linux-kernel - Re: how to read one udp packet with more than one recvfrom() calls?

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Message-ID: <AANLkTim_qtAO-muZk4q9N=_DFsvAcQprnLDx7XOe3aNg@mail.gmail.com>
Date:	Mon, 8 Nov 2010 15:12:00 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	ranjith kumar <ranjithproxy@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: how to read one udp packet with more than one recvfrom() calls?

On Mon, Nov 8, 2010 at 3:08 PM, ranjith kumar <ranjithproxy@...il.com> wrote:
> Hi,
>
> I  have implemented client and server programs using udp
> protocol(files are attached).
> UDP packet size is 500bytes.
>
> I want to read these 500bytes in two calls to recvfrom(). First time
> reading 100bytes and second time 400bytes.
> How to do this?
>
> When I tried to change the third argument of recvfrom(size_t len),
> from 500 to 100, first 100bytes are read correctly.
> But when I call recvfrom() second time with len=400, it is reading the
> first 400bytes of "next udp packet".
> Why? Isn't it possible to read one udp packet in two calls to
> recvfrom()/read()????
>

recvmsg(2):

       MSG_PEEK
              This  flag  causes the receive operation to return data from the
              beginning of the receive queue without removing that  data  from
              the queue.  Thus, a subsequent receive call will return the same
              data.

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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