realloc() & free() in C

What is realloc()?

realloc() modifies the size of an existing memory block created with malloc() or calloc().

• Can increase or decrease memory
• Keeps old data safe when expanding
• Returns new pointer (may change address)
• Used when exact memory size is unknown

Syntax:

ptr = realloc(ptr, new_size);

What is free()?

free() releases dynamically allocated memory, preventing memory leaks.

• Frees memory created by malloc(), calloc(), realloc()
• After freeing, pointer becomes invalid
• Must call free() after finishing memory usage

free(ptr);

Important Rules

• Never use free() on the same pointer twice
• Always check if realloc() returned NULL
• Do not access memory after freeing
• Use realloc() only on dynamic memory

5 Practical Examples

1. Increase Memory Using realloc()

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

int main(){
    int *p = malloc(2 * sizeof(int));

    p[0] = 10;
    p[1] = 20;

    p = realloc(p, 4 * sizeof(int));

    p[2] = 30;
    p[3] = 40;

    for(int i=0;i<4;i++)
        printf("%d ", p[i]);

    free(p);
    return 0;
}

2. Shrink Memory Using realloc()

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

int main(){
    int *p = malloc(5 * sizeof(int));

    for(int i=0;i<5;i++)
        p[i] = i + 1;

    p = realloc(p, 2 * sizeof(int));

    for(int i=0;i<2;i++)
        printf("%d ", p[i]);

    free(p);
    return 0;
}

3. realloc() Returning NULL

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

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

    int *temp = realloc(p, 100000000000);

    if(temp == NULL){
        printf("Reallocation failed!");
    } else {
        p = temp;
    }

    free(p);
    return 0;
}

4. Free Memory After Use

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

int main(){
    char *s = malloc(20);
    sprintf(s, "Hello C");

    printf("%s", s);

    free(s);
    return 0;
}

5. realloc() for Dynamic Array Growth

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

int main(){
    int n = 2;
    int *arr = malloc(n * sizeof(int));

    arr[0] = 5;
    arr[1] = 10;

    n = 5;
    arr = realloc(arr, n * sizeof(int));

    arr[2] = 15;
    arr[3] = 20;
    arr[4] = 25;

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

    free(arr);
    return 0;
}