I just tried the exact same code in Linux.
(It's an FTP client).
In Linux, if sendfile() is called with 0 size, then it immediately returns
zero.
No traffic is generated on the network at all.
I think this is simple & sane behavior to adapt in NuttX.
On Mon, Jun 19, 2023 at 4:02 AM Petro Karashchenko <
petro.karashche...@gmail.com> wrote:
> The sendfile() description across the OSes is not the same. The FreeBSD
> version https://man.freebsd.org/cgi/man.cgi?sendfile(2) for example has
> words "The nbytes argument specifies how many bytes of the file should be
> sent, with 0 having the special meaning of send until the end of file has
> been
> reached" while Linux version does not states anything about zero count, so
> this seems to be "system defined". We can try to experiment with the Linux
> version to see how zero bytes count is handled.
>
> Best regards,
> Petro
>
> On Sun, Jun 18, 2023, 11:10 PM Fotis Panagiotopoulos
> wrote:
>
> > Hi Alan,
> >
> > Thanks for checking this out.
> >
> > The resources you linked indicate (correctly) that a TCP segment may
> have 0
> > data size.
> > But also that usually this is not allowed for the users? I don't know, I
> > don't intend to use this.
> >
> > However, sendfile (not the plain send) would be completely meaningless
> with
> > a 0 size, I think.
> > Currently, NuttX will not send anything on the wire with size set to 0.
> Not
> > even an empty TCP segment.
> >
> > I don't really want to change the way it works, only improve its
> behavior.
> > As it is right now, resources are wasted, and the cryptic EAGAIN makes
> > troubleshooting harder.
> >
> > But what's best? Return zero? Change errno?
> >
> > On Sun, Jun 18, 2023 at 10:13 PM Alan C. Assis
> wrote:
> >
> > > Suggest, try to run you same application with this issue on Linux.
> > >
> > > I'm expecting at least a similar error on Linux.
> > >
> > > BR,
> > >
> > > Alan
> > >
> > > On 6/18/23, Alan C. Assis wrote:
> > > > Hi Fotis,
> > > >
> > > > On 6/18/23, Fotis Panagiotopoulos wrote:
> > > >> Hello,
> > > >>
> > > >> Due to a bug in my application, the code tried to use sendfile()
> with
> > > >> zero
> > > >> size.
> > > >>
> > > >> As I see, sendfile() will happily proceed, and it will block here:
> > > >>
> > https://github.com/apache/nuttx/blob/master/net/tcp/tcp_sendfile.c#L523
> > > >>
> > > >> Obviously, it will never manage to send any data (as the requested
> > size
> > > >> is
> > > >> 0),
> > > >> and after the timeout expires, it will return EAGAIN.
> > > >>
> > > >
> > > > Good finding!
> > > >
> > > > BTW, that is the default timeout value?
> > > >
> > > > I think conn->sconn.s_sndtimeo is only defined when people enable the
> > > > CONFIG_NET_SOCKOPTS.
> > > >
> > > >> This seems a bit wrong to me.
> > > >> Why not return immediately, but instead wait for the (inevitable)
> > > >> timeout?
> > > >> Why not return a more meaningful error code, or maybe just 0?
> > > >>
> > > >
> > > > I was going to suggest avoid sending a packet with 0 byte, but it is
> > > > supported:
> > > > https://www.quora.com/Can-TCP-send-0-byte-packets
> > > > https://github.com/golang/go/issues/10940
> > > >
> > > > See, technically it is not an error! :-)
> > > >
> > > >> I couldn't find what the standards dictate for this edge case, so
> > maybe
> > > >> the
> > > >> current behavior is correct.
> > > >>
> > > >> What do you think?
> > > >>
> > > >
> > > > I think it is correct, but most servers will just ignore it and you
> > > > will end-up with a timeout.
> > > >
> > > > BR,
> > > >
> > > > Alan
> > > >
> > >
> >
>
