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Dynamic Memory Allocation in C: A Comprehensive Guide

When you declare a variable or an array of a data type, the space occupied by them in the memory of the system remains constant throughout the program. At times, the constant space allocated during compile-time isn’t enough. In that case, the concept of dynamic memory allocation in C comes to the rescue. Learn more about dynamic memory allocation from this article. 


Table of Content:



What is Dynamic Memory Allocation in C?

Dynamic memory allocation involves the manual allocation and clearing of memory as per programming requirements. Dynamic memory in C gets managed and served with pointers pointing toward the newly allocated memory space in the heap area. 

This concept helps with memory allocation in C even during run-time without any hassle. While automatic memory management leverages the stack, dynamic memory allocation in C uses the heap.

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Understanding the Difference Between Static and Dynamic Memory Allocation

Here is a list of difference between static and dynamic memory allocation in C:

Static Memory Allocation Dynamic Memory Allocation
Constant or invariable memory is reserved at compile-time and cannot be modified.  Dynamic memory allocation in C is reserved at run-time and can be modified easily. 
Also called compile-time memory allocation in C.  Dynamic memory allocation in C is also called run-time memory allocation. 
Unable to allocate or deallocate a memory block during run-time.  Can support the allocation and deallocation of a memory block during run-time. 
Static memory allocation in C uses stack space.  Dynamic memory allocation in C uses heap space. 
Does not support memory reusability while the program runs. Therefore, it is less efficient.  Supports reusability while the program is running, which makes it highly efficient. 

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Dynamic Memory Allocation in C

The <stdlib.h> library comes with the following functions that support dynamic memory allocation:

The Malloc() Function

The malloc() function is primarily used for dynamic memory allocation in C. This function helps allocate a memory block dynamically. It conserves memory space of a particular size. It delivers a null pointer that points to the memory location. 

The returned pointer is usually of the void type. Therefore, you will be able to assign the C malloc() function to all pointers. 

Syntax of Malloc()

The syntax of malloc() dynamic memory allocation in C is as follows:

ptr = (cast_type *) malloc (byte_size);

The ptr refers to a pointer of cast_type. The C malloc() function delivers a pointer to the allocated memory of byte_size. 

The malloc() function in dynamic memory allocation is used with the character data type along with complex data types like structures. 

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C Calloc() Method

The calloc or contiguous allocation method helps with the dynamic memory allocation of a specified number of blocks of a particular type. The calloc method for dynamic memory allocation in C is quite similar to the malloc method. However, you will come across some primary differences between the two, which are as follows:

  • The calloc method comes with two arguments or parameters.
  • The calloc method initializes every block with a default value of 0. 


Syntax of C Calloc

The syntax for the calloc() method for dynamic memory allocation in c is as follows:

ptr = (cast-type*)calloc(n, element-size);

In this case, n refers to the number of elements. Meanwhile, element size describes the size of every element. 

In case the space isn’t enough, the allocation will fail and deliver a NULL pointer.

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The Free() Function

The free() method for dynamic memory allocation is primarily used for deallocation. It can help you dynamically deallocate the memory. The memory allocated using the different methods for memory allocation in C does not get deallocated on its own. Therefore, the free method can help with minimzing memory wastage by freeing it. 

Syntax of Free() Function

The syntax for the free() function for memory allocation in C is as follows:

An example of the free() function is as follows:
int main()
    // This pointer will hold the
    // base address of the block created
    int *ptr, *ptr1;
    int n, i;
    // Get the number of elements for the array
    n = 5;
    printf("Enter number of elements: %dn", n);
    // Dynamically allocate memory using malloc()
    ptr = (int*)malloc(n * sizeof(int));
    // Dynamically allocate memory using calloc()
    ptr1 = (int*)calloc(n, sizeof(int));
    // Check if the memory has been successfully
    // allocated by malloc or not
    if (ptr == NULL || ptr1 == NULL) {
        printf("Memory not allocated.n");
    else {
        // Memory has been successfully allocated
        printf("Memory successfully allocated using malloc.n");
        // Free the memory
        printf("Malloc Memory successfully freed.n");
        // Memory has been successfully allocated
        printf("nMemory successfully allocated using calloc.n");
        // Free the memory
        printf("Calloc Memory successfully freed.n");
    return 0;
Enter number of elements: 5
Memory successfully allocated using malloc.
Malloc Memory successfully freed.
Memory successfully allocated using calloc.
Calloc Memory successfully freed. 
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Malloc() Vs Calloc()

Here is a list of difference between Malloc() Vs Calloc() in detail.

Malloc() Method Calloc() Method
Used to allocate a single memory block during run-time Used to allocate multiple memory blocks during run-time
Can’t initialize the allocated memory block because it contains some garbage value Can initialize all the allocated memory blocks with zero value
Takes a single argument, which is the size of a memory block that should be allocated Takes two arguments, which is the number of elements and the size of one element to be allocated


Programmers benefit largely from learning about dynamic memory allocation in C++ and C. Dynamic memory allocation can be used with the malloc() and calloc() library functions. Meanwhile, the free() method is useful for freeing up memory space. 





The dynamic malloc memory allocation in C is used for allocating a memory block. It stores memory space of a specified size and makes the bull pointer point to the memory location.
The free() function for dynamic memory allocation in C is used for release or deallocation. Freeing memory in a program helps you make it more available for later.
You are allowed to reallocate memory dynamically with either malloc() or calloc(). You will have to use the realloc() prompt for the same.
Some potential issues or errors with dynamic memory allocation in c include memory leaks and fragmentation. The fragmentation associated with dynamic allocation in C can either be internal or external.
If you want to avoid memory leaks while working with dynamic memory allocation in C, you will have to make use of the free() statement. You can assign NULL to the pointer variable after using the free() statement for dynamic memory allocation to prevent dangling pointers.

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