realloc() Function

The realloc() function in C dynamically adjusts the size of an allocated memory block. It enables you to either expand or shrink a block of memory while preserving the existing data up to the minimum of the old and new sizes. The memory block may be moved to a new location during this process.

Syntax of realloc()

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void* realloc(void* ptr, size_t size);

Parameters

Parameter	Description
`ptr`	Pointer to a memory block previously allocated by `malloc()`, `calloc()`, or `realloc()`. It can also be `NULL`, in which case `realloc()` behaves like `malloc()`.
`size`	New size for the memory block in bytes.

Return Value

The function returns a pointer to the reallocated memory block, which might be the same as the original pointer or a new location. A return value of NULL indicates that either the requested size was zero (resulting in deallocation) or the memory allocation failed, in which case the original memory block remains unchanged.

Additional points: If the new size is larger than the original, the additional memory will have indeterminate values. Also, if realloc() fails to allocate the requested memory, the original block is not freed.

Examples for realloc()

Example 1: Expanding a Dynamically Allocated Array

This example demonstrates how to expand an existing memory block to accommodate additional elements.

Program

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

int main(void) {
    int *arr = malloc(5 * sizeof(int));
    if (arr == NULL) {
        return 1;
    }
    // Initialize array with values 1 to 5
    for (int i = 0; i < 5; i++) {
        arr[i] = i + 1;
    }
    // Expand array to hold 10 integers
    int *temp = realloc(arr, 10 * sizeof(int));
    if (temp == NULL) {
        free(arr);
        return 1;
    }
    arr = temp;
    // Initialize the new elements to 0
    for (int i = 5; i < 10; i++) {
        arr[i] = 0;
    }
    // Print the expanded array
    for (int i = 0; i < 10; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
    free(arr);
    return 0;
}

Explanation:

Allocated memory for an array of 5 integers and initialized it with values from 1 to 5.
Used realloc() to expand the memory block to hold 10 integers.
Initialized the additional elements (indices 5 to 9) to 0.
Printed the entire array to verify that the original data was preserved and the new elements were set correctly.

Program Output:

1 2 3 4 5 0 0 0 0 0

Example 2: Reducing the Size of an Allocated Memory Block

This example shows how to reduce the size of an allocated memory block while preserving the initial portion of the data.

Program

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

int main(void) {
    int *arr = malloc(10 * sizeof(int));
    if (arr == NULL) {
        return 1;
    }
    // Initialize array with values 1 to 10
    for (int i = 0; i < 10; i++) {
        arr[i] = i + 1;
    }
    // Reduce array size to hold only 5 integers
    int *temp = realloc(arr, 5 * sizeof(int));
    if (temp == NULL) {
        free(arr);
        return 1;
    }
    arr = temp;
    // Print the reduced array
    for (int i = 0; i < 5; i++) {
        printf("%d ", arr[i]);
    }
    printf("\n");
    free(arr);
    return 0;
}

Explanation:

Allocated memory for an array of 10 integers and initialized it with values from 1 to 10.
Used realloc() to shrink the memory block to hold only 5 integers.
Printed the first 5 elements to verify that the original data was preserved.

Program Output:

1 2 3 4 5

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C stdlib.h realloc() Function