Invalid file descriptor 0xff for FileWatching

[mzaffalon]

I am trying to connect to a machine running Linux from Windows

import LibSSH as ssh
session = ssh.Session("192.168.80.9"; user="root", log_verbosity=ssh.SSH_LOG_FUNCTIONS)

crashes with

Unhandled Task ERROR: ArgumentError: Passed file descriptor fd=Base.Libc.WindowsRawSocket(0xffffffffffffffff) is not a valid file descriptor

On my system the first two calls to ret = lib.ssh_connect(session_ptr) return SSH_AGAIN. Only after the first call session.fd is a positive number. Does _wait_loop(session) start too early?

I use LibSSH v0.7.1 and Julia v1.12.5.

[JamesWrigley]

Could you post the full stack trace?

And yeah, I could see thread safety being the issue. You could test it by starting Julia with -t 1,0 to only start one thread, or you could try #33 which has improved thread safety.

[mzaffalon]

This is with LibSSH v0.7.1 and starting as julia --project=.

julia> session = ssh.Session("192.168.80.9"; user="root", log_verbosity=ssh.SSH_LOG_FUNCTIONS)
[2026/03/08 19:19:08.217836, 3] ssh_config_parse_file:  Reading configuration data from C:\Users\nmrsu/.ssh/config
[2026/03/08 19:19:08.219831, 3] ssh_connect:  libssh 0.11.3 (c) 2003-2025 Aris Adamantiadis, Andreas Schneider and libssh contributors. Distributed under the LGPL, please refer to COPYING file for information about your rights, using threading threads_pthread
[2026/03/08 19:19:08.223821, 3] getai:  host 192.168.80.9 matches an IP address
[2026/03/08 19:19:08.226813, 3] ssh_socket_connect:  Nonblocking connection socket: 1416
[2026/03/08 19:19:08.227810, 3] ssh_connect:  Socket connecting, now waiting for the callbacks to work
[2026/03/08 19:19:08.229805, 4] ssh_socket_pollcallback:  Poll callback on socket 1416 (POLLOUT ), out buffer 0
[2026/03/08 19:19:08.231799, 3] ssh_socket_pollcallback:  Received POLLOUT in connecting state
[2026/03/08 19:19:08.232796, 4] socket_callback_connected:  Socket connection callback: 1 (0)
[2026/03/08 19:19:08.234790, 3] ssh_client_connection_callback:  session_state=2
[2026/03/08 19:19:08.235788, 3] ssh_socket_unbuffered_write:  Enabling POLLOUT for socket
[2026/03/08 19:19:08.236786, 4] ssh_socket_unbuffered_write:  wrote 23
[2026/03/08 19:19:08.237783, 3] ssh_connect:  current state : 2
Unhandled Task ERROR: ArgumentError: Passed file descriptor fd=Base.Libc.WindowsRawSocket(0xffffffffffffffff) is not a valid file descriptor
Stacktrace:
 [1] FileWatching._FDWatcher(fd::Base.Libc.WindowsRawSocket, readable::Bool, writable::Bool)
   @ FileWatching C:\Users\nmrsu\.julia\juliaup\julia-1.12.5+0.x64.w64.mingw32\share\julia\stdlib\v1.12\FileWatching\src\FileWatching.jl:418
 [2] _FDWatcher
   @ C:\Users\nmrsu\.julia\juliaup\julia-1.12.5+0.x64.w64.mingw32\share\julia\stdlib\v1.12\FileWatching\src\FileWatching.jl:413 [inlined]
 [3] _FDWatcher
   @ C:\Users\nmrsu\.julia\juliaup\julia-1.12.5+0.x64.w64.mingw32\share\julia\stdlib\v1.12\FileWatching\src\FileWatching.jl:405 [inlined]
 [4] poll_fd(s::RawFD, timeout_s::Float64; readable::Bool, writable::Bool)
   @ FileWatching C:\Users\nmrsu\.julia\juliaup\julia-1.12.5+0.x64.w64.mingw32\share\julia\stdlib\v1.12\FileWatching\src\FileWatching.jl:801
 [5] _safe_poll_fd(::RawFD, ::Vararg{Any}; kwargs::@Kwargs{readable::Bool, writable::Bool})
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\LibSSH.jl:174
 [6] _safe_poll_fd
   @ C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\LibSSH.jl:171 [inlined]
 [7] macro expansion
   @ .\lock.jl:376 [inlined]
 [8] _wait_loop(session::LibSSH.Session)
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:476
 [9] (::LibSSH.var"#7#8"{LibSSH.Session})()
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:79

caused by: InterruptException:
Stacktrace:
 [1] try_yieldto(undo::typeof(Base.ensure_rescheduled))
   @ Base .\task.jl:1157
 [2] wait()
   @ Base .\task.jl:1229
 [3] wait(c::Base.GenericCondition{ReentrantLock}; first::Bool)
   @ Base .\condition.jl:141
 [4] wait(session::LibSSH.Session)
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:450
 [5] connect(session::LibSSH.Session)
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:511
 [6]
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:221
 [7] Session
   @ C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:186 [inlined]
 [8] top-level scope
   @ REPL[2]:1
Some type information was truncated. Use `show(err)` to see complete types.

[mzaffalon]

Starting single threaded did not change anything. On the branch safety, it hangs here

julia> session = ssh.Session("192.168.80.9"; user="root", log_verbosity=ssh.SSH_LOG_FUNCTIONS)
[2026/03/08 19:28:56.657714, 3] ssh_config_parse_file:  Reading configuration data from C:\Users\nmrsu/.ssh/config
[2026/03/08 19:28:56.660669, 3] ssh_connect:  libssh 0.11.3 (c) 2003-2025 Aris Adamantiadis, Andreas Schneider and libssh contributors. Distributed under the LGPL, please refer to COPYING file for information about your rights, using threading threads_pthread
[2026/03/08 19:28:56.663662, 3] getai:  host 192.168.80.9 matches an IP address
[2026/03/08 19:28:56.664659, 3] ssh_socket_connect:  Nonblocking connection socket: 1488
[2026/03/08 19:28:56.665656, 3] ssh_connect:  Socket connecting, now waiting for the callbacks to work
[2026/03/08 19:28:56.667651, 4] ssh_socket_pollcallback:  Poll callback on socket 1488 (POLLOUT ), out buffer 0
[2026/03/08 19:28:56.668648, 3] ssh_socket_pollcallback:  Received POLLOUT in connecting state
[2026/03/08 19:28:56.670643, 4] socket_callback_connected:  Socket connection callback: 1 (0)
[2026/03/08 19:28:56.671643, 3] ssh_client_connection_callback:  session_state=2
[2026/03/08 19:28:56.672640, 3] ssh_socket_unbuffered_write:  Enabling POLLOUT for socket
[2026/03/08 19:28:56.674632, 4] ssh_socket_unbuffered_write:  wrote 23
[2026/03/08 19:28:56.675630, 3] ssh_connect:  current state : 2

ERROR: InterruptException:
Stacktrace:
 [1] poptask(W::Base.IntrusiveLinkedListSynchronized{Task})
   @ Base .\task.jl:1216
 [2] wait()
   @ Base .\task.jl:1228
 [3] wait(c::Base.GenericCondition{ReentrantLock}; first::Bool)
   @ Base .\condition.jl:141
 [4] wait(session::LibSSH.Session)
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:476
 [5] connect(session::LibSSH.Session)
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:617
 [6]
   @ LibSSH C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:236
 [7] Session
   @ C:\Users\nmrsu\zaffalon\lock-dumps\dev\LibSSH\src\session.jl:201 [inlined]
 [8] top-level scope
   @ REPL[2]:1
Some type information was truncated. Use `show(err)` to see complete types.

[mzaffalon]

Out of curiosity: what was the reason to wrap libssh instead of libssh2 which is shipped by default with Julia?

[JamesWrigley]

Interesting 🤔 I'll dig into it more next week. If you're down to have a go yourself, you could try modifying this loop body to notify waiters immediately if the file descriptor is not valid:

LibSSH.jl/src/session.jl

Line 475 in f80aebe

while !(@atomic session._waiter_stop_flag) && isopen(session)

Out of curiosity: what was the reason to wrap libssh instead of libssh2 which is shipped by default with Julia?

Libssh is much better documented and has more features (particularly server support) compared to libssh2. In fact, back when I started writing the bindings libssh2 didn't have any API docs at all.

[JamesWrigley]

Could you try again with latest master? I made some more thread safety fixes.

[mzaffalon]

Thank you for working on the issue. Since now it hangs while trying to connect to the Linux system, I tried the demo on the localhost and this too hangs

julia> session = ssh.Session("127.0.0.1", 2222)
[2026/05/09 18:04:50.195600, 3] ssh_config_parse_file:  Reading configuration data from C:\Users\nmrsu/.ssh/config
[2026/05/09 18:04:50.197594, 1] ssh_strict_fopen:  Failed to open a file C:/ProgramData/ssh/ssh_config for reading: No such file or directory
[2026/05/09 18:04:50.200587, 3] ssh_connect:  libssh 0.11.4 (c) 2003-2025 Aris Adamantiadis, Andreas Schneider and libssh contributors. Distributed under the LGPL, please refer to COPYING file for information about your rights, using threading threads_pthread
[2026/05/09 18:04:50.203579, 3] getai:  host 127.0.0.1 matches an IP address
[2026/05/09 18:04:50.205575, 3] ssh_socket_connect:  Nonblocking connection socket: 1228
[2026/05/09 18:04:50.207569, 3] ssh_connect:  Socket connecting, now waiting for the callbacks to work
[2026/05/09 18:04:50.208565, 4] ssh_socket_pollcallback:  Poll callback on socket 1228 (POLLOUT ), out buffer 0
[2026/05/09 18:04:50.210560, 3] ssh_socket_pollcallback:  Received POLLOUT in connecting state
[2026/05/09 18:04:50.211558, 4] socket_callback_connected:  Socket connection callback: 1 (0)
[2026/05/09 18:04:50.213552, 3] ssh_client_connection_callback:  session_state=2
[2026/05/09 18:04:50.214550, 3] ssh_socket_unbuffered_write:  Enabling POLLOUT for socket
[2026/05/09 18:04:50.215546, 4] ssh_socket_unbuffered_write:  wrote 23
[2026/05/09 18:04:50.217542, 3] ssh_connect:  current state : 2

I also had to comment out the block in LibSSH.jl starting with @compile_workload begin because the compilation hanged too.
This was on a Windows 10. On a Windows 11, there is no output at all. Can it be that the session is blocked by my organization?

[JamesWrigley]

Mmm not sure. Seems kinda unlikely if the LibSSH release worked 🤔 (insofar as it didn't hang)

We currently don't test windows on CI, I'll add that and see if I can reproduce it.

[JamesWrigley]

@DeniedPyro, thanks for the report, I closed that issue in favor of keeping everything in this one. I sneakily documented Windows as unsupported in v1 because I couldn't figure it out in time 🙈 I'll have another go this week-ish.

[JamesWrigley]

Could y'all try the branch in #48? It uses a custom build of libssh that fixes windows support.

[DeniedPyro]

@JamesWrigley Looks good on my side! it runs correctly!