Dynamic Memory – Reallocating Arrays

realloc changes the size of an existing allocation while preserving its content.

int *arr = malloc(5 * sizeof *arr);
/* ... fill ... */
arr = realloc(arr, 10 * sizeof *arr); // grow to 10 elements

If the block cannot be enlarged in place, realloc allocates a new block, copies the old data, frees the old block, and returns the new pointer. Always assign to a temporary variable first to avoid losing the original pointer on failure.

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📚 Example: Growing an Integer Buffer

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

int main(void) {
    size_t cap = 4, used = 0;
    int *buf = malloc(cap * sizeof *buf);
    if (!buf) return 1;
    for (int i = 0; i < 12; ++i) {
        if (used == cap) {
            size_t newCap = cap * 2;
            int *tmp = realloc(buf, newCap * sizeof *tmp);
            if (!tmp) { free(buf); return 1; }
            buf = tmp; cap = newCap;
        }
        buf[used++] = i * i;
    }
    for (size_t i = 0; i < used; ++i) printf("%d ", buf[i]);
    printf("\n");
    free(buf);
    return 0;
}