c memory allocation with examples

Dynamic Memory Allocation:

An ARRAY NAME is actually a POINTER to the FIRST ELEMENT within the

array.

It is possible to define the array as a pointer rather than as a conventional

Array.

A conventional array results in the beginning of program execution.

This can be over ruled if array is represented as a pointer variable.

A pointer variable representing an array requires some type of initial

memory assignment before the array elements are processed which is

called as DYNAMIC MEMORY ALLOCATION.

The DYNAMIC MEMORY ALLOCATION is handled using malloc() &

calloc() functions.

Whenever memory is allocated dynamically, the processed data finds a

place in the HEAP rather then STACK.

Hence the contents of the heap should be freed immediately after operation,

for which free() function is used.

malloc() & calloc() functions are defined to return a pointer to char,

hence the pointer that is returned should be type cast to the appropriate

pointer.

malloc() or calloc() if fail, return a NULL, if success return the address of

the memory chunk that was allocated.

Calloc() needs two arguments i.e number of memory spaces and the data

type to be allocated, and malloc() needs only one argument.

Stack v/s Heap Allocation:

The memory allocated as stack has unique name and address, therefore

memory location can be accessed by name or through address.

The memory allocated as heap does not contain a name, instead contains

only the starting address of the memory allocated, therefore can be

accessed through this address.

The memory allocated through heap can be local or global respective to its

functional definition.

If memory allocated is global then that memory can be accessed through a

different function by locating its address.

In static memory allocation the memory is allocated and freed by system.

In dynamic memory allocation it is the risk of the programmer.

Garbage Collection:

The process of collection of memory locations, which are not in use and

freeing that space is called as GARBAGE COLLECTION.

By default, memory allocated by malloc() contains garbage values and that

allocated by calloc() contains zeros.

<alloc.h> prototype need to be declared when using the Dynamic Memory

Allocation.

Sort a one-dimensional integer Array in ascending order using pointer notation:

Program

#include<stdio.h>

void main()

{

void sort(int, int *);

int k, n, *a;

printf("\nHow Many Numbers Will Be Entered : ");

scanf("%d", &n);

a = (int *) malloc(n * sizeof(int));

for(k = 0; k < n; k++)

{

printf("\nEnter A[%d] Index Element : ", k);

scanf("%d", a + k);

}

sort(n, a);

for (k = 0; k < n; k++)

printf("\n\nThe Sorted Data A[%d] : %d", k, *(a + k) );

free(a);

}

void sort(int n, int *a)

{

int k, j, temp;

for(k = 0; k < n - 1; k++)

{

for(j = k + 1; j < n; j++)

{

if(*(a + j) < *(a + k))

{

temp = *(a + k);

*(a + k) = *(a + j);

*(a + j) = temp;

}

}

}

return;

}

Pointer and Strings:

Illustration 1

Program

#include<stdio.h>

void main()

{

char str[80], *p;

int len = 0;

p = str;

printf("\nEnter The String To Find Its Length : ");

scanf("%s", str);

while(*p != '\0')

{

p++;

len ++;

}

printf("\nThe String is %s and its Length is : %d", str, len);

}

Illustration 2

Program

#include<stdio.h>

#define LEN 21

void main()

{

char *s1, *s2;

int flag = 1;

s1 = (char *) malloc(LEN * sizeof(char));

s2 = (char *) malloc(LEN * sizeof(char));

printf("\nEnter The String 1 : "); scanf("%s", s1);

printf("\n\nEnter The String 2 : "); scanf("%s", s2);

while(*s1 != '\0' && *s2 != '\0')

{

if(*s1 != *s2)

{

flag = 0; break;

}

s1++; s2++;

}

if(flag == 0)

printf("\nThe Strings are Not Equal.");

else

printf("\nThe Strings are Equal.");

}

Revisiting Sorting in ‘C’:

Insertion Sort

Program

#include<stdio.h>

#define N 5

void main()

{

int i, j, k, x, a[N];

printf("\nEnter The Element To Be Inserted ( -9999 to stop) : ");

scanf("%d", &x);

i = 0;

while(x != -9999 && i < N)

{

k = i - 1;

while((x < a[k]) && (k >= 0) )

{

a[k + 1] = a[k];

--k;

}

a[k + 1] = x;

printf("\n\nEnter The Element To Be Inserted (To Stop Enter -9999) : ");

scanf("%d", &x);

++i;

}

printf("\nThe Final Sorted Array : ");

for(j = 0; j < i; j++)

printf("%5d", a[j]);

}