Subj : Win32: the PREFER_POLL code paths call WSAPoll() in ways it rejects (W
To   : GitLab note in main/sbbs
From : Rob Swindell
Date : Sun Aug 02 2026 19:57:21

https://gitlab.synchro.net/main/sbbs/-/issues/1212#note_9899

The semantics point is right, and it's why this issue recommends against flipping the define rather than for it — `WSAPoll(NULL, 0, t)` returns `WSAEINVAL` where POSIX `poll()` treats it as a sleep, and `POLLPRI` is rejected outright.

On the capacity point I couldn't reproduce a 64-descriptor limit. Probe built with the **default** `FD_SETSIZE` (64), polling 512 descriptors where the only readable one is placed last:

```
FD_SETSIZE = 64 (default)
WSAPoll(512 fds) = 1  err=0  first ready index = 511 (listener is 511)
```

If `WSAPoll()` were backed by a 64-entry `fd_set`, index 511 could not be reported at all. Whatever it does internally, its observable capacity isn't `FD_SETSIZE`.

Tested only on Windows 11 26200 (MSVC 19.44) — if the recollection is from Win7/Win8-era `WSAPoll()`, it may well have been accurate then and changed since. That's its own argument for keeping Windows on `select()` while older versions are supported, which is where this issue lands anyway.

Worth noting the linked page is also what sanctions the `FD_SETSIZE` fix that just landed in 031df44f62:

> If an application is designed to be capable of working with more than 64 sockets using the **select** and **WSAPoll** functions, the implementor should define the manifest **FD_SETSIZE** in every source file before including the *Winsock2.h* header file. One way of doing this may be to include the definition within the compiler options in the makefile. For example, you could add "-DFD_SETSIZE=128" as an option to the compiler command line for Microsoft C++.

and that it puts the provider's own ceiling at available memory, with `FD_SETSIZE` affecting "only … the **FD_XXX** macros". The `FD_SET and select` heading grouping `WSAPoll` in with `select` is probably the source of the confusion — `WSAPoll()` doesn't take an `fd_set` at all.

<details>
<summary>wsapoll_scale.c — the 512-descriptor probe</summary>

```c
/* Is WSAPoll() itself limited to FD_SETSIZE (64) descriptors, as it would be
   if it were implemented over Windows select()?
   Built WITHOUT redefining FD_SETSIZE, so FD_SETSIZE is the default 64. */
#include <winsock2.h>
#include <ws2tcpip.h>
#include <stdio.h>

#pragma comment(lib, "ws2_32.lib")

#define N 512

int main(void)
{
	WSADATA wd;
	WSAPOLLFD fds[N];
	SOCKET pad[N], lsock, csock;
	struct sockaddr_in sa;
	u_long nb = 1;
	int i, len, r, ready_idx = -1;

	WSAStartup(MAKEWORD(2, 2), &wd);
	printf("FD_SETSIZE = %d (default)\n", (int)FD_SETSIZE);

	/* N-1 permanently-quiet UDP sockets */
	for (i = 0; i < N - 1; i++) {
		pad[i] = socket(AF_INET, SOCK_DGRAM, 0);
		memset(&sa, 0, sizeof sa);
		sa.sin_family = AF_INET;
		sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
		sa.sin_port = 0;
		if (bind(pad[i], (struct sockaddr *)&sa, sizeof sa) != 0) {
			printf("bind failed at %d, err=%d\n", i, WSAGetLastError());
			return 1;
		}
		fds[i].fd = pad[i];
		fds[i].events = POLLIN;
		fds[i].revents = 0;
	}

	/* the ONLY readable descriptor, placed last - far beyond 64 */
	lsock = socket(AF_INET, SOCK_STREAM, 0);
	memset(&sa, 0, sizeof sa);
	sa.sin_family = AF_INET;
	sa.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
	sa.sin_port = 0;
	bind(lsock, (struct sockaddr *)&sa, sizeof sa);
	listen(lsock, 5);
	len = sizeof sa;
	getsockname(lsock, (struct sockaddr *)&sa, &len);
	csock = socket(AF_INET, SOCK_STREAM, 0);
	ioctlsocket(csock, FIONBIO, &nb);
	connect(csock, (struct sockaddr *)&sa, sizeof sa);
	Sleep(300);

	fds[N - 1].fd = lsock;
	fds[N - 1].events = POLLIN;
	fds[N - 1].revents = 0;

	r = WSAPoll(fds, N, 2000);
	for (i = 0; i < N; i++) {
		if (fds[i].revents != 0) {
			ready_idx = i;
			break;
		}
	}
	printf("WSAPoll(%d fds) = %d  err=%d  first ready index = %d (listener is %d)\n",
	    N, r, WSAGetLastError(), ready_idx, N - 1);

	return 0;
}
```

</details>

— *Authored by Claude (Claude Code), on behalf of @rswindell*
--- SBBSecho 3.37-Linux
 * Origin: Vertrauen - [vert/cvs/bbs].synchro.net (1:103/705)

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