This document discusses dynamic memory allocation in C using library functions like malloc(), calloc(), realloc() and free(). It explains that dynamic allocation allows memory to be requested and freed at runtime rather than being fixed at compile time. Memory for a program comes from the stack or heap, with the heap being used for global variables and dynamically allocated memory. Examples are provided to demonstrate allocating memory for an array of floats using malloc(), allocating and initializing memory using calloc(), resizing allocated memory using realloc(), and freeing memory using free().

1. Dynamic Memory Allocation

2. Basic Idea
• Many a time we face situations where data is dynamic in nature.
• – Amount of data cannot be predicted beforehand.
• – Number of data items keeps changing during program execution.
• Such situations can be handled more easily and effectively using
dynamic memory management techniques.
• C language requires the number of elements in an array to be
specified at compile time.
• – Often leads to wastage of memory space or program failure.
• Dynamic Memory Allocation
• – Memory space required can be specified at the time of execution.
• – C supports allocating and freeing memory dynamically using library
routines.

3. Memory Allocation Process in C
• Memory allocated to any program is either from Stack or Heap.
• Stack memory is allocated to local variable, function call parameters etc.
• Heap memory is allocated to global variables or the variables which request
for memory space and asking for memory space is called dynamic allocation
of memory.
• Four basic functions which belong to stdlib.h or malloc.h or alloc.h for
dynamic memory allocation –
1. malloc()
2. calloc()
3. realloc()
4. free()

9. Example
void main(){
int i,N;
float *height;
float sum=0,avg;
printf("Input no. of studentsn");
scanf("%d", &N);
height = (float *) malloc(N * sizeof(float));
printf("Input heights for %d students n",N);
for (i=0; i<N; i++)
scanf ("%f", &height[i]);
for(i=0;i<N;i++)
sum += height[i];
avg = sum / (float) N;
printf("Average height = %f n“,avg);
free (height);
}

11. Calloc()
• Calloc function allocates the block of bytes from
heap.
• Syntax: void * calloc (num, size)
• This function takes two arguments that specify
the number of elements to be reserved, and the
size of each element in bytes
• It allocates memory block equivalent to num*
size .

12. Example2: calloc and malloc

15. Realloc Example
void main() {
int* array,*array1;
int n, i;
printf("Enter the number of elements: ");
scanf("%d", &n);
array = (int*) calloc(n,sizeof(int));
for (i=0; i<n; i++) {
printf("Enter number %d: ", i);
scanf("%d", &array[i]);
}
array1 = (int*) realloc(array,(n+1)*sizeof(int));
if(array1)
printf("Memory Allocated");
printf("nThe Dynamic Array is: n");
for (i=0; i<(n+1); i++)
printf("The value of %d is %dn", i, array1[i]);
}

16. O/P?
#include <malloc.h>
#include <stdio.h>
void main() {
int* ptr;
ptr=(int *)malloc(sizeof(int));
*ptr=5;
printf("%d ",*ptr);
free(ptr);
*ptr=7;
printf("%d ",*ptr);
}