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C : Dynamic memory allocation

Tutorial by:      Date: 2016-04-27 05:09:42

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The concept of dynamic memory allocation in c language enables the C programmer to allocate memory at runtime. Dynamic memory allocation in c language is possible by 4 functions of stdlib.h header file.

  1. malloc()
  2. calloc()
  3. realloc()
  4. free()

Before learning above functions, let's understand the difference between static memory allocation and dynamic memory allocation.

static memory allocation dynamic memory allocation
memory is allocated at compile time. memory is allocated at run time.
memory can't be increased while executing program. memory can be increased while executing program.
used in array. used in linked list.

Now let's have a quick look at the methods used for dynamic memory allocation.

malloc() allocates single block of requested memory.
calloc() allocates multiple block of requested memory.
realloc() reallocates the memory occupied by malloc() or calloc() functions.
free() frees the dynamically allocated memory.

malloc() function in C

The malloc() function allocates single block of requested memory.

It doesn't initialize memory at execution time, so it has garbage value initially.

It returns NULL if memory is not sufficient.

The syntax of malloc() function is given below:

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

Let's see the example of malloc() function.

 
  1. #include <stdio.h>  
  2. #include <stdlib.h>  
  3. void main(){  
  4.     int n,i,*ptr,sum=0;  
  5.     printf("Enter number of elements: ");  
  6.     scanf("%d",&n);  
  7.     ptr=(int*)malloc(n*sizeof(int));  //memory allocated using calloc  
  8.     if(ptr==NULL)                       
  9.     {  
  10.         printf("Sorry! unable to allocate memory");  
  11.         exit(0);  
  12.     }  
  13.     printf("Enter elements of array: ");  
  14.     for(i=0;i<n;++i)  
  15.     {  
  16.         scanf("%d",ptr+i);  
  17.         sum+=*(ptr+i);  
  18.     }  
  19.     printf("Sum=%d",sum);  
  20.     free(ptr);  
  21. }  

Output:

Enter elements of array: 3
Enter elements of array: 10
10
10
Sum=30

calloc() function in C

The calloc() function allocates multiple block of requested memory.

It initially initialize all bytes to zero.

It returns NULL if memory is not sufficient.

The syntax of calloc() function is given below:

 
  1. ptr=(cast-type*)calloc(number, byte-size)  

Let's see the example of calloc() function.

 
  1. #include <stdio.h>  
  2. #include <stdlib.h>  
  3. void main(){  
  4.     int n,i,*ptr,sum=0;  
  5.     printf("Enter number of elements: ");  
  6.     scanf("%d",&n);  
  7.     ptr=(int*)calloc(n,sizeof(int));  //memory allocated using calloc  
  8.     if(ptr==NULL)                       
  9.     {  
  10.         printf("Sorry! unable to allocate memory");  
  11.         exit(0);  
  12.     }  
  13.     printf("Enter elements of array: ");  
  14.     for(i=0;i<n;++i)  
  15.     {  
  16.         scanf("%d",ptr+i);  
  17.         sum+=*(ptr+i);  
  18.     }  
  19.     printf("Sum=%d",sum);  
  20.     free(ptr);  
  21. }  

Output:

Enter elements of array: 3
Enter elements of array: 10
10
10
Sum=30

realloc() function in C

If memory is not sufficient for malloc() or calloc(), you can reallocate the memory by realloc() function. In short, it changes the memory size.

Let's see the syntax of realloc() function.

 
  1. ptr=realloc(ptr, new-size)  

free() function in C

The memory occupied by malloc() or calloc() functions must be released by calling free() function. Otherwise, it will consume memory until program exit.

Let's see the syntax of free() function.

 
  1. free(ptr)  

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