dynamic memory allocation in c

1. 1
Dynamic Memory Allo
cation
Group 4
Software Engineering

2. 2
Problem with Arrays
 Sometimes
 Amount of data cannot be predicted beforehand
 Number of data items keeps changing during program
execution
 Example: Seach for an element in an array of N elements
 One solution: find the maximum possible value of N and
allocate an array of N elements
 Wasteful of memory space, as N may be much smaller
in some executions
 Example: maximum value of N may be 10,000, but a p
articular run may need to search only among 100 elem
ents
 Using array of size 10,000 always wastes memory i
n most cases

3. 3
Better Solution
 Dynamic memory allocation
 Know how much memory is needed after the program
is run
 Example: ask the user to enter from keyboard
 Dynamically allocate only the amount of memory nee
ded
 C provides functions to dynamically allocate me
mory
 malloc, calloc, realloc

4. 4
Memory Allocation Functions
 malloc
 Allocates requested number of bytes and returns a
pointer to the first byte of the allocated space
 calloc
 Allocates space for an array of elements, initializes
them to zero and then returns a pointer to the mem
ory.
 free
 Frees previously allocated space.
 realloc
 Modifies the size of previously allocated space.
 We will only do malloc and free

5. C Library - <stdlib.h>
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6. 6

7. 7
Allocating a Block of Memory
 A block of memory can be allocated using the fu
nction malloc
Reserves a block of memory of specified size
and returns a pointer of type void
It returns a pointer that can be type-casted to
any pointer type. It points to the first byte of th
e allocated memory, else returns NULL.
 General format:
type *p;
p = (type *) malloc (byte_size);

8. 8
Example
p = (int *) malloc(100 * sizeof(int));
 A memory space equivalent to 100 times t
he size of an int bytes is reserved
The address of the first byte of the allocate
d memory is assigned to the pointer p of ty
pe int
p
400 bytes of space

9. 9
Contd.
 cptr = (char *) malloc (20);
Allocates 20 bytes of space for the pointer cptr
of type char
 sptr = (struct stud *) malloc(10*sizeof(struct stu
d));
Allocates space for a structure array of 10 elem
ents. sptr points to a structure element of type
struct stud
Always use sizeof operator to find number of bytes for
a data type, as it can vary from machine to machine

10. 10
Points to Note
 malloc always allocates a block of contiguou
s bytes
The allocation can fail if sufficient contiguo
us memory space is not available
If it fails, malloc returns NULL
if ((p = (int *) malloc(100 * sizeof(int))) == NULL)
{
printf (“n Memory cannot be allocated”);
exit();
}

11. 11
Using the malloc’d Array
 Once the memory is allocated, it can be used with pointer
s, or with array notation
 Example:
int *p, n, i;
scanf(“%d”, &n);
p = (int *) malloc (n * sizeof(int));
for (i=0; i<n; ++i)
scanf(“%d”, &p[i]);
The n integers allocated can be accessed as *p, *(p+1), *(p+
2),…, *(p+n-1) or just as p[0], p[1], p[2], …,p[n-1]

12. 12
#include <stdio.h>
#include <stdlib.h>
int main() {
int *ptr, i , n1, n2;
printf("Enter size: ");
scanf("%d", &n1);
ptr = (int*) malloc(n1 * sizeof(int));
printf("Amount of allocated memory: %i bytesnnn", n1 * sizeof(int));
printf("Addresses of previously allocated memory:n");
for(i = 0; i < n1; ++i)
printf("%pcn",ptr + i);
printf("nEnter the new size: ");
scanf("%d", &n2);
// rellocating the memory
ptr = realloc(ptr, n2 * sizeof(int));
printf("Addresses of newly allocated memory:n");
for(i = 0; i < n2; ++i)
printf("%pcn", ptr + i);
free(ptr);
return 0;
}

13. 13
Releasing the Allocated Space: fre
e()
 An allocated block can be returned to the syste
m for future use by using the free function
 To deallocate the memory we use the General s
yntax:
free (ptr);
where ptr is a pointer to a memory block which
has been previously created using malloc
 Note that no size needs to be mentioned for the
allocated block, the system remembers it for ea
ch pointer returned

14. 14

15. 15

16. 16
malloc( )-ing array of structures
typedef struct{
char name[20];
int roll;
float SGPA[8], CGPA;
} person;
void main()
{
person *student;
int i,j,n;
scanf("%d", &n);
student = (person *)malloc(n*sizeof(person));
for (i=0; i<n; i++) {
scanf("%s", student[i].name);
scanf("%d", &student[i].roll);
for(j=0;j<8;j++) scanf("%f", &student[i].SGPA[j]);
scanf("%f", &student[i].CGPA);
}
}

17. calloc()
Description
The C library function void *calloc(size_t nitems, size_t size) allocates the requ
ested memory and returns a pointer to it. The difference in malloc and calloc is
that malloc does not set the memory to zero where as calloc sets allocated me
mory to zero.
Declaration
Following is the declaration for calloc() function.
void *calloc(size_t nitems, size_t size)
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18. // Program to calculate the sum of n numbers entered by the user
// The calloc() function allocates memory and initializes all bits to zero.
#include <stdio.h>
#include <stdlib.h>
int main() {
int n, i, *ptr, sum = 0;
printf("Enter number of elements: ");
scanf("%d", &n);
ptr = (int*) calloc(n, sizeof(int));
printf("Amount of allocated memory: %i bytesnnn", n * sizeof(int));
if(ptr == NULL) {
printf("Error! memory not allocated.");
exit(0);
}
printf("Enter elements: ");
for(i = 0; i < n; ++i) {
scanf("%d", ptr + i);
sum += *(ptr + i);
}
printf("Sum = %d", sum);
free(ptr);
return 0;
}

19. realloc()
Description
The C library function void *realloc(void *ptr, size_t size) attempts to resize the
memory block pointed to by ptr that was previously allocated with a call to mallo
c or calloc.
Declaration
Following is the declaration for realloc() function.
void *realloc(void *ptr, size_t size)
Parameters
ptr − This is the pointer to a memory block previously allocated with malloc, call
oc or realloc to be reallocated. If this is NULL, a new block is allocated and a po
inter to it is returned by the function.
size − This is the new size for the memory block, in bytes. If it is 0 and ptr point
s to an existing block of memory, the memory block pointed by ptr is deallocate
d and a NULL pointer is returned.
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20. Return Value
This function returns a pointer to the newly allocated memory, or NULL
if the request fails.
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21. 21
#include <stdio.h>
#include <stdlib.h>
int main() {
int *ptr, i , n1, n2;
printf("Enter size: ");
scanf("%d", &n1);
ptr = (int*) malloc(n1 * sizeof(int));
printf("Amount of allocated memory: %i bytesnnn", n1 * sizeof(int));
printf("Addresses of previously allocated memory:n");
for(i = 0; i < n1; ++i)
printf("%pcn",ptr + i);
printf("nEnter the new size: ");
scanf("%d", &n2);
// rellocating the memory
ptr = realloc(ptr, n2 * sizeof(int));
printf("Addresses of newly allocated memory:n");
for(i = 0; i < n2; ++i)
printf("%pcn", ptr + i);
free(ptr);
return 0;
}

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