Dynamic memory allocation in C

Dynamic memory allocation in C:

     The process of allocating memory during program execution is called dynamic memory allocation.

This is use full in the situations when the size of array we declare initially is fixed and cannot be modified . Dynamic memory allocation allows a program to obtain variable amount of memory space, while running or to release space when no space is required.

Dynamic memory allocation functions in C:

C language offers 4 dynamic memory allocation functions. They are library functions under "stdlib.h".

1. malloc()
2. calloc()
3. realloc()
4. free()

S.no	Function	Syntax
1	malloc ()	Allocates requested size of bytes and returns a pointer first byte of allocated space Syntax : ptr=(cast-type*)malloc(byte-size) Here, ptr is pointer of cast-type.                                     Example : ptr=(int*)malloc(100*sizeof(int));
2	calloc ()	Allocates space for an array elements, initializes to zero and then returns a pointer to memory Syntax : ptr=(cast-type*)calloc(n, element-size); Here, ptr is pointer of cast-type. Example :ptr=(float*)calloc(25, sizeof(float));
3	realloc ()	Change the size of previously allocated space Syntax : ptr=realloc(ptr-name, newsize); Here, ptr is pointer of cast-type. Example :ptr=realloc(ptr, 40);
4	ree ()	deallocate the previously allocated space   Syntax : free (pointer_name); Example :free(ptr); Here, ptr is pointer to the memory that is dynamically created..

1. malloc() function in C:

• malloc () function is used to allocate single block of 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 a pointer if it has created a memory otherwise returns a null pointer if it couldn’t allocate requested amount of memory.

·      Syntax : ptr=(cast-type*)malloc(byte-size)

           Here, ptr is pointer of cast-type.

·         Example : ptr=(int*)malloc(100*sizeof(int));

If the size of int is 2 bytes The statement will allocate 200 and the pointer points to the address of first byte of memory.

2. calloc() function in C:

·      calloc() allocates multiple blocks of memory each of same size and sets all bytes to zero.

• calloc () function returns a pointer if it has created a memory otherwise returns a null pointer if it couldn’t allocate requested amount of memory.

·      Syntax : ptr=(cast-type*)calloc(n, element-size);

Here, ptr is pointer of cast-type.

·         Example :ptr=(float*)calloc(25, sizeof(float));

This statement allocates contiguous space in memory for an array of 25 elements each of size of float, i.e, 4 bytes.

3. 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.

·      Syntax : ptr=realloc(ptr-name, newsize);

Here, ptr is pointer of cast-type.

• Example : ptr=(int*)malloc(100*sizeof(int));

If the size of int is 2 bytes The statement will allocate 200 and the pointer points to the address of first byte of memory.

          ptr=realloc(ptr, 150* sizeof(int));

The statement will reallocate memory of 300 bytes  and returns pointer points to the address of first byte of memory otherwise returns a null pointer if it couldn’t allocate requested amount of memory.

 4. free() function in C:

• free () function frees the allocated memory by malloc (), calloc (), realloc () functions and returns the memory to the system.

·         Syntax :      free (pointer_name);

·      Example :   free(ptr);

Here, ptr is pointer to the memory that is dynamically created..

Example program for malloc() function in C:

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

int main()

{

     char *mem_allocation;

     /* memory is allocated dynamically */

     mem_allocation = (char*)malloc( 20 * sizeof(char) );

     if( mem_allocation== NULL )

     {

        printf("Couldn't able to allocate requested memory\n");

     }

     else

     {

        strcpy( mem_allocation,"fresh2refresh.com");

     }

     printf("Dynamically allocated memory content : " \

            "%s\n", mem_allocation );

     free(mem_allocation);

}

Example program for calloc() function in C:

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

int main()

{

     char *mem_allocation;

     /* memory is allocated dynamically */

     mem_allocation = calloc( 20, sizeof(char) );

     if( mem_allocation== NULL )

     {

        printf("Couldn't able to allocate requested memory\n");

     }

     else

     {

         strcpy( mem_allocation,"fresh2refresh.com");

     }

         printf("Dynamically allocated memory content   : " \

                "%s\n", mem_allocation );

         free(mem_allocation);

}

Example program for realloc() and free() functions in C:

#include <stdio.h>

#include <string.h>

#include <stdlib.h>

int main()

{

    char *mem_allocation;

    /* memory is allocated dynamically */

    mem_allocation = malloc( 20 * sizeof(char) );

    if( mem_allocation == NULL )

    {

        printf("Couldn't able to allocate requested memory\n");

    }

    else

    {

       strcpy( mem_allocation,"fresh2refresh.com");

    }

    printf("Dynamically allocated memory content  : " \

           "%s\n", mem_allocation );

    mem_allocation=realloc(mem_allocation,100*sizeof(char));

    if( mem_allocation == NULL )

    {

        printf("Couldn't able to allocate requested memory\n");

    }

    else

    {

        strcpy( mem_allocation,"space is extended upto " \

                               "100 characters");

    }

    printf("Resized memory : %s\n", mem_allocation );

    free(mem_allocation);

}