Dynamic memory allocation in C

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

• memory is allocated at compile time.
• memory can't be increased while executing

      the program.

• used in the array. 

Dynamic memory allocation

• memory is allocated at run time.
• memory can be increased while executing the program.
• used in the linked list.

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

• malloc(): allocates a single block of requested memory.
• calloc(): allocates multiple blocks 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 a 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:

ptr=(cast-type*)malloc(byte-size)
  
Let's see the example of malloc() function.

#include<stdio.h>  
#include<stdlib.h>  
int main(){  
  int n,i,*ptr,sum=0;    
    printf("Enter number of elements: ");    
    scanf("%d",&n);    
    ptr=(int*)malloc(n*sizeof(int));  //memory allocated using malloc    
    if(ptr==NULL)                         
    {    
        printf("Sorry! unable to allocate memory");    
        exit(0);    
    }    
    printf("Enter elements of array: ");    
    for(i=0;i<n;++i)    
    {    
        scanf("%d",ptr+i);    
        sum+=*(ptr+i);    
    }    
    printf("Sum=%d",sum);    
    free(ptr);     
return 0;  
}    
Output:

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

calloc() function in C
The calloc() function allocates multiple blocks of requested memory.

It initially initializes all bytes to zero.

It returns NULL if memory is not sufficient.

The syntax of calloc() function is given below:

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

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

#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));  //memory allocated using calloc    
    if(ptr==NULL)                         
    {    
        printf("Sorry! unable to allocate memory");    
        exit(0);    
    }    
    printf("Enter elements of array: ");    
    for(i=0;i<n;++i)    
    {    
        scanf("%d",ptr+i);    
        sum+=*(ptr+i);    
    }    
    printf("Sum=%d",sum);    
    free(ptr);    
return 0;  
}    
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.

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.

free(ptr)