Re: [Qemu-devel] [qemu-devel]The problem of QMP command getfd.

[Li Zhang]

On 2013年01月29日 00:30, Eric Blake wrote:
> On 01/28/2013 07:16 AM, Li Zhang wrote:
> > > 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.
> You can also look at how libvirt uses this code, for an example of what
> it takes to use SCM_RIGHTS.
You are right, I saw that in libvirt.
Thanks for your suggestions.
> > > 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().
> > Thanks a lot for your detailed explanation and suggestions.
> > I will look into the code to understand how it works.
> Additionally, using the 'getfd' command is not very robust; new code
> should be preferring to use the 'add-fd' command family, as fdsets are
> less prone to leaked fds; this also uses SCM_RIGHTS.  However, libvirt
> does not yet have example code for using 'add-fd' (I'm slowly working on
> adding that).
Nice. Thanks for your information. :)
>