fgetwln(FILE *stream, size_t *lenp)
{
	struct filewbuf *fb;
	wint_t wc;
	size_t wused = 0;

	/* Try to diminish the possibility of several fgetwln() calls being
	 * used on different streams, by using a pool of buffers per file. */
	fb = &fb_pool[fb_pool_cur];
	if (fb->fp != stream && fb->fp != NULL) {
		fb_pool_cur++;
		fb_pool_cur %= FILEWBUF_POOL_ITEMS;
		fb = &fb_pool[fb_pool_cur];
	}
 	fb->fp = stream;
 
 	while ((wc = fgetwc(stream)) != WEOF) {
		if (!fb->len || wused >= fb->len) {
 			wchar_t *wp;
 
 			if (fb->len)
				fb->len *= 2;
			else
				fb->len = FILEWBUF_INIT_LEN;

			wp = reallocarray(fb->wbuf, fb->len, sizeof(wchar_t));
			if (wp == NULL) {
				wused = 0;
				break;
			}
			fb->wbuf = wp;
		}

		fb->wbuf[wused++] = wc;

		if (wc == L'\n')
			break;
	}

	*lenp = wused;
	return wused ? fb->wbuf : NULL;
}

The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.