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Data Types and Storage Classes in C

A programming language should deal with different types of data. The type of data can be a Whole number, Real number or Character string. C language comes with predefined data types like int, long, float, double and char. We use the word define when we are initializing or assigning a constant value to a variable.

Different Types of Buckets hold different types of Data. A bucket can be thought of like a memory location. C Programming Data Types and Storage Classes Tutorial

C Data Types

Basic C data types are as follows.

1. int – Integer data type
2. short – Integer data type smaller than int
3. long – Integer data type bigger than int
4. float – Real number data type
5. double – Real number data type bigger than float
6. long double – Real number data type bigger than double
7. char – Character data type

1) C Int Data Type

Integers are whole numbers without any decimal points. Size of an int variable is 4 bytes maximum in a 32 bit compiler. In 16 bit old compilers, size of int is only 2 bytes. ‘u’ is appended to define unsigned int variable. To represent signed variables, keyword ‘signed’ is not required. You can use if you want. Compiler will not through error.

int range

Range of signed int variable is from -2147483648 to 2147483647.

Range of unsigned int variable is from 0 to 4294967295.

10, 35, 876, 001 are int type.

0.5, 10.7, 22.5, 123.456789 are not int type. These are float or double type.

Declaring an Int Variable

int k;

Defining an int Variable

int k=10;
int p;
p=25;
unsigned int a= 334u;

Format Specifier for Reading using Scanf

To read or scan signed int values, format Specifier ‘%d’ is used.

To scan unsigned in values, format specifier ‘%u’ is used.

Format Specifier for Printing using Printf

To print signed int values, format specifier ‘%d’ is used.

To print unsigned int values, format specifier ‘%u’ is used.

2) C Short Data Type

Size of a short variable is 2 bytes which is half the size of an int variable.

short range

Range of signed short variable is from -32768 to 32767.

Range of unsigned short variable is from 0 to 65535.

Declaring and Defining Short Variable

short pp = 123;
short qq;
qq = 456;

Format Specifier for Reading using Scanf

To read or scan signed short values, format Specifier ‘%d’ is used.

To scan unsigned short values, format specifier ‘%u’ is used.

Format Specifier for Printing using Printf

To print signed short values, format specifier ‘%d’ is used.

To print unsigned short values, format specifier ‘%u’ is used.

3) C Long Data Type

Size of a long variable is 4 bytes. Letter ‘l’ or ‘L’ is used to define signed long variable. Letters ‘lu’ or ‘ul’ is used to define unsigned long variable.

long range

Range of signed long variable is from -2147483648 to 2147483647.

Range of unsigned long variable is from 0 to 4294967295.

Declaring and Defining long variable

long ab = 45L;
unsigned long p = 5678912ul;

Format Specifier for Reading using Scanf

To read or scan signed long values, format Specifier ‘%ld’ is used.

To scan unsigned long values, format specifier ‘%lu’ is used.

Format Specifier for Printing using Printf

To print signed long values, format specifier ‘%ld’ is used.

To print unsigned long values, format specifier ‘%lu’ is used.

4) C Float Data Type

Float numbers are simply real numbers with decimal points. You can append letter ‘f’ to define a floating point constant. Size of a float variable is 4 bytes.

3.1415 is a double data type constant.

3.1415f is a float data type constant. Use an explicit ‘f’ letter.

Declaring and Defining float variable

float ab = 3.14f;
float p = (float)5678.1234;
float q;
q = 45.678f;

float range

Range of floating point variable can be from -3.4e38 to +3.4e38.

Format Specifier for Reading using Scanf

To read or scan float values, format Specifier ‘%f’ is used.

Format Specifier for Printing using Printf

To print float values, format specifier ‘%f’ is used.

5) C Double Data Type

C double data type is two times the size of a float variable. It can hold a real number with 8 bytes memory. Any real number without any appending letter is a double data type constant.

3.14 is a double data type constant.

Declaring and Defining double variable

double score= 98.12345678;

double range

Range of double variable can be from -1.7e308 to +1.7e308.

Format Specifier for Reading using Scanf

To read or scan double values, format Specifier ‘%lf’ is used.

Format Specifier for Printing using Printf

To print double values, format specifier ‘%lf’ is used.

6) C Long Double Data Type

C long double data type is two times the size of a double variable. It can hold a real number with 10 bytes memory. Any real number with an appending letter ‘l’ or ‘L’ is a long double constant.

3.14L is a long double data type constant.

Declaring and Defining double variable

long double score= 98.12345678L;

long double range

Range of long double variable can be from -1.7e4932 to +1.7e4932.

Format Specifier for Reading using Scanf

To read or scan long double values, format Specifier ‘%Lf’ is used.

Format Specifier for Printing using Printf

To print long double values, format specifier ‘%Lf’ is used.

7) C Char Data Type

C char data type is used to represent or store letters and symbols. Its size is only 1 byte.

ASCII – American Standard Code for Information Interchange defined integer codes for each letter, number and symbol.

Declaring and Defining char variable

char group= 'M';

char range

Range of signed char variable can be from -128 to + 127.

Range of unsigned char variable can be from 0 to + 255.

Format Specifier for Reading using Scanf

To read or scan character values, format Specifier ‘%c’ is used.

Format Specifier for Printing using Printf

To print character values, format specifier ‘%c’ is used.

Storage Classes in C

A Storage Class in C language decides the following.

• Where to store a variable in memory
• Default value to be assigned to the variable at the time of creation.
• Scope of a variable within which the variable can store value and outside which the variable is not accessible.
• Life of the variable or the amount of time variable is kept for use.

Each and every variable in C language requires both Data Type and Storage Class. By default, ‘auto’ storage class is applied to a variable.

Storage Classes in C

There are four types of Storage Classes in C language. Keywords used to declare or define C storage classes are auto, register, static and extern.

1. Auto Storage Class or Automatic Storage Class
2. Register Storage Class
3. Static Storage Class
4. Extern Storage Class

1. Auto Storage Class in c

An ‘auto’ storage variable is stored in memory. This memory is called Stack Memory.

Default value of an auto storage variable is garbage value or random value.

Scope of an auto variable is local to the block where it is defined.

Life of an auto variable is till the execution control stays within the block where it is defined.

In the below example, variable ‘a’ is declared without ‘auto’ keyword. But the compiler treats variable like auto variable only. There is no need to explicitly mention ‘auto’ storage class. It is applied to variables by default.

int main()
{
   int a; //same as auto int a;
   auto int b;
   printf("%d %d", a, b);
   return 0;
}

As there is no value assigned to variables, output of the above program is just garbage values. A garbage value is simply a random number of declared type. Here the random value is int type.

int main()
{
  auto int a=20;
  {
    int a=10;
    printf("%d",a);
  }
  printf("%d",a);

  return 0;
}
//output = 10 20

In the above example, outer a=20 is different from inner a=10. Curly braces define a block in C language. So, a=10 is cleared from memory after the block. So after printing a=10, execution control goes to outer printf to print a=20. After completion of the block, scope and life of an ‘auto’ variable dies.

In the above infographic image, an auto variable is compared with an AUTO CLOSE BOOK. For each new person, page number of the book is new and random. There is no stored value between function calls.

2. Register Storage Class in c

A Register storage variable is similar to ‘auto’ storage variable. Only difference between an auto variable and register variable is the memory location where the variable is stored. Register variables are stored in CPU Registers. These registers are extra memory locations along with RAM memory locations. CPU registers allow fast reading and writing of data. So processor need not ask RAM to read and write data, thus saving time.

Default value of a Register storage variable is garbage value.

Scope of a Register storage variable is local to the block where it is defined.

Life of a Register variable is till the execution control stays within the block where it is defined.

int main()
{
   register int a=1;
   while(a<10)
   {
	 printf("%d ", a);
	 a++;
   }
}

Register storage class variables are often used with loop counters or iteration variables to reduce execution time on repetitive works. Also, if a variable is used at many places in a program, then it is better to declare that variable under Register storage class to save time.

The number of CPU registers are always limited. So there is no guarantee that the storage class ‘register’ ensures storing the variable in CPU registers. You do not get any error. CPU treats the variable like an auto storage class variable and uses stack memory without any warning.

Also note that the size of a CPU register may be 16 bit or 32 bit. If the variable size is more than the size of the register, CPU automatically places the variables in auto storage class. Suppose you want to store a float variable in Register storage class of 16 bit register CPU. Even if you declare explicitly like register float abc=1.2f;, it is treated like an automatic storage class variable.

3. Static Storage Class in c

A static storage class variable retains its value between function calls. This is called persistence. These static variables die only with the program execution. In case of auto storage variable and register storage variable, value is reset to default after the block or function execution ends. So these auto and register variables can not retain value between function calls.

Static variables are stored in Data Segment Memory instead of Stack Memory.

Default value of a static variable is Zero.

Scope of a static variable is with in the block or function where it is declared.

Life of a static variable exists till the program execution ends.

Storage Classes in C Example 1:

In the image above with a computer and books on a table, a static variable is like a book with a pen. You can keep pen to maintain page number. Even if you close the book (function or block ends), pen maintains the book number. Next time you open the book (subsequent function or block calls), last page number is maintained.

Storage Classes in C Example 2:

If you open many windows in a computer and press sleep button, the computer goes to sleep. Once you wake the PC again, all programs are restored with the previous state. Similarly, the value of a static variable persists between function calls. Once you shutdown (program execution ends) the computer, all programs (variables) are removed from memory.:

Example Program:

int main()
{
  display();
  display();
}
void display()
{
  //invoked only for very first function call
  static int a=10;
  
  static int c;
  //executed everytime fresh as it is just an assignment
  c=30;

  //invoked fresh every time
  auto int b=20;

  a++; b++; c++;
  printf("%d %d %d\n", a, b, c);
}
//output
//11 21 31
//12 21 31

4. Extern Storage Class in c

Extern or External storage class variables are called global variables. These variables are available to all functions of that C Program. These variables are defined outside of all functions. Variables that are declared inside a function or block are called local variables.

Extern variables are stored in Memory or RAM.

Default value of an extern storage class variable is Zero like static variable.

Scope of an extern variable is global or the entire program.

Life of an extern variable is until the program execution ends.

When to use external variables?

Declare and define an extern variable when you want to make the variable available to all functions of the C program. Also, if you want to make the variable available to functions outside the C program where you defined it.

To use external variables in your program, you need to do two things.

1. Use include<> statement to import the C files into your program.
2. Declare the extern variable in other C files with an ‘extern’ keyword.

Example 1:

int stock=5;
int main()
{
  display();
  stock++;
  display();
}
void display()
{
  printf("Stock=%d\n",stock);
}
//output
//Stock=5;
//Stock=6;
//variable stock is available to both main() and display() functions.

Example 2:

To use a variable that is not defined until the execution point use ‘extern’ keyword.

int main()
{
  extern int stock;
  printf("Stock=%d\n",stock);
}
extern int stock=5;

In other programming languages, the order of definition of global variables is not important.

Example 3:

You can make your program variable available to included C Files using the extern keyword.

#include<display.c>
void showme();

int stock=10;
int main()
{
  printf("Stock=%d\n",stock);
  showme();
}

//display.c file
extern int stock;
void showme()
{
  stock++;
  printf("Stock=%d", stock);
}

//output
// Stock=10
// Stock=11

In other programming languages, the order of definition of global variables is not important.

[WpProQuiz 22]

Data Types and Storage Classes in c interview MCQ

1) What is a C Storage Class.?

A) C Storage decides where to or which memory store the variable.

B) C Storage Class decides what is the default value of a variable.

C) C Storage Class decides what is the Scope and Life of a variable.

D) All the above.

Answer [=] D

2) Every C Variable must have.?

A) Type

B) Storage Class

C) Both Type and Storage Class

D) Either Type or Storage Class

Answer [=] C

Explanation:

Yes. A C Variable should have both Type and Storage Class.

Eg.

int a=5; //By default its Storage Class is auto.
auto int b=10;

3) Find a C Storage Class below.

A) static

B) auto

C) register & extern

D) All the above

Answer [=] D

4) What is the default C Storage Class for a variable.?

A) static

B) auto

C) register

D) extern

Answer [=] B

Explanation:

int a=5; // auto is by default
auto int b=10; // storage class auto is explicitly mentioned.

5) Choose a right answer.

A) auto variable is stored in ‘Memory’. static variable is stored in ‘Memory’. extern variable is stored in ‘Memory’. register variable is stored in ‘Memory’.

B) auto variable is stored in ‘Memory’. static variable is stored in ‘Memory’. extern variable is stored in ‘Memory’. register variable is stored in ‘Register’.

C) auto variable is stored in ‘Register’. static variable is stored in ‘Register’. extern variable is stored in ‘Register’. register variable is stored in ‘Memory’.

D) auto variable is stored in ‘Register’. static variable is stored in ‘Register’. extern variable is stored in ‘Register’. register variable is stored in ‘Register’.

Answer [=] B

Explanation:

Only a register variable is stored in CPU Memory called Registers. Other variables are stored in RAM.

6) A register variable is stored in a Register. Where does a Register Present in a Computer.?

A) RAM ( Random Access Memory )

B) ROM ( Read Only Memory )

C) CPU (Central Processing Unit )

D) DMA ( Direct Memory Access )

Answer [=] C

Explanation:

Yes. Registers are part of a CPU to store variable whose value changes frequently. Loop variables for example.

register int i=1;
while(i <= 10)
{
	printf("i=%d\n", i); 
}

7) Variables of type auto, static and extern are all stored in .?

A) ROM

B) RAM

C) CPU

D) Compiler

Answer [=] B

8) Find a right answer.

A) Default value of auto variable = Garbage Value Default value of static = Garbage Value Default value of extern = Garbage Value Default value of register = Garbage Value

B) Default value of auto variable = zero Default value of static = zero Default value of extern = zero Default value of register = zero

C) Default value of auto variable = Garbage Default value of static = zero Default value of extern = zero Default value of register = Garbage

D) Default value of auto variable = zero Default value of static = Garbage Default value of extern = Garbage Default value of register = zero

Answer [=] C

9) Find a correct statement.

A) Scope of auto variable = local to block or function Scope of register variable = local to block or function Scope of static variable = local to block or function Scope of extern variable = global or available to all functions and blocks

B) Scope of auto variable = global or available to all functions and blocks Scope of register variable = global or available to all functions and blocks Scope of static variable = global or available to all functions and blocks Scope of extern variable = local to block or function

C) Scope of auto variable = global or available to all functions and blocks Scope of register variable =  local to block or function Scope of static variable = global or available to all functions and blocks Scope of extern variable = local to block or function

D) Scope of auto variable = local to block or function Scope of register variable = global or available to all functions and blocks Scope of static variable = local to block or function Scope of extern variable = global or available to all functions and blocks

Answer [=] A

10) Choose a correct statement.

A) Life of an auto variable = persists between function calls Life of an register variable = with in the block or function Life of an static variable = persists between function calls Life of an extern variable = until program ends

B) Life of an auto variable = persists between function calls Life of an register variable = until program ends Life of an static variable = persists between function calls Life of an extern variable = with in the block or function

C) Life of an auto variable = until program ends Life of an register variable = until program ends Life of an static variable = until program ends Life of an extern variable = until program ends

D) Life of an auto variable = with in the block or function Life of an register variable = with in the block or function Life of an static variable = persists between function calls Life of an extern variable = until program ends

Answer [=] D

11) Which among the following is a Local Variable.?

A) register

B) auto

C) static

D) extern

Answer [=] B

12) Which among the following is a Global Variable.?

A) auto

B) register

C) static 

D) extern

Answer [=] D

Explanation:

#include<stdio.h>

#include<myutils.c>

void myshow();
int a=10; // a is a global variable.
//Notice that there is no 'extern' keyword.
void main()
{
	printf("a=%d ", a);
	a = a + 1;
	myshow();
	show();

}

void show()
{
	printf("a=%d ", a);
}

//myutils.c file
//here we used extern to inform compiler
//about possible definition somewhere
extern int a;
void myshow()
{
	printf("a=%d ", a);
	a++;

}

//Output is 10 11 12

13) Choose a correct statement about static variable.

A) A static global variable can be accessed in other files.

B) A static global variable can be used only in a file in which it is declared.

C) A static global variable can not be declared without extern keyword.

D) Default value of a static variable is -1.

Answer [=] B

Explanation:

#include<stdio.h>
static int a=15;
void myshow();
int main()
{
	printf("%d", a);
	myshow();
	return 0;
}


//myutils.c file
extern int a;
void myshow()
{
	printf("%d", a);
}


//Compiler error.
//You can not use static global varible outside of file

14)

register float a = 3.14f;

Choose right statement.

A) Variable a is stored in CPU registers for fast access.

B) Variable a is converted to int and then stored in a CPU register.

C) register Storage Class is ignored and treated as

auto float a = 3.14f;

D) You get a compiler error as you can not store non integer value in a CPU register.

Answer [=] C

Explanation:

Yes. CPU register can not store anything more than 16 bits or 2 bytes. You will not any compiler errors. It will be treated just like an auto Storage Class variable.

15) What is the difference between Declaration and Definition.?

A) Declaration does allocate memory for a variable. Definition does allocate memory for a variable.

B) Declaration does allocate memory for a variable. Definition does not allocate memory for a variable.

C) Declaration does not allocate memory for a variable. Definition does allocate memory for a variable.

D) Declaration does not allocate memory for a variable. Definition does not allocate memory for a variable.

Answer [=] C

Explanation:

int a; //Definition. Allocates memory.

extern int b; //Does not allocate memory.

16) Choose a right statement.

A) A non static global variable can not be used in included files.

B) A non static global variable can be used or referred to inside included files.

C) A non static global variable does not live till the end of program execution.

D) None of the above

Answer [=] B

17) Choose a right statement.

A) Redeclaration of a variable is Ok.

B) Redefinition of a variable is not Ok.

C) Definition of a variable uses memory blocks.

D) All the above.

Answer [=] D

18) Choose a correct statement.

A) Register variables are usually fast retrieving variables.

B) Static variables are usually maintain their values between function calls.

C) Auto variables release their memory after the block or function where they are declared.

D) All the above.

Answer [=] D

19) Choose a right statement.

A) Variables of type auto are stored in Stack memory.

B) Variable of type Static are stored in Segmented Memory.

C) Variables of type register are stored in Micro Processor Memory.

D) All the above.

Answer [=] D

20) Choose a right statement.

A) Variables of type auto are initialized fresh for each block or function call.

B) Variables of type static are initialized only first time the block or function is called.

C) Variables of type register are initialized each time the block or function is executed.

D) All the above.

Answer [=] D