Menu
×
   ❮   
HTML CSS JAVASCRIPT SQL PYTHON JAVA PHP HOW TO W3.CSS C C++ C# BOOTSTRAP REACT MYSQL JQUERY EXCEL XML DJANGO NUMPY PANDAS NODEJS R TYPESCRIPT ANGULAR GIT POSTGRESQL MONGODB ASP AI GO KOTLIN SASS VUE DSA GEN AI SCIPY AWS CYBERSECURITY DATA SCIENCE
     ❯   

C Reallocate Memory


Reallocate Memory

If the amount of memory you reserved is not enough, you can reallocate it to make it larger.

Reallocating reserves a different (usually larger) amount of memory while keeping the data that was stored in it.

You can change the size of allocated memory with the realloc() function.

The realloc() function takes two parameters:

int *ptr2 = realloc(ptr1, size);
  • The first parameter is a pointer to the memory that is being resized.
  • The second parameter specifies the new size of allocated memory, measured in bytes.

The realloc() function tries to resize the memory at ptr1 and return the same memory address. If it cannot resize the memory at the current address then it will allocate memory at a different address and return the new address instead.

Note: When realloc() returns a different memory address, the memory at the original address is no longer reserved and it is not safe to use. When the reallocation is done it is good to assign the new pointer to the previous variable so that the old pointer cannot be used accidentally.

Example

Increase the size of allocated memory:

int *ptr1, *ptr2, size;

// Allocate memory for four integers
size = 4 * sizeof(*ptr1);
ptr1 = malloc(size);

printf("%d bytes allocated at address %p \n", size, ptr1);

// Resize the memory to hold six integers
size = 6 * sizeof(*ptr1);
ptr2 = realloc(ptr1, size);

printf("%d bytes reallocated at address %p \n", size, ptr2);
Try it Yourself »

NULL Pointer & Error Checking

The realloc() function returns a NULL pointer if it is not able to allocate more memory. This is very unlikely, but it is worth keeping in mind when you need your code to be failproof.

The following example checks whether realloc() is able to resize the memory or not, by checking for a NULL pointer:

Example

Check for a NULL pointer:

int *ptr1, *ptr2;

// Allocate memory
ptr1 = malloc(4);

// Attempt to resize the memory
ptr2 = realloc(ptr1, 8);

// Check whether realloc is able to resize the memory or not
if (ptr2 == NULL) {
  // If reallocation fails
  printf("Failed. Unable to resize memory");
} else {
  // If reallocation is sucessful
  printf("Success. 8 bytes reallocated at address %p \n", ptr2);
  ptr1 = ptr2;  // Update ptr1 to point to the newly allocated memory
}
Try it Yourself »

Note: You should always include error checking (if pointer == NULL) when allocating memory.

Note: You should also always free, or release, allocated memory when you are done using it. This is important to make sure that your program behaves as expected, but it will also make it more maintainable and efficient.

To free memory, simply use the free() function:

Example

Free allocated memory:

// Free allocated memory
free(ptr1);
Try it Yourself »

You will learn more about how to free allocated memory and why this is important in the next chapter.



×

Contact Sales

If you want to use W3Schools services as an educational institution, team or enterprise, send us an e-mail:
sales@w3schools.com

Report Error

If you want to report an error, or if you want to make a suggestion, send us an e-mail:
help@w3schools.com

W3Schools is optimized for learning and training. Examples might be simplified to improve reading and learning. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content. While using W3Schools, you agree to have read and accepted our terms of use, cookie and privacy policy.

Copyright 1999-2024 by Refsnes Data. All Rights Reserved. W3Schools is Powered by W3.CSS.