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It seems that the return code of WSAPoll() does not include the count of array items with revents == POLLNVAL. In the case where all of them are POLLNVAL, instead of returning 0 (which usually indicates a timeout) it returns -1 and WSAGetLastError() == WSAENOTSOCK.
This does not match the MSDN documentation which claims that the return code is the number of descriptors for which revents is non-zero. But it arguably does agree with the FreeBSD and MacOSX man pages which say that it returns the number of descriptors that are "ready for I/O".
BTW, the implementation of select_poll() assumes that the return code of poll() (if non-negative) is equal to the number of non-zero revents fields. But select_have_broken_poll() considers a MacOSX poll() implementation to be good even in cases where this assumption is not true:
static int select_have_broken_poll(void)
{
 int poll_test;
 int filedes[2];
 struct pollfd poll_struct = { 0, POLLIN|POLLPRI|POLLOUT, 0 };
 if (pipe(filedes) < 0) {
 return 1;
 }
 poll_struct.fd = filedes[0];
 close(filedes[0]);
 close(filedes[1]);
 poll_test = poll(&poll_struct, 1, 0);
 if (poll_test < 0) {
 return 1;
 } else if (poll_test == 0 && poll_struct.revents != POLLNVAL) {
 return 1;
 }
 return 0;
}
Note that select_have_broken_poll() == FALSE if poll_test == 0 and poll_struct.revents == POLLNVAL.