Difference between revisions of "Advance-C/C2/Storage-class-specifiers/English"

• Storage class specifiers
• auto keyword
• static keyword
• extern keyword
• register keyword with the help of examples.

System Requirements

• Ubuntu Operating system version 11.10 and
• gcc Compiler version 4.6.1 on Ubuntu

Prerequisites

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Introduction

• 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.

storage_specifier data_type variable _name

Types

• auto

• static

• extern

• register

'auto'

• 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.

I have a code file; let us go through it.

Note that our filename is auto.c

void increment(void);

{

increment();

increment();

increment();

increment();

return 0;

}

In the main function, increment function is called 4 times.

Then we have return 0 statment.

Let us see the function definition

{

auto int i = 0 ;

printf ( "%d ", i ) ;

i++;

}

It has a local scope.

Then we display value of i using printf.

Value of i is incremented here.

Type: gcc auto.c -o auto

Press Enter.

Type: ./auto

Let us initialize the auto variable i above the main function.

And paste it over here.

Click on Save

./auto

Press the uparrow key twice.

Press Enter

file-scope declaration of i specifies auto

This is because an auto variable is local to the function.

We cannot initialize it globally.

global initialization

Highlight the global initialization.

Come back to our program.

Delete this; paste it over here.

Click on Save and execute on the terminal.

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.

Although we have studied about static variable in the previous tutorials.

I will explain it here briefly.

'static'

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.

I will edit the same code file.

Come back to our program.

Save the file as static.c

Now I will just change the filename as static.

Click on Save.

auto int i =0;

to

static int i=0;

• the variable i to static
• int i equal to zero

Click on Save.

Type:

gcc static.c -o static -o stat

Execute the file on the terminal.

Type:

gcc space static.c space -o space stat

Press Enter

./stat

./stat

Press Enter

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.

extern variable

• 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.

I have a code file; let us go through it.

Note that our filename is extern.c

int x = 10 ;

{

extern int y;

In the main function I have declared an extern integer variable y.

printf ( "The value of y is %d",y ) ;

return 0;

}

int y = 50 ;

This the return statement.

Type: gcc extern.c -o ext

gcc extern.c -o ext

Press Enter

Type: ./ext

Press Enter

The value of x is 10

The value of y is 50

We can define it anywhere in the program.

Both the statements are justified.

• 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.

I have a code file.

Let us go through it.

Note that the file name is register.c

This variable will be directly stored in the register memory.

printf("n%d\n",i);

return 0;

}

This will display the value of i.

gcc register.c -o register

Type: ./register

On the terminal, type:

gcc space register.c space -o space register

Press Enter.

Type: ./register

Press Enter.

Values stored in register memory 1 2 3 4 5

Let us summarize.

Summary

• Storage class specifiers
• auto keyword
• static keyword
• extern keyword
• register keyword

• Write a program to print the sum of first 5 numbers

• Declare both the keywords auto and static in the program

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