complete macos g_poll:

gint
g_poll (GPollFD *fds,
	guint    nfds,
	gint     timeout)
{
  struct timeval tv;
  fd_set rset, wset, xset;
  GPollFD *f;
  int ready;
  int maxfd = 0;

  FD_ZERO (&rset);
  FD_ZERO (&wset);
  FD_ZERO (&xset);

  for (f = fds; f < &fds[nfds]; ++f)
    if (f->fd >= 0)
      {
	if (f->events & G_IO_IN)
	  FD_SET (f->fd, &rset);
	if (f->events & G_IO_OUT)
	  FD_SET (f->fd, &wset);
	if (f->events & G_IO_PRI)
	  FD_SET (f->fd, &xset);
	if (f->fd > maxfd && (f->events & (G_IO_IN|G_IO_OUT|G_IO_PRI)))
	  maxfd = f->fd;
      }

  tv.tv_sec = timeout / 1000;
  tv.tv_usec = (timeout % 1000) * 1000;

  ready = select (maxfd + 1, &rset, &wset, &xset,
		  timeout == -1 ? NULL : &tv);
  if (ready > 0)
    for (f = fds; f < &fds[nfds]; ++f)
      {
	f->revents = 0;
	if (f->fd >= 0)
	  {
	    if (FD_ISSET (f->fd, &rset))
	      f->revents |= G_IO_IN;
	    if (FD_ISSET (f->fd, &wset))
	      f->revents |= G_IO_OUT;
	    if (FD_ISSET (f->fd, &xset))
	      f->revents |= G_IO_PRI;
	  }
      }

  return ready;
}

Looks like (unsurprisingly) FreeBSD has a similar implementation.

https://github.com/freebsd/freebsd-src/blob/f51e9d0e0988df58c94db586ab5c8b5fd091c004/sys/sys/select.h#L65