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C Dynamic Memory Allocation

C Dynamic Memory Allocation

In this tutorial, We will learn about dynamically allocate memory in your C program using standard library functions: malloc(), calloc(), free() and realloc().

Introduction

In an array, we have to declare the size of an array before you use it. Hence, the array size we declared may be insufficient or more than required to hold data. To solve this issue, you can allocate memory dynamically. This technique allows us to obtain more memory when required and release it when not necessary. In C program, dynamic allocation achieve by 4 function which comes under <sdlib.h> library.
  • malloc(): Allocates requested size of bytes and returns a pointer first byte of allocated space
  • calloc(): Allocates space for array elements, initializes to zero and then returns a pointer to memory
  • free(): deallocate the previously allocated space
  • realloc(): Change the size of previously allocated space

C malloc()

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

Example
ptr = (int*) malloc(100 * sizeof(int));
C calloc()
ptr = (cast-type*)calloc(n, element-size);

Example
ptr = (float*) calloc(25, sizeof(float));


C free()
free(ptr);
C realloc()
ptr = realloc(ptr, newsize);
Example #1
 #include <stdio.h>  
 #include <stdlib.h>  
 int main()  
 {  
   int num, i, *ptr, sum = 0;  
   printf("Enter number of elements: ");  
   scanf("%d", &num);  
   ptr = (int*) calloc(num, sizeof(int));  
   if(ptr == NULL)  
   {  
     printf("Error! memory not allocated.");  
     exit(0);  
   }  
   printf("Enter elements of array: ");  
   for(i = 0; i < num; ++i)  
   {  
     scanf("%d", ptr + i);  
     sum += *(ptr + i);  
   }  
   printf("Sum = %d", sum);  
   free(ptr);  
   return 0;  
 }

Example #2
 #include <stdio.h>  
 #include <stdlib.h>  
 int main()  
 {  
   int *ptr, i , n1, n2;  
   printf("Enter size of array: ");  
   scanf("%d", &n1);  
   ptr = (int*) malloc(n1 * sizeof(int));  
   printf("Address of previously allocated memory: ");  
   for(i = 0; i < n1; ++i)  
      printf("%u\t",ptr + i);  
   printf("\nEnter new size of array: ");  
   scanf("%d", &n2);  
   ptr = realloc(ptr, n2);  
   for(i = 0; i < n2; ++i)  
      printf("%u\t", ptr + i);  
   return 0;  
 }
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