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

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Time
Narration
00:01 Welcome to the Spoken Tutorial on Storage class specifiers.
00:07 In this tutorial, We will learn about:
  • Storage class specifiers
  • auto keyword
  • static keyword
  • extern keyword
  • register keyword, with the help of examples.
00:22 For this tutorial, I am using:
  • Ubuntu Operating system version 11.10 and
  • gcc Compiler version 4.6.1 on Ubuntu.
00:34 To follow this tutorial, you should be familiar with 'C' tutorials.
00:41 If not, for relevant tutorials, please visit our website which is as shown.
00:47 I will start with an introduction to storage class specifiers.
00:52 Specifiers tell the compiler- * where to store a variable
00:57 * How to store the variable
00:59 * What is the initial value of the variable
01:03 * Life time of the variable.
01:06 The Syntax is: storage_specifier data_type variable _name
01:13 Types of storage class specifiers are:
  • auto
  • static
  • extern
  • register .
01:21 Let us start with auto keyword.
01:24 auto keyword declares an automatic variable.
01:28 It has a local scope.
01:30 Keywords are not initialized automatically.
01:34 You should explicitly initialize keywords while declaring.
01:39 Storage space of keywords is CPU memory.
01:43 Let us see an example. I have a code file; let us go through it.
01:49 Note that our filename is auto.c.
01:54 We have declared a function as “increment()”.
01:58 This is the main() function.
02:00 In the main() function, increment() function is called 4 times.
02:06 Then we have the return 0 statement.
02:10 Let us see the function definition.
02:14 Here, we have declared variable 'i' as auto int. It has a local scope.
02:21 Then we display value of 'i' using printf.
02:26 Value of i is incremented here.
02:30 Let us open the terminal by pressing Ctrl+Alt+T keys simultaneously on your keyboard.
02:38 Type: gcc space auto.c space hyphen o space auto. Press Enter.
02:48 Type: dot slash auto
02:51 The output is zero.
02:54 Now come back to our program.
02:57 Let us initialize the auto variable i above the main() function.
03:02 I will cut this declaration and initialization from here and paste it over here.

Click on Save.

03:14 Let us execute on the terminal. Press the up-arrow key twice.

Press Enter.

03:22 We get an error: "file-scope declaration of 'i' specifies 'auto'" .
03:29 This is because an auto variable is local to the function.
03:34 We cannot initialize it globally.
03:37 Let us fix the error. Come back to our program.
03:42 Delete this; paste it over here.
03:47 Click on Save and execute on the terminal.
03:52 Press the up-arrow key. Recall the previous command.
03:57 Press Enter. Type: dot slash auto. Press Enter.
04:03 Yes, it is working! The output is zero.
04:07 This is because we have initialized the value of 'i' as zero.
04:13 Now, let us see static variable.
04:16 Although we have studied about static variable in the previous tutorials, I will explain it here briefly.
04:24 'static' variables are initialized to zero.
04:28 They are not destroyed even after program control exits from the block.
04:35 Value of the variable persists between different function calls.
04:41 Storage space is CPU memory.
04:45 Let us see an example. I will edit the same code file.
04:51 Come back to our program.
04:54 Press Ctrl + Shift + S keys simultaneously.
05:01 Now, I will just change the filename as static. Click on Save.
05:10 Now, I will change the initialization of the variable 'i' to static int i equal to zero

Click on Save.

05:23 Let us see what happens. Execute the file on the terminal.
05:30 Type: gcc space static.c space hyphen o space stat. Press Enter.
05:41 Type: dot slash stat. Press Enter.
05:46 The output is displayed as: "0, 1, 2, 3"
05:51 This is because, static variables are global variables.
05:56 The scope of static variable is local to the function, they are defined in.
06:03 They do not lose their value between function calls.
06:08 Now, let us learn about extern keyword.
06:12 Scope of extern variable is throughout the main program.
06:17 Definition for extern variable might be anywhere in the 'C' program.
06:23 extern variables are initialized to zero, by default.
06:28 They can be accessed by all functions in the program.
06:33 These are stored in CPU memory.
06:36 Let us see an example.
06:38 I have a code file; let us go through it.
06:42 Note that our filename is "extern.c".
06:47 I have initialized a variable as integer variable 'x' to 10.
06:54 This is the main() function. In the main() function, I have declared an extern integer variable y.
07:03 Using the printf statements, we will display the values of x and y.

This is the return statement.

07:12 We will initialize 'y' to 50 after the main() function close.
07:18 Now, switch to the terminal and let us see what will be the output.
07:24 Type: gcc space extern.c space hyphen o space ext. Press Enter.
07:35 Type: dot slash ext. Press Enter.
07:40 The output is displayed as:

The value of x is 10 The value of y is 50

07:48 As we studied, the value of the extern keyword is through out the main program.
07:55 We can define it anywhere in the program.
07:59 Both the statements are justified.
08:02 Now, let us move on to register keyword.
08:06 register variables will be accessed faster than normal variables.
08:13 They are stored in register memory rather than main memory.
08:19 Limited number of variables can be used since register size is very low.
08:25 16 bits, 32 bits or 64 bits.
08:30 Let us see an example now. I have a code file. Let us go through it.
08:37 Note that the file name is register.c
08:42 Here, we have declared register integer variable.
08:47 This variable will be directly stored in the register memory.
08:53 This is the 'for' loop that displays the value of 'i' from 1 to 5.
08:59 This will display the value of 'i'.
09:03 Let us execute the program and see.
09:07 On the terminal, type: gcc space register.c space hyphen o space register
09:17 Press Enter. Type: dot slash register. Press Enter.
09:25 You can see, the output is displayed as: "Values stored in register memory 1 2 3 4 5".
09:34 This brings us to the end of this tutorial. Let us summarize.
09:39 In this tutorial, we learnt-
  • Storage class specifiers
  • auto keyword
  • static keyword
  • extern keyword
  • register keyword
09:52 As an assignment, write a program to print the sum of first 5 numbers.
09:59 Declare both the keywords auto and static in the program.
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Contributors and Content Editors

PoojaMoolya, Pratik kamble, Sandhya.np14