realloc() and free() in C
In the previous lessons, you learned how to allocate memory using
malloc() and calloc().
Now comes the most critical responsibility of a C programmer: managing memory correctly.
This lesson will teach you:
- How to resize allocated memory
- How to safely release memory
- How to avoid dangerous memory mistakes
What Is realloc()?
realloc stands for re-allocation.
It is used when:
- You need more memory than initially allocated
- You want to reduce unused memory
Instead of creating new memory from scratch, realloc modifies existing memory.
Basic Syntax of realloc()
ptr = realloc(ptr, new_size);
Important rule:
- The pointer passed to realloc must already be allocated
Example: Expanding Memory
#include <stdio.h>
#include <stdlib.h>
int main() {
int *arr = (int*) malloc(2 * sizeof(int));
arr[0] = 10;
arr[1] = 20;
arr = realloc(arr, 4 * sizeof(int));
arr[2] = 30;
arr[3] = 40;
for(int i = 0; i < 4; i++) {
printf("%d ", arr[i]);
}
free(arr);
return 0;
}
Explanation:
- Initial memory holds 2 integers
- realloc expands it to 4 integers
- Old data is preserved
How realloc Works Internally
When realloc is called:
- If adjacent memory is available → memory expands in place
- If not → new memory is allocated
- Old data is copied
- Old memory is freed automatically
You don’t see this process — but it happens underneath.
Reducing Memory Size
realloc can also reduce memory size:
ptr = realloc(ptr, 2 * sizeof(int));
This frees extra unused memory.
What Is free()?
free() releases dynamically allocated memory back to the system.
free(ptr);
After calling free:
- The memory is no longer valid
- Using it causes undefined behavior
Best Practice After free()
Always reset pointer after freeing:
free(ptr);
ptr = NULL;
This prevents accidental reuse.
Common Memory Mistakes
- Forgetting to call free()
- Using memory after free()
- Calling free() twice
- Losing pointer reference
These mistakes cause memory leaks and crashes.
Real-World Analogy
Think of memory like a rented apartment:
- malloc / calloc → rent apartment
- realloc → upgrade to bigger apartment
- free → vacate apartment
If you don’t vacate, rent keeps accumulating.
Mini Practice
- Allocate memory for 2 integers
- Expand it to 5 using realloc
- Store values
- Free memory safely
Quick Quiz
Q1. What does realloc do?
Q2. Does realloc preserve old data?
Q3. What happens if free() is not called?
Q4. Is it safe to use pointer after free()?
Q5. Why set pointer to NULL after free()?