Re: [Xen-devel] [help] problem with `tools/xenstore/xs.c: xs_talkv()`

[éç <aware.why@xxxxxxxxx>]

On Thu, Aug 9, 2012 at 4:21 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:

On Thu, 2012-08-09 at 08:07 +0100, 马磊 wrote:
>
>
> On Wed, Aug 8, 2012 at 4:32 PM, 马磊 <aware.why@xxxxxxxxx> wrote:
>         Hi all,
>
>             In  xen-4.1.2 src/tools/xenstore/xs.c: xs_talkv function,
>          there are several lines as follow:
>
>
>          437        mutex_lock(&h->request_mutex);
>
>
>
>          438
>
>
>
>          439        if (!xs_write_all(h->fd, &msg, sizeof(msg)))
>
>
>
>          440                goto fail;
>
>
>
>          441
>
>
>
>          442        for (i = 0; i < num_vecs; i++)
>
>
>
>          443                if (!xs_write_all(h->fd,
>         iovec[i].iov_base, iovec[i].iov_len))
>
>
>
>          444                        goto fail;
>
>
>
>          445
>
>
>
>          446        ret = read_reply(h, &msg.type, len);
>
>
>
>          447        if (!ret)
>
>
>
>          448                goto fail;
>
>
>
>          449
>
>
>
>          450        mutex_unlock(&h->request_mutex);
>
>
>
>
>         The above seems to tell me that after writing to h->fd , the
>         read_reply invoking read_message which immediatelly  read from
>         hd->fd?
>         What did it mean by this?
>
>
>
>
>         Thanks
>
> If hd->fd refers to a socket descriptor, it's understandable that
> writing and then imediatelly reading, in which case the fd is returned
> by get_handle(xs_daemon_socket(), flags).
>
>
> But when fd is retrived by get_handle(xs_domain_dev(), flags), it
> means to write to a file and then read from the same file imediatelly.
> Dose it have something to do with the internal communication
> protocol?!

Yes, the xenstore protocol involves both writing messages and reading
replies, but that seems trivially obvious so I'm afraid I really have no
idea what your question is nor what is confusing you. Perhaps describing
in more detail what you are trying to achieve will help?

Reading http://wiki.xen.org/wiki/Asking_Xen_Devel_Questions might also
help you consider what it is you are asking.


>
>
> Thanks for replying.
>
>
>
>
>
>
The final read and write operations are achieved by:

read(fd, data, len); 

write(fd, data, len); 

Maybe my confusing lies in this point that what's the distinction between the read and write operations on a socket file, the /proc/xen/xenbus, a regular file?

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