On Mon, Jan 28, 2013 at 03:27:01PM +0800, Li Zhang wrote:
> Hi all,
>
> I am trying to executing qmp command getfd according to qmp-commands.hx.
>
> { "execute": "getfd", "arguments": { "fdname": "fd1" } }
>
> Every time, it returns the error.
>
> {
> "error": {
> "class": "GenericError",
> "desc": "No file descriptor supplied via SCM_RIGHTS"
> }
> }
>
> This error is related with SCM_RIGHTS.
> I don't know about SCM at all.
>
> Can anyone give some hints about this?
The "getfd" command allows the client to pass a file descriptor to QEMU
and give it a name. Later that file descriptor can be used by QEMU.
The QMP client must send '{ "execute": "getfd", "arguments": { "fdname":
"fd1" } }' together with SCM_RIGHTS CMSG. You are getting the error
because you sent the "getfd" command but forgot to include a file
descriptor using SCM_RIGHTS.
SCM_RIGHTS is a feature of UNIX domain sockets. It allows one process
to pass a file descriptor to another process through the UNIX domain
socket. See "man 7 unix" and "man 3 cmsg" for details.
Look at monitor.c:qmp_getfd() to understand how this works. The QEMU
code to receive a passed file descriptor is in
qemu-char.c:unix_process_msgfd().
Stefan
