Computer Programming Unit –3
POINTERS INTRODUCTION
Pointers are important in c-language. Some tasks are performed more easily with pointers such as dynamic memory allocation, cannot be performed without using pointers. So it’s very necessary to learn pointers and to become a perfect c programmer.
As you know, every variable is a memory location and every memory location has address which is defined and accessed using ampersand (&) operator.
#include
Pointers: pointer is a variable that stores address of another variable.
Pointer are used to allocate memory dynamically i.e at runtime in c-language.
Pointer variable might belonging to any of the data type such as int, float, char, double, short, e.t.c.
Syntax : data_type * variable_name;
Example : int *p; char * ch;
Where * is used to denote the pointers. P is the pointer variable and not a normal variable.
Key points to remember about pointer in c
Normal variable stores the value whereas pointer variable stores the address of the variable. The content of the C pointer always be a whole number i.e. address. Always C pointer is initialized to null, i.e. int *p = null. The value of null pointer is 0. & symbol is used to get the address of the variable. * symbol is used to get the value of the variable that the pointer is pointing to. 1 Prepared by Gajjala.varaprasad , JNTU Anantapur
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If a pointer in C is assigned to NULL, it means it is pointing to nothing. Two pointers can be subtracted to know how many elements are available between these two pointers. But, Pointer addition, multiplication, division are not allowed. The size of any pointer is 2 byte.
Pointer is more efficient in handling array and structure. It reduces the length of the program and execution time.
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Pointers are one of the most distinct and exciting features of c-language. It provides power and flexibility to the language.
DECLARING A POINTER
A variable is a declared as a pointer that holds a memory address.
Syntax for pointer declaration
Datatype *pointer_name;
Explanation:
Data type: type of variable that pointer points to. Or data type whose value is stored in pointer name.
Asterisk: it is called as indirection operator. It is also called as value at address operator.
Pointer_Name: Must follow be all rules of variable. It Must be any c-identifier
Ways of Declaring Pointer Variable:
* can appears anywhere between Pointer_name and Data Type.
int *p; or int* p; or int * p;
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Example of Declaring Integer Pointer : int n = 20; int *ptr; Example of Declaring Character Pointer : char ch = 'A'; char *cptr; Example of Declaring Float Pointer : float fvar = 3.14; float *fptr;
Example program for pointers in C:
Initialization of Pointers or initializing pointers
Pointer initialization is the process of assigning address of the variable to pointer variable. Pointer variable contains address of variable of same data type. In c-language address operator ‘&’ is used to determine the address of variable i.e it is used to get the address of variable.
int a=10; int *ptr; // pointer declaration ptr = &a; // pointer initialization
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Computer Programming Unit –3 int *ptr = &a; // initialization & declaration %p is a control specifier, which is used for displaying the address in hex format. int *p1; double *p2; char *p3; float *p4;
Note : if you need a pointer to store the address of integer variable, then the data type of pointer should be int. By using *pointer we can access the value of variable
example : double a=10; double *p; p=&a;
Accessing variable through pointer:
We can access the value of pointer using the de-reference operator and address of variable can be printed using address operator.
Program : accessing value and address of Pointer
#include
} Output : Address of i = 65524 Address of i = 65524 Address of j = 65522 Address of j = 65522
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POINTER OPERATORS
C Pointers – Operators that are used with Pointers The operators & and * that are used with Pointers in C. The & operator is also known as “Address of” Operator.
Pointer Operator in C Program :
#include
In order to create pointer to a variable we use “*” operator and to find the address of variable we use “&” operator.
[box]Don’t Consider “&” and “*” operator as Logical AND and Multiplication Operator in Case of Pointer.
Important Notes :
1. ‘&’ operator is called as address Operator. 2. ‘*’ is called as ‘Value at address’ Operator. 3. ‘Value at address’ Operator gives ‘Value stored at Particular address. 4. ‘Value at address’ is also called as ‘Indirection Operator’.
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Pointer Operators : Live Program
#include
Output: The address of n is 1002 The Value of n is 20 The Value of n is 20
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Important point to note is: The data type of pointer and the variable must match, an int pointer can hold the address of int variable, and similarly a pointer declared with float data type can hold the address of a float variable.
The * Operator is also known as Value at address operator.
If you need a pointer to store the address of integer variable then the data type of the pointer should be int.
int *p1; /*Pointer to an integer variable*/ double *p2; /*Pointer to a variable of data type double*/ char *p3; /*Pointer to a character variable*/ float *p4; /*pointer to a float variable*/ double a = 10; double *p; p = &a; Note:
*p would give us the value of the variable a. The following statement would display 10 as output. printf("%d", *p);
Similarly if we assign a value to *pointer like this:
*p = 200;
It would change the value of variable a. The statement above will change the value of a from 10 to 200.
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Accessing a variable to pointer
Null Pointer
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PASSING POINTER TO A FUNCTION IN C PROGRAMMING (OR) PASSING POINTER AS FUNCTION PARAMETERS
When we pass address to a function, the parameters receiving the address should be pointer. Just like any other argument, pointers can also be passed to a function as an argument.
Let’s take an example to understand how this is done. When pointer is passed to function as an argument, address of memory location is passed instead of value. This is because, pointers stores the location of the memory and not the value. This method is known as call by reference or call by address in c.
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ACCESSING AN ARRAY USING POINTER:
This program declares the array of five element and the elements of that array are accessed using pointer. As we know that array is collection of data items that belongings to same data type in c. By using pointers that can be declared and accessed very easily. When an array is declared, the compiler allocates a base address and sufficient amount of storage to contain all the elements of the array in contiguous memory locations. void main { int *p,sum,i; int x[5]={5,9,6,3,7}; i=0; p=x; sum=0; printf(“Element \t Value \t Address \n\n”); while(i<5) { printf(“x[%d] %d %u \n”, i, *p, p); sum=sum+*p; i++; p++; } Printf(“\n sum=%d\n”, sum); Printf(“\n &x[0]=%u\n”, &x[0]); Printf(“\n p= %u\n”, p); }
ACCESSING STRING MEMBERS USING A POINTER
A string is array of characters, terminated with a null characters. Similar to arrays, we can use a pointers to access the individual characters in a string.
void main() { char *name; int length; char *cptr = name; Name = “DELHI”; while(*cptr!=‘\0’) { Printf(“%c is stored at address %u\n”,*cptr,cptr); cptr++; } Length=cptr-name; Printf(“length of the string = %d\n”, Length); getch(); }
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POINTER EXPRESSION
Arithmetic operations between two or more pointers are not possible. But pointers can be used to perform arithmetic operations on the value they point to
Pointer expressions: pointer variables can be used in expressions like variables. For example if p1 and p2 are properly declared and initialized pointers, then these statements are valid.
e.g: a = *p1 * *p2 / *p3; b = 10* - *p3 / *p2;
Pointer increment is valid in c.
Ex: P++ or p=p1+2 are valid statements. When pointer is incremented, it increases it value by length of the data type. It points to memory location or data type.
(1) Characters – 1 byte (2) integer – 2 bytes (3) Float – 4 bytes (4) double – 8 bytes like other variables pointer variables can be used in expressions
If p1 and p2 are properly declared and initialized pointers, then the following statements are valid:
Y=*p1**p2; Sum=sum+*p1; Z=5*-*p2/ *p1; *p2=*p2+10; *p1=*p1+*p2; *p1=*p2-*p1;
NOTE:
In the third statement there is a blank space between ‘/’ and * because the symbol /*is considered as beginning of the comment and therefore the statement fails.
2) if p1 and p2 are properly declared and initialized pointers then, ‘C’ allows adding integers to a pointer variable.
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Example: Now, int a=5, b=10; P1=p1+1=1000+2=1002; int *p1,*p2; P1=p1+2=1000+ (2*2) =1004; p1=&a; P1=p1+4=1000+ (2*4) =1008; p2=&b; P2=p2+2=3000+ (2*2) =3004;
P2=p2+6=3000+ (2*6) =3012;
Here addition means bytes that pointer data type hold are subtracted number of times that is subtracted to the pointer variable.
3) If p1 & p2 are properly declared and initialized, pointers then ‘C’ allows to subtract integers from pointers. From the above example,
P1=p1-1=1000-2=998; P1=p1-2=1000-4=996; P1=p1-4=1000-8=992; P2=p2-2=3000-4=2996; P2=p2-6=3000-12=2988;
Here the subtraction means byte that pointer data type hold are subtracted number of times that is subtracted to the pointer variable.
4) If p1 & p2 are properly declared and initialize pointers, and both points to the elements of same type. “Subtraction of one pointer from another pointer is also possible".
5) Pointer can also be used with increment and decrement operators.
Ex: int a=10; int *b; b=&a;
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Write a c program to show the effect of increment on pointers variables. Display the memory locations of integer, character, and floating point numbers before and after increment of pointers.
Void main () { int x, *x1; char y, *y1; float z, *z1; clrscr(); printf(“Enter integer, character, float”); scanf(“%d%c%f”,&x,&y,&z); x1 = &x; y1 = &y; z1=&z; printf(“Address of x= %u“, x1); printf(“Address of y= %u“, y1); printf(“Address of z= %u“, z1); x++; y++; z++; printf(“After Increment in Pointers”); printf(“Address of x= %u“, x1); printf(“Address of y= %u“, y1); printf(“Address of z= %u“, z1); printf(“Size of Data types”); printf(“value of x= %d“, sizeof(x)); printf(“value of y= %d“, sizeof(y)); printf(“value of z= %d“, sizeof(z)); printf(“Values of variables”); printf(“value of x= %d“, x); printf(“value of y= %c“, y); printf(“value of z= %f“, z); getch(); }
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POINTERS AND ARRAYS
Array name itself is an address or pointer. It points to the address of the first element. The elements of the array together with their address can be displayed by using name itself. Array elements are always stored in contiguous memory locations.
When an array is declared, compiler allocates sufficient amount of memory to contain all the elements of the array. Base address i.e address of the first element of the array is also allocated by the compiler. Suppose we declare an array arr, int arr[5]={ 1, 2, 3, 4, 5 };
Assuming that the base address of arr is 1000 and each integer requires two bytes, the five elements will be stored as follows:
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Here variable arr will give the base address, which is a constant pointer pointing to the element, arr[0]. Therefore arr is containing the address of arr[0] i.e 1000. In short, arr has two purpose - it is the name of an array and it acts as a pointer pointing towards the first element in the array. arr is equal to &arr[0] //by default
We can declare a pointer of type int to point to the array arr.
int *p; p = arr; or p = &arr[0]; //both the statements are equivalent.
Now we can access every element of array arr using p++ to move from one element to another.
NOTE: You cannot decrement a pointer once incremented. p-- Won’t work.
POINTERS AND TWO-DIMENSIONAL ARRAYS
A matrix can represent two dimensional elements of an array. Here, the argument of row number and second column number. To display the elements of two-dimensional array using pointer it is essential to have ‘&’ operator as pre-fix with an array name followed by elements, otherwise compiler shows an error.
Write a program to display array elements and their address using pointers.
4.22
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ARRAY OF POINTERS
C language also supports array of pointers. It is nothing but a collection of addresses. Here we store address of variables for which we have to declare an array as a pointer. We can also have array of pointers. Pointers are very helpful in handling character array with rows of varying length.
Write a program to store addresses of different elements of an array-using array of pointers.
4.25
MULTIPLE INDIRECTIONS (POINTERS TO POINTERS)
C allows the pointer to point to another pointer.
It has declaration is similar to normal pointer but have more asterisk sign before them.
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Normally, a pointer contains address of variable. When we define a pointer to pointer, the first pointer contains the address of the second pointer, which points to the location that contains actual value.
Pointer is known as a variable containing address of another of another variable. The pointer variable also has an address. The pointer variable containing address of another pointer variable is called as pointer to pointer.
Ex: **p;
#include
VOID POINTERS
Pointer can also be declared as void type. Void pointer cannot be dereferencing without explicit type conversion. This is because, being void the compiler cannot determine the size of the object that points to, though void pointer declaration is possible, void variables declaration is not allowed.
4.38
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MEMORY ALLOCATION
There are two types of memory allocation
Static memory
Dynamic memory
Static memory Allocation
Memory is allocated at compile time that is called static memory. Every static variable comes under static variable.
int arr[5].
Here it is fixed number of integer’s elements stored using array. It comes under static memory. Here we cannot store sixth element of array. So it is called static array. Because these are comes under fixed size.
Dynamic memory Allocation
The process of memory allocation during program execution is called dynamic memory allocation. The only way to allocated dynamically memory through pointers.
Dynamic memory Allocation means size of array and variable will increase and decreased using pointers that is called dynamic memory allocation.
C language does not have any technique to allocate memory dynamically there are 4 library functions under stdlib.h for dynamic memory allocation.
malloc( ) calloc ( ) realloc( ) free( )
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malloc() function in C:
malloc () function is used to allocate space in memory during the execution of the program. malloc () does not initialize the memory allocated during execution. It carries garbage value. malloc () function returns null pointer if it couldn’t able to allocate requested amount of memory. malloc() function is used for allocating block of memory at runtime. This function reserves a block of memory of given size and returns a pointer of type void. This means that we can assign it to any type of pointer using typecasting. If it fails to allocate enough space as specified, it returns a NULL pointer.
Syntax: void* malloc(byte-size)
Example using malloc() :
int *x; x = (int*)malloc(50 * sizeof(int)); //memory space allocated to variable x free(x); //releases the memory allocated to variable x
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#include
Output
Dynamically allocated memory content : king college of engineering
C calloc()
The name calloc stands for "contiguous allocation".
The only difference between malloc() and calloc() is that, malloc() allocates single block of memory whereas calloc() allocates multiple blocks of memory each of same size and sets all bytes to zero.
By using calloc() we can create the memory dynamically at initial stage. calloc() required 2 arguments of type count, size-type. Count will provide number of elements; size-type is data type size. calloc() will creates the memory in blocks format. Initial value of the memory is zero.
Syntax void *calloc(number of items, element-size)
ptr=(cast-type*)calloc(number, byte-size) ;
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#include
Output:
Dynamically allocated memory content : king college of engineering
realloc() function in C:
realloc () function modifies the allocated memory size by malloc () and calloc () functions to new size. If enough space doesn’t exist in memory of current block to extend, new block is allocated for the full size of reallocation, then copies the existing data to new block and then frees the old block. By using realloc() we can create the memory dynamically at middle stage. Generally by using realloc() we can reallocation the memory. Realloc() required 2 arguments of type void*, size_type. Void* will indicates previous block base address, size-type is data type size. Realloc() will creates the memory in bytes format and initial value is garbage.
Syntax
void* realloc(pointer, new-size)
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Example using realloc() : int *x; x=(int*)malloc(50 * sizeof(int)); x=(int*)realloc(x,100); //allocated a new memory to variable x
#include
Output
Dynamically allocated memory content : fresh2refresh.com Resized memory : space is extended upto 100 characters
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C free()
Dynamically allocated memory created with either calloc() or malloc() doesn't get freed on its own. You must explicitly use free() to release the space. syntax of free() free(ptr);
This statement frees the space allocated in the memory pointed by ptr.
Example programs
Example 2: Using C calloc() and free()
#include
int main( ) { int num, i, *ptr, sum = 0; printf("Enter number of elements: "); scanf("%d", &num); ptr = (int*) calloc(num, sizeof(int)); if(ptr == NULL) { printf("Error! memory not allocated."); exit(0); } printf("Enter elements of array: "); for(i = 0; i < num; ++i) { scanf("%d", ptr + i); sum += *(ptr + i); } printf("Sum = %d", sum); free(ptr); return 0; }
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Example 3: Using realloc( )
#include
Example 1: Using C malloc() and free()
#include
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Example: Find Largest Element Using Dynamic Memory Allocation - calloc()
#include
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PROBLEMS WITH POINTERS
Dangling pointers Memory leaks Double-deallocation/heap corrupting
Dangling pointers: A pointer pointing to a memory location that has been deleted (or freed) is called dangling pointer.
1. Dangling pointers arise when an object is deleted or de-allocated, without modifying the value of the pointer, so that the pointer still points to the memory location of the de-allocated memory. 2. In short pointer pointing to non-existing memory location is called dangling pointer.
Example
// The pointer pointing to local variable becomes // dangling when local variable is static. #include
Output: A garbage Address The above problem doesn’t appear (or p doesn’t become dangling) if x is a static variable.
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MEMORY LEAKS
Memory leaks In computer science, a memory leak occurs when a computer program incorrectly manages memory allocations.
Memory leak occurs when programmers create a memory in heap and forget to delete it. Memory leaks are particularly serious issues for programs like daemons and servers which by definition never terminate.
/* Function with memory leak */ #include
void f() { int *ptr = (int *) malloc(sizeof(int));
/* Do some work */
return; /* Return without freeing ptr*/ } MEMORY OVERWRITE
Since p has been allocated to 10 bytes, if some code snippet tries to write a value to p that is 11 bytes, then this operation will silently, without telling you, eat up one byte from some other location. Let's assume pointer q represents this memory.
Figure 2. Original contents of q
Figure 3. Overwritten contents of q
As a result, the pointer q will have contents that were never expected. Even if your module is coded well, it might behave incorrectly due to a coexisting module doing some memory overwriting. The example code snippet below can also explain this scenario. char *name = (char *) malloc(11);
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In this example, the memcpy operation is trying to write 11 bytes to p, whereas it has been allocated only 10 bytes.
Heap memory corrupt:
Attempting to free memory already freed. Freeing memory that was not allocated. Attempting to read/write memory already freed. Attempting to read/write to memory which was never allocated. Memory allocation error. Reading/writing to memory out of the bounds of a dynamically allocated array
UNDERSTANDING THE SCOPE OF FUNCTIONS
Scope: the scope refers to the visibility of variables. In other words, the scope of variable defines the section of code in which variable is visible scope of variable can be broadly categorized in to three categories.
Local scope Global scope Function scope
Local variables or Local Scope
Variables that are declared inside a function or block are called local variables. The variables which are declared inside function definition are called local variables. By using the local variables, we can access the particular function only.
Ex: Local variable is a variable, which is declared inside the main( ) int main () {
int a, b, c; /* local variable declaration */
a = 10; /* actual initialization */
b = 20;
c = a + b;
printf ("value of a = %d, b = %d and c = %d\n", a, b, c);
return 0;
}
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Global Variables The variables which are declared outside of the function definition are called global variable. A global variable can be used in all functions.
Global variables are defined outside a function, usually on top of the program. Global variables hold their values throughout the lifetime of your program and they can be accessed inside any of the functions defined for the program.
A global variable can be accessed by any function. That is, a global variable is available for use throughout your entire program after its declaration.
#include
int a, b; /* local variable declaration */
a = 10; /* actual initialization */ b = 20; g = a + b;
printf ("value of a = %d, b = %d and g = %d\n", a, b, g);
return 0; }
SCOPE RULES
A scope in C is a region of the program where the defined variable can have its existence and beyond that, variable can't be accessed.
In C programming, Scope of an identifier is the part of the program where the identifier may directly be accessible. C scope rules tells rule of the scope of a variable that what is the scope of that variable. For example, local scope, global scope or just a formal parameters. The scope of a variable is a set of statements or block of statements
Types of scope: A scope may be three types
Block scope Function scope Program scope
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Block scope: A Block is a set of statements enclosed within left and right braces ({ and } respectively). Blocks may be nested in C (a block may contain other blocks inside it). A variable declared in a block is accessible in the block and all inner blocks of that block, but not accessible outside the block. int main( ) { int x = 10, y = 20; // The outer block contains declaration of x and y, so // following statement is valid and prints 10 and 20 printf("x = %d, y = %d\n", x, y); // y is declared again, so outer block y is not accessible // in this block int y = 40;
x++; // Changes the outer block variable x to 11 y++; // Changes this block's variable y to 41
printf("x = %d, y = %d\n", x, y); // This statement accesses only outer block's variables printf("x = %d, y = %d\n", x, y); return 0; }
Output: x = 10, y = 20 x = 11, y = 41 x = 11, y = 20
Block Scope i.e Local Scope of variable is used to evaluate expression at block level. Variable is said to have local scope / block scope if it is defined within function or local block. In short we can say that local variables are in block scope.
Important Points about Block Scope:
1. Block Scope is also called Local Scope. 2. It can be used only within a function or a block. 3. It is not visible outside the block. 4. Variables defined within local scope are called as Local variables.
Function scope:
A label (and only a label) declared inside a function is in scope everywhere in that function, in all nested blocks, before and after its own declaration. Note: a label is declared implicitly, by using an otherwise unused identifier before the colon character before any statement.
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Computer Programming Unit –3 void f() {
goto label; // label in scope even though declared later label:
goto label; // label ignores block scope } void g() { goto label; // error: label not in scope in g() }
A function itself is a block. Parameters and local variables of function follow the same block scope rules.
Program scope:
When variables are declared in the function and all those variables are valid within a function. These variables are unknown to other functions in program. Global variables and local variables are valid in entire program.
Type qualifiers
The keywords which are used to modify the properties of a variable are called type qualifiers. In C programming language, type qualifiers are the keywords used to modify the properties of variables. Using type qualifiers, we can change the properties of variables. C programming language provides two type qualifiers and they are as follows.
const volatile
1. CONST KEYWORD:
Constants are also like normal variables. But, only difference is, their values can’t be modified by the program once they are defined. They refer to fixed values. They are also called as literals. They may be belonging to any of the data type.
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#include
i = 15 ; x = 100 ; // creates an error
printf("i = %d\nx = %d", i, x ) ;
}
Syntax: const data_type variable_name; (or) const data_type *variable_name;
Please refer C – Constants topic in this tutorial for more details on const keyword.
2. VOLATILE KEYWORD:
When a variable is defined as volatile, the program may not change the value of the variable explicitly. But, these variable values might keep on changing without any explicit assignment by the program. These types of qualifiers are called volatile. For example, if global variable’s address is passed to clock routine of the operating system to store the system time, the value in this address keep on changing without any assignment by the program. These variables are named as volatile variable.
Syntax: volatile data_type variable_name; (or) volatile data_type *variable_name;
C – Storage Class Specifiers
Storage class specifiers in C language tells the compiler where to store a variable, how to store the variable, what is the initial value of the variable and life time of the variable. Every variable in C programming has two properties: type and storage class.
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Type refers to the data type of a variable. And, storage class determines the scope and lifetime of a variable.
There are 4 types of storage class:
1. automatic 2. external 3. register 4. static
Local Variable or Automatic (Auto) : keyword is auto
The variables declared inside the function are automatic or local variables.
The scope of this auto variable is within the function only. It is equivalent to local variable. All local variables are auto variables by default.
#include
increment();
increment();
increment();
increment();
return 0; }
void increment(void) {
auto int i = 0 ;
printf ( "%d ", i ) ;
i++; }
Output
0 0 0 0
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2. Example program for extern variable in C:
The scope of this extern variable is throughout the main program. It is equivalent to global variable. Definition for extern variable might be anywhere in the C program.
#include
Output : The value of x is 10 The value of y is 50
3.Example program for register variable in C:
Register variables are also local variables, but stored in register memory. Whereas, auto variables are stored in main CPU memory. Register variables will be accessed very faster than the normal variables since they are stored in register memory rather than main memory. But, only limited variables can be used as register since register size is very low. (16 bits, 32 bits or 64 bits)
#include
Computer Programming Unit –3
for (i=0;i<5;i++) { // Accessing each variable printf("value of arr[%d] is %d \n", i, arr[i]); } return 0; }
Output : value of arr[0] is 10 value of arr[1] is 20 value of arr[2] is 30 value of arr[3] is 40 value of arr[4] is 50
4. Example program for static variable in C:
Static variables retain the value of the variable between different function calls.
//C static example #include
Output:
0 1 2 3
Return Statement:
A function may or may not return a value. A return statement returns a value to the calling function and assigns to the variable in the left side of the calling function.
37 Prepared by Gajjala.varaprasad , JNTU Anantapur
Computer Programming Unit –3
If a function does not return a value, the return type in the function definition and declaration is specified as void.
The return statement is used to return some value or simply pass the control to the calling function. The return statement can be used in the following two ways.
1. return; 2. return expression;
The first form of the return statement is used to terminate the function and pass the control to the calling function. No value from the called function is returned when this form of the return statement is used.
The following program demonstrates the use of the first form of the return statement.
#include
#include
38 Prepared by Gajjala.varaprasad , JNTU Anantapur
Computer Programming Unit –3
} void function1( ) {
Printf(“First \n”);
} void function2( ) { printf(“Second \n”); return;
} int function3( ) { printf(“Third – 1\n”); return 0; printf(“Third – 2\n”);
} int function4( ) { printf(“Fourth\n”); return 0;
}
39 Prepared by Gajjala.varaprasad , JNTU Anantapur