Advance-C/C2/Storage-class-specifiers/English

Title of script: Storage class specifiers in C

Author: Ashwini Patil

Keywords: Video tutorial, storage class, auto, static, extern, register.

Visual Cue	Narration
Display Slide 1	Welcome to the spoken tutorial on Storage class specifiers.
Display Slide 2	In this tutorial, We will learn about Storage class specifiers auto keyword static keyword extern keyword register keyword with the help of examples.
Display Slide 3 System Requirements	For this tutorial I am using Ubuntu Operating system version 11.10 and gcc Compiler version 4.6.1 on Ubuntu
Display Slide 4 Prerequisites spoken-tutorial.org	To follow this tutorial you should be familiar with C and C++ tutorials. If not, for relevant tutorials please visit our website, which is as shown.
Display Slide 5 Introduction	I will start with an introduction to storage class specifiers. Specifiers tell the compiler where to store a variable. How to store the variable. What is the initial value of the variable. Life time of the variable.
Display Slide 6	Syntax: storage_specifier data_type variable _name
Display Slide 7 Types	Types of storage class specifiers are: auto static extern register
Display Slide 8 'auto'	Let us start with auto keyword. Auto keyword declares an automatic variable. It has a local scope. Keywords are not initialized automatically. You should explicitly initialize keywords while declaring Storage space of keywords is CPU memory.
	Let us see an example. I have a code file; let us go through it. Note that our filename is auto.c
#include<stdio.h> void increment(void);	We have declared a function as “increment”.
int main() { increment(); increment(); increment(); increment(); return 0; }	This is the main function. In the main function, increment function is called 4 times. Then we have return 0 statment. Let us see the function definition
void increment(void) { auto int i = 0 ; printf ( "%d ", i ) ; i++; }	Here we have declared variable i as auto int. It has a local scope. Then we display value of i using printf. Value of i is incremented here.
Press Ctrl+Alt+T	Let us open the terminal by pressing Ctrl+Alt+T keys simultaneously on your keyboard.
Compile Type: gcc auto.c -o auto	Type: gcc auto.c -o auto Press Enter.
Execute Type: ./auto	Type: ./auto
	The output is zero.
	Now come back to our program. Let us initialize the auto variable i above the main function.
	I will cut this declaration and initialization from here. And paste it over here. Click on Save
gcc auto.c -o auto ./auto	Let us execute on the terminal. Press the uparrow key twice. Press Enter
	We get an error: file-scope declaration of i specifies auto This is because an auto variable is local to the function. We cannot initialize it globally.
Delete global initialization Highlight the global initialization.	Let us fix the error. Come back to our program. Delete this; paste it over here. Click on Save and execute on the terminal.
	Press the up arrow key. Recall the previous command. Press Enter. Type: ./auto Press Enter. Yes it is working! The output is zero. This is because we have initialized the value of i as zero.
	Now let us see static variable. Although we have studied about static variable in the previous tutorials. I will explain it here briefly.
Display Slide 9 'static'	'static' variables are initialized to zero. They are not destroyed even after program control exits from the block. Value of the variable persists between different function calls. Storage space is CPU memory.
Point to the program.	Let us see an example. I will edit the same code file. Come back to our program.
press ctrl + shft + s keys. Save the file as static.c	Press Ctrl + Shft + S keys simultaneously. Now I will just change the filename as static. Click on Save.
Change auto int i =0; to static int i=0;	Now, I will change the initialization of the variable i to static int i equal to zero Click on Save.
On the terminal Type: gcc static.c -o static -o stat	Let us see what happens. Execute the file on the terminal. Type: gcc space static.c space -o space stat Press Enter
Type: ./stat	Type: ./stat Press Enter
	The output is displayed as: 0, 1, 2, 3 This is because static variables are global variables. The scope of static variable is local to the function they are defined in. They do not lose their value between function calls.
Display Slide 10 extern variable	Now let us learn about extern keyword. Scope of extern variable is throughout the main program. Definition for extern variable might be anywhere in the C program. extern variables are initialized to zero, by default. They can be accessed by all functions in the program. These are stored in CPU memory.
	Let us see an example. I have a code file; let us go through it. Note that our filename is extern.c
#include<stdio.h> int x = 10 ;	I have initialized a variable as integer variable x to 10.
int main() { extern int y;	This the main function. In the main function I have declared an extern integer variable y.
printf ( "The value of x is %d \n", x ) ; printf ( "The value of y is %d",y ) ; return 0; } int y = 50 ;	Using the printf statements we will display the values of x and y. This the return statement.
Highlight int y = 50 ;	We will initalize y to 50 after the main function close.
On the terminal	Now switch to the terminal and let us see what will be the output.
Compile Type: gcc extern.c -o ext	Type: gcc extern.c -o ext Press Enter
Execute Type: ./ext	Type: ./ext Press Enter
Output	The output is displayed as: The value of x is 10 The value of y is 50
Point to the statements.	As we studied, the value of the extern keyword is throughtout the main program. We can define it anywhere in the program. Both the statements are justified.
	Now let us move on to register keyword.
Display Slide 11	Register variables will be accessed faster than normal variables. They are stored in register memory rather than main memory. Limited number of variables can be used since register size is very low. 16 bits, 32 bits or 64 bits.
	Let us see an example now. I have a code file. Let us go through it. Note that the file name is register.c
register int i;	Here we have declared register integer variable. This variable will be directly stored in the register memory.
for(i=1; i<=5; i++) printf("n%d\n",i); return 0; }	This is the for loop that displays the value of i from 1 to 5. This will display the value of i.
Type: gcc register.c -o register Type: ./register	Let us execute the program and see. On the terminal, type: gcc space register.c space -o space register Press Enter. Type: ./register Press Enter.
	You can see the output is displayed as: Values stored in register memory 1 2 3 4 5
	This brings us to the end of this tutorial. Let us summarize.
Display Slide 12 Summary	In this tutorial, we learnt- Storage class specifiers auto keyword static keyword extern keyword register keyword
Display Slide 13	As an assignment, Write a program to print the sum of first 5 numbers Declare both the keywords auto and static in the program
Display Slide 14	Watch the video available at the link shown below It summarises the Spoken Tutorial project If you do not have good bandwidth, you can download and watch it
Display Slide 15 Spoken Tutorial Workshops	The Spoken Tutorial Project Team Conducts workshops using spoken tutorials Gives certificates to those who pass an online test For more details, please write to contact@spoken-tutorial.org
Display Slide 15 Acknowledgement	Spoken Tutorial Project is a part of the Talk to a Teacher project It is supported by the National Mission on Education through ICT, MHRD, Government of India More information on this Mission is available at: http://spoken-tutorial.org\NMEICT-Intro
	This is Ashwini Patil from IIT Bombay. Thank you for joining.

Contributors and Content Editors

Ashwini, Nancyvarkey