POLLHUP polling

This page is a work in progress

Recently I've been trying to listen on multiple sockets at once for events like reading or writing. This has worked fine, but as I've tried to listen for a socket close event I've found the documentation poor and often contradictory. This page is my attempt to figure this topic out.

POSIX

POSIX specifies two functions for waiting on multiple file descriptors:

select which originates from the BSD sockets API

poll which originates from the System V STREAMS API

select doesn't specify anything to do with sockets being closed, so that's not too useful.

poll initially looks like it's better but it leaves a lot of questions after reading:

Does POLLHUP apply to sockets?
Can the events field be 0 and still get POLLHUP events?
Under what conditions do we even get return events?

I highly recommend trying to read the poll standard and sussing out these details.

As far as I can tell nobody has brought up either of these questions during the standards process. At least in these sources:

The only talk of this issue I've found online is the 2001 email "poll() and events==0" which has no clear answers.

Existing lore

Richard Kettlewell's poll() and EOF has a test results for the case of setting POLLIN and closing a socket with no data to read. This is the most common case of POLLHUP use I see online.

The test results give us some interesting data:

The first is that POLLIN is often set if a socket is closed. If set alone this means a POLLIN poll without any return events may mean you still can't read from the socket
The second is that POLLHUP is often not set on older systems. This means programs polling POLLOUT may miss socket closures altogether. This is one reason why systems are moving to set POLLHUP instead of just POLLIN
The third is that neither are always set. Cygwin and very early Linux systems only set POLLHUP. This seems like a bug to me especially as most online advice recommend watching POLLIN for socket closures when writing portable programs

This data doesn't check what poll returns if there's still data in the socket to read but the connection has closed. That case may have entirely different results.

Another issue is what to return if a socket was never connected in the first place?

Test code

I wrote this quick program that tests what happens when you poll a closed socket with or without data with various event flags.

/* compile and run with gcc test.c -oTEST && ./TEST */

#include <stdio.h>
#include <errno.h>
#include <poll.h>
#include <sys/socket.h>
#include <unistd.h>
#include <string.h>

short do_test(short events, const char *data) {
  /* create a socket pair, one end for parent, one end for child */
  int sockets[2];
  int rc;
  rc = socketpair(AF_UNIX, SOCK_STREAM, 0, sockets);
  if(rc == -1) { printf("socket err %i\n", errno); return 1; }
  if(data) {
    rc = write(sockets[1], data, strlen(data));
    if(rc == -1) { printf("write err %i\n", errno); return 1; }
  }
  rc = close(sockets[1]); /* close one end */
  if(rc == -1) { printf("close err %i\n", errno); return 1; }
  /* poll for events */
  struct pollfd fd;
  fd.fd = sockets[0]; /* watch other end */
  fd.events = events;
  fd.revents = 0;
  rc = poll(&fd, 1, 10000);
  if(rc < 1) { printf("poll err %i\n", errno); return 1; }
  printf("do_test events %x data %p: %0x\n", events, data, fd.revents);
}
  
void do_multitest(const char* str) {
  do_test(0, str);
  do_test(POLLIN, str);
  do_test(POLLOUT, str);
  do_test(POLLIN|POLLOUT, str);
  do_test(POLLIN|POLLOUT|POLLPRI|POLLHUP|POLLERR|POLLNVAL, str);
}

int main(void) {
  printf("POLLIN: %x POLLOUT: %x POLLHUP: %x\n", POLLIN, POLLOUT, POLLHUP);
  do_multitest(NULL);
  do_multitest("hello world");
  return 0;
}

It's a bit nasty but it seems to get the job done to verify behaviour.

Linux

On Linux 5.19 it seems that polling a closed socket will return:

POLLIN if checking
POLLOUT if checking
POLLHUP always, even if checking for no events

This is regardless of whether there is data in the socket to read.

This behaviour is not POSIX compatible as POLLHUP and POLLOUT are supposedly mutually exclusive.

The Linux poll man page specifies the following:

POLLIN means there is data to read
POLLOUT means data can be written
POLLHUP indicates a pipe or socket has the peer end closed
The events bitmask can be empty

FreeBSD

On FreeBSD 13.1 it seems that polling a closed socket will return:

POLLIN if checking
POLLHUP always

This is regardless of whether there is data in the socket to read.

The FreeBSD poll man page has less information than the POSIX standard.

It's not documented but the events bitmask can be empty.

NetBSD

On NetBSD 9.3 it seems that polling a closed socket will return:

POLLIN if checking
POLLOUT if checking

This is regardless of whether there is data in the socket to read.

The NetBSD poll man page specifically notes that "Sockets produce POLLIN rather than POLLHUP when the remote end is closed"

Leaving the events bitmask empty causes poll to timeout.

OpenBSD

On OpenBSD 7.1 it seems that polling a closed socket will return:

POLLIN if checking
POLLHUP always

This is regardless of whether there is data in the socket to read.

This behavior is identical to FreeBSD's behaviour.

The OpenBSD poll man page specifically notes that POLLERR can indicate a socket disconnection.

It's not documented but the events bitmask can be empty.

Windows

cygwin:

$ ./a.exe

POLLIN: 1 POLLOUT: 4 POLLHUP: 10

poll err 0

do_test events 1 data 0x0: 1

do_test events 4 data 0x0: 4

do_test events 5 data 0x0: 5

do_test events 3f data 0x0: 5

poll err 0

do_test events 1 data 0x10040307c: 1

do_test events 4 data 0x10040307c: 4

do_test events 5 data 0x10040307c: 5

do_test events 3f data 0x10040307c: 5

wsapoll

select-based emulation

cygwin

minix

my dosbox code :(