Hello,

I need to write a UDP application. The first byte in the packet tells
me what type of packet it is. Since I have to use UDP I have to use
recvfrom and sendto. If I do a recvfrom to read the first byte the
rest of the buffer is destroyed. I heard there is an UDP socket option
I can set that will let me read the first byte without destorying the
rest of the buffer. Is this true? If so what is it?

Thanks,
Ben

On 25 May 2006, bofh1234@hotmail.com wrote:

> I need to write a UDP application. The first byte in the packet
> tells me what type of packet it is. Since I have to use UDP I
> have to use recvfrom and sendto. If I do a recvfrom to read the
> first byte the rest of the buffer is destroyed. I heard there is
> an UDP socket option I can set that will let me read the first
> byte without destorying the rest of the buffer. Is this true? If
> so what is it?

Perhaps you're thinking of the recvfrom() flag MSG_PEEK? But I've
never used it and don't know how it behaves with UDP.

Dave

--
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t.i.k.t.i.n [at] a.d.v.a.n.c.e.d.r.e.l.a.y [dot] c.o.m

Yes that it is. My brain turned off for a minute. Thanks.

bofh1234@hotmail.com wrote:
> Yes that it is. My brain turned off for a minute. Thanks.

Since you are going to have to be able to (virtually) strip-off that
first byte anyway when you read the datagram the second time, you
might as well read the entire datagram to begin with.

rick jones

BTW, UDP does not always mean having to use sendto() and recvfrom().
If you "connect()" the UDP endpoint to the one remote IP/port with
which you are communicationg, you can use send() and recv().

All this and more are covered in books like "Unix Network Programming"
by Stevens, Fenner and Rudof. IMO must reading for anyone writing
network applications.

--
The glass is neither half-empty nor half-full. The glass has a leak.
The real question is "Can it be patched?"
these opinions are mine, all mine; HP might not want them anyway...
feel free to post, OR email to rick.jones2 in hp.com but NOT BOTH...

In article <ZTldg.1088$FY7.294@news.cpqcorp.net>,
Rick Jones <rick.jones2@hp.com> wrote:

> bofh1234@hotmail.com wrote:
> > Yes that it is. My brain turned off for a minute. Thanks.
>
> Since you are going to have to be able to (virtually) strip-off that
> first byte anyway when you read the datagram the second time, you
> might as well read the entire datagram to begin with.

It sounds like the type byte can be used to determine how big the rest
of the datagram is, so that he can read it into an appropriately sized
buffer.

Although he could simply set his buffer size to the maximum of any of
the types, and read into that.

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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