Re: msghdr problem in tcp_sendmsg

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Thu, 8 Sep 2005, Serge Goodenko wrote:

> Hello!
>
> I trace the kernel networking code (ver 2.4.25).
> I send simple message (say, "hello") using simple client and see how
> tcp_sendmsg function works.
> And what I see is that there's NO my message (e.g. "hello") in the msghdr
> structure that tcp_sendmsg receives,
> e.g. in (char*)msg->msg_iov->iov_base, although debugger shows that
> msg->msg_iov->iov_len = 6, e.g. equals to "hello" message length (+1).
> Can anyone explain where I am wrong or what I do not understand?
>
> I am using the UML kernel and standard gdb debugger.
>

Did you build the proper 'struct iovec' ?  It gets a pointer to
your "hello", not the string itself as member iov_base.
If you look at this with the debugger, you need to dereference
this pointer. The string can be anywhere.

When receiving, that struct member gets a pointer to your buffer,
not the buffer itself. This means that struct iovec is always
the sizeof(void *) + sizeof(size_t) in length.

>
> Serge Goodenko,
> MIPT, Russia
> -

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13 on an i686 machine (5589.54 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
I apologize for the following. I tried to kill it with the above dot :

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