mrb_io_sysread(mrb_state *mrb, mrb_value io)
{
  struct mrb_io *fptr;
  mrb_value buf = mrb_nil_value();
  mrb_int maxlen;
  int ret;

  mrb_get_args(mrb, "i|S", &maxlen, &buf);
  if (maxlen < 0) {
    mrb_raise(mrb, E_ARGUMENT_ERROR, "negative expanding string size");
  }
  else if (maxlen == 0) {
    return mrb_str_new(mrb, NULL, maxlen);
  }

  if (mrb_nil_p(buf)) {
    buf = mrb_str_new(mrb, NULL, maxlen);
  }

  if (RSTRING_LEN(buf) != maxlen) {
    buf = mrb_str_resize(mrb, buf, maxlen);
  } else {
    mrb_str_modify(mrb, RSTRING(buf));
  }

  fptr = (struct mrb_io *)io_get_open_fptr(mrb, io);
  if (!fptr->readable) {
    mrb_raise(mrb, E_IO_ERROR, "not opened for reading");
  }
  ret = read(fptr->fd, RSTRING_PTR(buf), (fsize_t)maxlen);
  switch (ret) {
    case 0: /* EOF */
      if (maxlen == 0) {
        buf = mrb_str_new_cstr(mrb, "");
      } else {
        mrb_raise(mrb, E_EOF_ERROR, "sysread failed: End of File");
      }
      break;
    case -1: /* Error */
      mrb_sys_fail(mrb, "sysread failed");
      break;
    default:
      if (RSTRING_LEN(buf) != ret) {
        buf = mrb_str_resize(mrb, buf, ret);
      }
      break;
  }

  return buf;
}

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.