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libssh 0.8.5 under Windows 10
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Hello world,
I developping a putty like terminal with libssh and I got a problem in the bsd_poll() function:
Sometimes it hangs.

/* compute fd_sets and find largest descriptor */

nfds 1
fds[0] {fd=2712 events=0 revents=784 }

rc remains to -1
if (max_fd == SSH_INVALID_SOCKET || rc == -1) {

    errno = EINVAL;
    return -1;

and the function bsd_poll() returns -1

To workaround this, I put the program into sleep for 200ms when it read 0 data length, but in that case the program is too slow.
What is wrong ?
Many thanks.

Event Timeline

also seen with libssh 0.8.2

Hi Andreas,

I have done a mix between 0.8.5 from
and yours

Some logs to confirm the new code:
ssh_connect: libssh 0.8.90 (c) 2003-2018 Aris Adamantiadis, Andreas Schneider and libssh contributors. Distributed under the LGPL, please refer to COPYING file for information about your rights, using threading threads_winlock
ssh_socket_connect: Nonblocking connection socket: 2648
ssh_connect: Socket connecting, now waiting for the callbacks to work
ssh_connect: Actual timeout : 10000
ssh_socket_pollcallback: Poll callback on socket 2648 (POLLOUT ), out buffer 0
ssh_socket_pollcallback: Received POLLOUT in connecting state
socket_callback_connected: Socket connection callback: 1 (0)
ssh_socket_unbuffered_write: Enabling POLLOUT for socket
ssh_socket_pollcallback: Poll callback on socket 2648 (POLLOUT ), out buffer 0
ssh_socket_pollcallback: Poll callback on socket 2648 (POLLIN ), out buffer 0
callback_receive_banner: Received banner: SSH-2.0-OpenSSH_7.1
ssh_client_connection_callback: SSH server banner: SSH-2.0-OpenSSH_7.1
ssh_analyze_banner: Analyzing banner: SSH-2.0-OpenSSH_7.1
ssh_analyze_banner: We are talking to an OpenSSH client version: 7.1 (70100)

Same issue if I reduce the wait on empty read to 1 ms (It’s better ! :) before if was occuring at 5 ms)

static int bsd_poll(ssh_pollfd_t *fds, nfds_t nfds, int timeout)

/* compute fd_sets and find largest descriptor */
for (rc = -1, max_fd = 0, i = 0; i < nfds; i++) {
    if (fds[i].fd == SSH_INVALID_SOCKET) {

#ifndef _WIN32

if (fds[i].fd >= FD_SETSIZE) {
    rc = -1;


    if (fds[i].events & (POLLIN | POLLRDNORM)) {
        FD_SET (fds[i].fd, &readfds);
    if (fds[i].events & (POLLOUT | POLLWRNORM | POLLWRBAND)) {
        FD_SET (fds[i].fd, &writefds);
    if (fds[i].events & (POLLPRI | POLLRDBAND)) {
        FD_SET (fds[i].fd, &exceptfds);
    if (fds[i].fd > max_fd &&
            (fds[i].events & (POLLIN | POLLOUT | POLLPRI |
                              POLLRDNORM | POLLRDBAND |
                              POLLWRNORM | POLLWRBAND))) {
        max_fd = fds[i].fd;
        rc = 0;

if (max_fd == SSH_INVALID_SOCKET || rc == -1) {
    errno = EINVAL;
    return -1;

I put a breakpoint on the line “errno = EINVAL ;” launch my terminal program on debug and watch some variables at this point...
max_fd 0
rc -1

nfds 1
fds[0] {fd=2628 events=0 revents=784 }

I’ve tried to force the select() or timeout or change the -1 to 0 but the polling seems to be broken at this point.

My new questions :
I notice these lines in the log (verbosity = 4 SSH_LOG_FUNCTIONS)
grow_window: growing window (channel 43:0) to 1280000 bytes

channel_rcv_data: Channel receiving 81 bytes data in 0 (local win=1279757 remote win=2097152)
channel_default_bufferize: placing 81 bytes into channel buffer (stderr=0)
channel_rcv_data: Channel windows are now (local win=1279676 remote win=2097152)
channel_request: Channel request shell success

Is it possible to enlarge this window (buffer I suppose...) with the c++ API ?

Comment on the bsd_poll() function

  • This is a poll(2)-emulation using select for systems not providing a native
  • poll implementation. *
  • Keep in mind that select is terribly inefficient. The interface is simply not
  • meant to be used with maximum descriptor value greater, say, 32 or so. With
  • a value as high as 1024 on Linux you'll pay dearly in every single call.
  • poll() will be orders of magnitude faster. */

Does it mean there are problems with select() on Windows ?
Where to find a poll implementation for Windows (Visual Studio 2017...) ? (if any exists ?)

Many thanks.

Hi Andreas (best wishes for this new year and new release :) ),
I've update my project with this version:

and the same issue occurs.
something goes wrong in the bsd_poll() and I'm not able to fix it.

Many thanks.

Hi Andreas,

As I can't go further, I hack the thing upon the API.
I remove the C++ wrapper to be in pure C.
This way I can read/write the channel and session structures to pach the error and fix the problem.
I remark that in this case of error, the data is really present on the server side.
Here my ugly (but working) code :

ssh_session m_pSession
ssh_channel m_pChannel

m_pSession and m_pChannel are successfuly created elsewere...

int SendSocket(const void *data, size_t len)

int iRet = -1 ;

if (m_pChannel != NULL)
  iRet = ssh_channel_write(m_pChannel, data, len) ;
  if (iRet == -1)
    if (m_pSession->session_state != SSH_SESSION_STATE_AUTHENTICATED)
      m_pSession->session_state = SSH_SESSION_STATE_AUTHENTICATED ;
      Sleep(500) ;
      // write 0 !
      iRet = ssh_channel_write(m_pChannel, data, 0) ;
      m_pSession->session_state = SSH_SESSION_STATE_AUTHENTICATED ;
      iRet = len ;
  iRet = -1 ;

return iRet ;


With this trick, no problem...

I haven't had time to look into this issue again. However the session state is internal, how did you get access to it? Also the C++ wrapper is unmaintained I really know someone who uses it for complex stuff.

Some time ago, we use WSAPoll() on Windows but it had a lot of issues that's why we stopped using it. I guess the best would be to move to a library which provides different event loops like libverto.