in C/C++.It also stores variable in memory.The default initial value is zero.The value of the variable persists between different function calls.We can use a static datatype  in following manner:-

  

#include <stdio.h>

void func() {
        static int x = 0; // x is initialized only once across three calls of func()
        printf("%d\n", x); // outputs the value of x
        x = x + 1;
}

int main(int argc, char * const argv[]) {
        func(); // prints 0
        func(); // prints 1
        func(); // prints 2
        return 0;
}

(5)typedef:-typedef keyword provide us to rename a variable datatype into a short and meaningful way.

Ex:-

                typedef unsigned long int ULI;

                 ULI a;

13)_declspec:-It is a storage class modifier  used with many extended attribute like dllimport and dllexport.

7)mutable:-This keyword can only be applied to non-static and non-const data members of a class. If a data member is declared mutable, then it is legal to assign a value to this data member from a const member function.

mutable member-variable-declaration;

8)thread_local:-The variable is allocated when the thread begins and deallocated when the thread ends. Each thread has its own instance of the variable. Only variables declared thread_local have this storage duration. thread_local can only be declared for global variables, plus those declared with static or extern.It is declared as following:-


 thread_local unsigned int rage = 1


Here are some inportant notes about storage class specifiers:-

C program instructions get stored in code/text segment
Register variables are stored in Register. 
The memory created dynamically are stored in Heap.
Local Variables/arrays (Except static) are stored in Stack.  
 Global, extern & static variables/arrays are stored in data segment.

 a) Uninitialized static/global/extern variables are stored in the BSS(Block started by symbol) of the data     segment .ie high address..
               (Note: Uninitialised static variable is automatically initialised to zero)
               (Note: Uninitialised global variable is automatically initialised to zero)
               (Note: Uninitialised extern variable is automatically initialised to zero)
 b) Initialized static/global/extern variables are stored in data section of the data segment. ie low address
               (Initialised static also includes zero initialised static variable)

const local variable(initialised) is stored on stack.
const local var (uninitialized) is NOT POSSIBLE. It must be initialized when declared.
const global var (initialized) is stored in data section of data segment.
const global var (uninitialized) is NOT POSSIBLE. It must be initialized when declared.
-----------
null pointer ->   .bss
static pointer (uninitialised)-> (initialized to null by default) -> null pointer ->   .bss
command line arguments (stack)
function parameters (stack)

----------
Data in the bss segment is initialized by the kernel to zero before the program starts executing.
------------
Default storage class specifiers:
For external declarations (outside a function) the default storage class specifier will be extern and for internal declarations(inside a function) it will be auto.

default storage class specifier for functions  is always extern.
We can understand this more easily by following details:-

Object	Storage Class Specifiers	Storage Duration (Life Time)	Linkage	Scope (Accesibility/Visibility)	Storage Area
Internal variable	auto	Automatic	No	Function/Block/Compound Stmt	SS:stack			Initialized with garbage value
Internal variable	register	Automatic	No	Function/Block/Compound Stmt	Register
Internal variable	static	static	No	Function/Block/Compound Stmt	DS	Uninitialized	DS:BSS	Initialized with zero
Internal variable	extern	static (???)	external (???)	Function/Block/Compound Stmt	None	Initialized	DS:data
Internal variable	none (default auto)	Automatic	No	Function/Block/Compound Stmt	SS:stack			Initialized with garbage value
Internal Function	auto	static (???)	external (???)	Project (multiple files)	CS
Internal Function	register	invalid	invalid	invalid	invalid
Internal Function	static	invalid	invalid	invalid	invalid
Internal Function	extern	static (???)	external (???)	Project (multiple files)	CS
Internal Function	none (default extern)	static (???)	external (???)	Project (multiple files)	CS
External variable	auto	static	external	Project (multiple files)	DS
External variable	register	invalid	invalid	invalid	invalid
External variable	static	static	internal	File	DS	Uninitialized	DS:BSS	Initialized with zero
External variable	extern	static	external	Project (multiple files)	DS/None	Initialized	DS:data
External variable	none (default extern)	static	external	Project (multiple files)	DS
External Function	auto	static	external	Project (multiple files)	CS
External Function	register	invalid	invalid	invalid	invalid
External Function	static	static	internal	File	CS				Storage Area
External Function	extern	static	external	Project (multiple files)	CS			extern int i;	None	intialized to 5
External Function	none (default extern)	static	external	Project (multiple files)	CS			int i=5;
	thread_local	thread	No					extern int i=6;	DS:data	intialized to 6
	dynamic mem allocation	Dynamic	No		SS:Heap
								extern int i;	None	intialized to 0
		Automatic	No					int i;
		Thread	No
		Dynamic	No