X11-Forwarding

[Klaus Schlitt <klaus.schlitt@xxxxxx>]

Hi,
I want to forward the X11 output from the remote application (e.g.
xterm) to my local computer.
From the libssh tutorial, I have used this code fragment to do so:

    ssh_channel x11channel;
    rc = ssh_channel_request_pty(channel);
    if (rc != SSH_OK) return rc;
    rc = ssh_channel_change_pty_size(channel, 80, 24);
    if (rc != SSH_OK) return rc;

    int displayNr = 100;
    rc = ssh_channel_request_x11(channel, 0, NULL, NULL, displayNr);
    if (rc != SSH_OK) return rc;

    rc = ssh_channel_request_shell(channel);
    if (rc != SSH_OK) return rc;

    x11channel = ssh_channel_accept_x11(channel, 3000);
    if (x11channel == NULL) return -1;

My callback function, which is called at the time,
ssh_channel_accept_x11() is executed, looks like:
static ssh_channel my_channel_open_request_x11_callback(ssh_session
session, const char* originator_address, int originator_port, void*
userdata)
{
    ssh_channel channel = ssh_channel_new(session);
    return channel;
}

My expectation was, that the ssh_channel, provided by
ssh_channel_accept_x11(), must be used then to do the ssh request from
where the X11 output will be redirected to my local computer.
Problem:
After the specified timeout (3000 ms in my example), the function
ssh_channel_accept_x11() returns NULL, although the return value of the
callback function was not NULL.

From the console log "channel" shows the newly created ssh channel from
the callback function:
[2022/04/18 10:33:34.651892, 2] grow_window: growing window (channel
43:0) to 1280000 bytes
session            = 0x16C9138
originator_address = ::1
originator_port    = 40528
userdata           = 016D9ED8
channel            = 0x16D9E60
[2022/04/18 10:33:40.551005, 2] grow_window:  growing window (channel
44:3) to 1280000 bytes
[2022/04/18 10:33:43.951259, 1] ssh_channel_accept:  No channel request
of this type from server
[2022/04/18 10:38:10.603076, 1] channel_request:  Channel request exec
failed

The problem seems to be:
"No channel request of this type from server"

What is wrong with my implementation?
The complete example can be found here:
http://kschlitt.bplaced.net/LibsshTestX11.cpp

My goal is to open the X11 window on my local computer where cygwin-x is
installed.
    rc = ssh_channel_request_exec(channel, "xterm -display
localhost:100.0");

Is my approach correct for this goal?
If I use the same display number, which other cygwin xterm windows have
already set, then the X11 windows is opened on my local computer by
executing the command ("xterm").
Problem with this solution:
How to know the display number to use for this purpose?

Thanks,
Klaus