recvfrom a large packet

[bernd@petrovitsch.priv.at (Bernd Petrovitsch)]

On Die, 2012-11-06 at 16:08 +0200, Victor Buciuc wrote:
> On Tue, Nov 6, 2012 at 1:35 PM, devendra.aaru <devendra.aaru@gmail.com> wrote:
> > if i do a recvfrom (sk, buf, 10000, 0, &addr, &len), shall i recv all the data
> > i mean the 10000 bytes?

Not necessarily. Consider the case that the other side sends only 9000
bytes.

> > since fragmentation happen in the ip layer and assembled happen in the
> > ip layer it doesnt matter for the upper layer about the packet size.
[....]
> > i wrote a test code and it seems to be working.

You tried one case and then you think it is in all cases the same?
You are very optimistic .....

> > is there any problem will come if i turn on firewall.
[...]

Try it. "Turn on firewall" is also not very precise ....

>    I think a signal can interrupt recvfrom. If you already had some
> data copied in the buffer then it will return something. You should

No, if a signal interrupts the recvfrom(), recvfrom() returns -1 (and
errno == EINTR).
Thus it cannot report any partially received packet and so there is
nothing written to the buffer.

> always check what you get in the result returned by recvfrom. If
> you're not satisfied with what you got you can always call again. (I
> assumed it's UDP we're talking about).

That is actually the only robust way:
1) append the read data to a buffer
2) check if enough is there to handle a packet. If not goto 1)
3) handle the first packet and delete it.
4) goto 2)

Kind regards,
	Bernd
-- 
Bernd Petrovitsch                  Email : bernd at petrovitsch.priv.at
                     LUGA : http://www.luga.at