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C Reallocate Memory

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✅ What is realloc()?

The function realloc() is used to change the size of memory that was allocated using malloc() or calloc().

It can:

  • Increase the memory block size

  • Decrease the memory block size

  • Move the memory block to a new location (if needed)

📌 Syntax

pointer = realloc(pointer, new_size);

Where:

  • pointer → existing allocated memory

  • new_size → new required size in bytes

🧠 Important Behavior Notes

Action Behavior
Increase size Keeps previous data and adds new space (uninitialized).
Decrease size Keeps only the data that fits.
Memory move If resizing requires a new block, old data is copied automatically.
Failure Returns NULL — original pointer remains unchanged.

🔍 Example: Expanding an Array Dynamically

#include <stdio.h>
#include <stdlib.h>

int main() {
int *arr = (int*) malloc(3 * sizeof(int));

arr[0] = 10;
arr[1] = 20;
arr[2] = 30;

// Resize to store 5 integers
arr = (int*) realloc(arr, 5 * sizeof(int));

arr[3] = 40;
arr[4] = 50;

for (int i = 0; i < 5; i++)
printf("%d ", arr[i]);

free(arr);
return 0;
}

Output:

10 20 30 40 50

❗ Safe Usage of realloc() (Recommended Method)

To avoid memory loss if realloc() fails:

int *temp = realloc(arr, new_size);
if (temp != NULL) {
arr = temp;
} else {
printf("Reallocation failed!\n");
}

📍 Example: Safe Reallocation

int *temp = realloc(arr, 100 * sizeof(int));
if(temp == NULL) {
printf("Error: Could not reallocate memory.\n");
return 1;
}
arr = temp;

🧹 Remember to Free Memory

Always call:

free(arr);

when you are done using dynamically allocated memory.

🧾 Key Points to Remember

  • realloc() resizes memory previously allocated with malloc() or calloc().

  • It preserves existing data automatically.

  • It can move memory to a new location if needed.

  • Always check if realloc() returns NULL before assigning.

  • Always free() memory after use.

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Sanjit Sinha — Web Developer | PHP • Laravel • CodeIgniter • MySQL • Bootstrap Founder, CodeCapsule — Student projects & practical coding guides. Email: info@codecapsule.in • Website: CodeCapsule.in

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