C realloc()

How it works

realloc can resize an allocated block, move it, or fail. For a nonzero requested size, failure returns null while leaving the old allocation available. Store the result in a temporary pointer so the owner is not lost on failure. After success the old pointer and interior aliases must no longer be used; retain only the new pointer. Bytes beyond the previous allocation size are not initialized. This example never calls realloc with zero, avoiding a difficult boundary that should be handled as an explicit release operation.

Run it locally

Use a hosted C17 compiler in a separate lab folder. The command assumes a driver named cc; substitute gcc, clang or zig cc as appropriate. C does not currently run in Bhylu’s browser playground. Apache, PHP and MySQL are not needed for these C programs.

cc -std=c17 -Wall -Wextra -Wpedantic -Werror reallocation.c -o lesson

On Windows, use -o lesson.exe and run .\lesson.exe in PowerShell. On Unix-like systems run ./lesson. Math-library linking differs by toolchain; GCC on many Unix systems needs -lm after the source.

Download the complete C source. File-writing labs must run in an empty private directory, away from existing reports.

Example

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

int main(void) {
    size_t old_count = 2;
    size_t new_count = 4;
    int *values = malloc(old_count * sizeof *values);
    if (values == NULL) { return 1; }
    values[0] = 10; values[1] = 12;
    if (new_count > SIZE_MAX / sizeof *values) { free(values); return 1; }
    int *grown = realloc(values, new_count * sizeof *values);
    if (grown == NULL) { free(values); return 1; }
    values = grown;
    for (size_t i = old_count; i < new_count; ++i) { values[i] = 14; }
    printf("Readings: %d %d %d %d\n", values[0], values[1], values[2], values[3]);
    free(values);
    return 0;
}

Expected result

Readings: 10 12 14 14

The previous two readings survive a successful resize. Only the new elements are initialized in the second loop. On failure, the old block is still freed before returning an error. A dynamic collection should retain both its used length and capacity and update them only after successful growth. Reallocating on every insertion can cause unnecessary copying.