Dynamic Memory Allocation in C

Function	Task
sizeof	The sizeof() function returns the memory size of the requested variable
malloc	Allocates memory requests size of bytes and returns a pointer to the Ist byte of allocated space
calloc	Allocates space for an array of elements initializes them to zero and returns a pointer to the memory
free	Frees previously allocated space
realloc	Modifies the size of previously allocated space.

sizeof()

The sizeof() function returns the memory size of requested variable. This call should be used in the conjunction with the calloc() function call, so that only the necessary memory is allocated, rather than a fixed size. Consider the following,

struct date {
int hour, minute, second;
};

int x;

x = sizeof( struct date );

malloc()

A block mf memory may be allocated using the function called malloc. The malloc function reserves a block of memory of specified size and return a pointer of type void. This means that we can assign it to any type of the pointer. It takes the following form:

ptr=(cast-type*)malloc(byte-size);

ptr is a pointer of type cast-type the malloc returns a pointer (of cast type) to an area of memory with the size byte-size. The following is the example of using malloc function

x=(int*)malloc(100*sizeof(int));

calloc()

Calloc is another memory allocation function that is normally used to request the multiple blocks of storage each of same size and then sets all bytes to zero. The general form of calloc is:

ptr=(cast-type*) calloc(n,elem-size);

The above statement allocates contiguous space for n blocks each size of the elements size bytes. All bytes are initialized to zero and a pointer to the first byte of allocated region is returned. If there is not enough space a null pointer is also returned.

realloc()

The memory allocated by using calloc or malloc might be insufficient or excess sometimes in both the situations we can change the memory size already allocated with the help of the function called realloc. This process is called the reallocation of memory. The general statement of reallocation of memory is :

ptr=realloc(ptr,newsize);

free()

Compile time storage of a variable is allocated and released by the system in accordance with its storage class. With the dynamic runtime allocation, it is our responsibility to release the space when it is not required at all.When the storage is limited,the release of storage space becomes important . When we no longer need the data we stored in a block of memory and we do not intend to use that block for the storing any other information, Using the free function,we may release that block of memory for future use.

free(ptr);

ptr is a pointer that has been created by using calloc or malloc.

The following program illustrate the reallocation of memory using realloc() and malloc()

/*Example program for reallocation*/

#include< stdio.h >
#include< stdlib.h >
define NULL 0
main()
{
char *buffer;
/*Allocating memory*/
if((buffer=(char *) malloc(10))==NULL)
{
printf("Malloc failed\n");
exit(1);
}
printf("Buffer of size %d created \n",_msize(buffer));
strcpy(buffer,Bangalore);
printf(\nBuffer contains:%s\n,buffer);
/*Reallocation*/
if((buffer=(char *)realloc(buffer,15))==NULL)
{
printf("Reallocation failed\n");
exit(1);
}
printf("\nBuffer size modified".\n);
printf("\nBuffer still contains: %s\n",buffer);
strcpy(buffer,Mysore);
printf("\nBuffer now contains:%s\n",buffer);
/*freeing memory*/
free(buffer);
}

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