Dynamic memory allocation

Very often we face situations in programming where the data is dynamic in

nature i.e. the number of data items keep changing during execution of the

program. For example, in an employee database, the number of employees

continually increase and decrease as and when names are added or deleted.

Accordingly, we need to manage the memory space. This can be done effectively

using dynamic data structures in conjunction with dynamic memory management

techniques.

The advantage of dynamic data structures is that it provides flexibility in

adding, deleting or rearranging data items at run time. Dynamic memory

management techniques allow us to allocate additional memory space or release

unwanted space at run time, thus optimizing the use of storage space.

Dynamic memory allocation



A good example to illustrate the importance of dynamic memory allocation is
arrays. The number of elements in an array needs to be specified at compile

time, but this may not be possible at all times and even if it is possible we

usually allocate more space than required. If the size of the array is given

wrongly, it may cause failure of the program or more commonly, wastage of memory

space. Thus instead of specifying the size of the array at compile time, if we

could do it at run time when we are absolutely sure of the required size, the

program would be more efficient. The process of allocating memory at run time is

known as dynamic memory allocation. Although C does not inherently have this

facility, there are four library routines known as memory management functions

that can be used for allocating and freeing memory during program execution.

They are:

malloc: it allocates the requested size of bytes and returns a pointer

to the first byte of the allocated space.



calloc: it allocates space for an array of elements, initializes them to
zero and then returns a pointer to memory.



free: frees previously allocated space.


realloc: modifies the size of previously allocated space.





Allocating memory


The malloc function can be used to allocate a block of memory. It reserves a
block of memory of specified size and returns a pointer of type void. This means

that we can assign it to any type of pointer. Malloc allocates a block of

contiguous bytes of memory. It takes the following form:



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

Here ptr is of type cast-type and malloc returns a pointer of cast-type to an

area of memory with size byte-size. For example:



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





On execution of this statement, a memory space equivalent to 100 times the size
of int data type bytes is reserved and the address of the first byte of memory

allocated is assigned to the pointer x of type int. Similarly



y = (char *)malloc(10);





allocates 10 bytes of space for the pointer y of type char.

Note: the storage space allocated dynamically has no name and

therefore its contents can be accessed only through a pointer.

Malloc can also be used to allocate space for complex data types such as

structures. Example:



z = (struct store *)malloc(sizeof(struct store));

where z is a pointer of type struct store.



Lets take a look at a program that illustrates the use of pointers.


/* Program that uses a table of integers whose size will be specified
interactively at run time */




#include



#define NULL 0


main()


{


int *p, *table, size;





printf("\nEnter the size of the table :");


scanf("%d",&size);





if((table=(int *)malloc(size * sizeof(int)))==NULL) /* Memory allocation */


{


printf("\nNo space available");


exit(1);


}





printf("\n Address of the first byte is %u\n",table);


printf("\n Input the values of the table :");


for(p=table;p
{



scanf("%d",p);


}


for(p=table;p
{



printf("\n The number %d is stored at the memory location %u",*p,p);


}


}