Dynamic memory allocation
The process of
allocation memory at run time is called dynamic memory allocation. Its
advantages are:
1.
When programmer actually need to use memory space at that
time dynamically allocates so save memory space.
2.
Programmer doesn’t need to know in advance how many
variables are needed.
There are mainly four
types function available for dynamic memory allocation.
malloc()
It allocates request
size of bytes and returns a pointer to the first byte of the allocated space.
Syntax
ptr-var=(cast type) malloc(byte size);
Here, ptr-var is a
pointer of type cast-type. It returns a pointer to an area of the memory with
size byte-size.
Ex,
int *x;
x=(int *)malloc(10*sizeof(int));
In above example , a
memory space 100*size of int(200) bytes is reserved and the address of the
first byte of the memory allocated is assigned to the pointer a of type of int.
calloc()
This function allocates
space for an array of elements. This function is normally used for derived data
type such as array and structure.
malloc() allocates
single block of storage space and calloc() allocates multiple block of storage.
Syntax:
ptr=(cast-type)calloc(n,element-size);
Here, ptr is a pointer
of type cast. The first argument n is number of variable for what need to allocate
space. The second argument element-size is
the size of each element in byte.
If allocation of memory space is successful , all bytes are initialized to zero and a
pointer to the first byte of the allocated region is returned. If there is no
enough space NULL pointer is returned.
Ex,
struct emp
{
int no;
char *name;
float age;
}
-----------
-----------
struct emp *e;
e=(struct emp
*)calloc(10,sizeof(struct emp));
Here, array of 10
structure object is create.
Program : Create
array of structure dynamically.
#include<stdio.h>
#include<conio.h>
#include<string.h>
struct stud
{
int
sno;
char
sname[15];
int
m1,m2,m3,total;
};
void main()
{
struct
stud *s;
int
i,n;
clrscr();
printf("\n
Enter total number of student : ");
scanf("%d",&n);
s=(struct
stud *)calloc(n,sizeof(struct stud));
for(i=0;i<n;i++)
{
printf("\n
----- student = %d ",i+1);
printf("\n
Enter No :");
scanf("%d",&s->sno);
fflush(stdin);
printf("\n
Enter Name:");
gets(s->sname);
printf("\n
Enter M1 M2 M3:");
scanf("%d
%d %d",&s->m1,&s->m2,&s->m3);
//calculation
s->total=s->m1
+ s->m2 +s->m3;
s++;
}
clrscr();
s=s-n;
printf("\n
output");
printf("\n
_______________________________________________________");
printf("\n
\t\t\t Mark sheet ");
printf("\n
_______________________________________________________");
for(i=0;i<n;i++)
{
printf("\n
address = %u",s);
printf("\n\n
No:%d",s->sno);
printf("\n
Name :%s",s->sname);
printf("\n\n m1
\t m2 \t m3 \t
total");
printf("\n%4d\t%4d\t%4d\t%5d",s->m1,s->m2,s->m3,s->total);
printf("\n_____________________________________________________");
s++;
}
free(s);
getch();
}
realloc()
When need to change the
size of previously allocated memory space then realloc() is used. It is known
as reallocation of memory.
Syntax:
ptr=realloc(ptr,newsize)
Here, ptr is the
pointer to the original block of memory. The newsize specifies the size in byte
, the function returns pointer to the first byte of new memory block.The
newsize may be larger or smaller than the size.
If the function is
unsuccessful in locating additional space , it returns as a NULL pointer and
original block is lost.
Ex,
ptr=(int
*)malloc(20*sizeof(int));
-------
-------
ptr=(char
*)realloc(30*sizeof(int));
Program : Use of realloc()
#include<stdio.h>
#include<conio.h>
void main()
{
int
*a,i,n,sum=0;
clrscr();
printf("\n\n
Enter how many number : ");
scanf("%d",&n);
a=(int
*)malloc(n*sizeof(int));
for(i=0;i<n;i++)
{
printf("\n
Enter The Value Of a[%d] : ",i);
scanf("%d",&a[i]);
}
for(i=0;i<n;i++)
{
printf("\n
a[%d] : %d",i,a[i] );
sum=sum+a[i];
}
printf("\n
Sum = %d",sum);
printf("\n\n
Enter new number to reallocate : ");
scanf("%d",&n);
a=(int
*)realloc(a,n);
for(i=0;i<n;i++)
{
printf("\n
Enter The Value Of a[%d] : ",i);
scanf("%d",&a[i]);
}
for(i=0;i<n;i++)
{
printf("\n
a[%d] : %d",i,a[i] );
sum=sum+a[i];
}
printf("\n
sum after reallocation : %d",sum);
getch();
}
free()
The release of storage
space becomes important when the storage is limited.free() function is used to
release(deallocate) that block of memory
which is created by malloc() and calloc().
Sytax:
free(ptr);
Here, the ptr is the
pointer variable which previously allocated dynamic memory.
Ex,
ptr=(int
*)malloc(20*sizeof(int));
free(ptr);
