malloc, calloc, realloc, free

The heap vs the stack

Local variables live on the stack and vanish automatically. The heap is memory you request explicitly with malloc-family functions, and it stays reserved until you free it.

malloc + free

malloc(n) returns raw, uninitialised memory (or NULL on failure). Always pair it with free once done.

int *arr = malloc(5 * sizeof(int));
if (arr == NULL) {
    return 1; // allocation failed
}

for (int i = 0; i < 5; i++) arr[i] = i * i;
printf("%d\n", arr[4]);

free(arr);
arr = NULL;

Output:

16

calloc vs realloc

calloc(n, size) allocates and zeroes the memory. realloc(ptr, newSize) grows or shrinks an existing block, copying old data over — always reassign its result, since the block may move.