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.
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).