netdev - Re: recv(2), MSG_TRUNK and kernels older than 2.6.22

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Message-ID: <1279620171.2498.30.camel@edumazet-laptop>
Date:	Tue, 20 Jul 2010 12:02:51 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Roy Marples <roy@...ples.name>
Cc:	netdev@...r.kernel.org
Subject: Re: recv(2), MSG_TRUNK and kernels older than 2.6.22

Le mardi 20 juillet 2010 à 11:24 +0200, Eric Dumazet a écrit :
> Le mardi 20 juillet 2010 à 10:08 +0100, Roy Marples a écrit :
> > On 20/07/2010 09:54, Eric Dumazet wrote:
> > > Is it for the dhcpcd problem we talk about few week ago, disturbed by
> > > new 64bit stats ?
> > 
> > Yes
> > 
> > >
> > > Why do you want to have a fixed size of 256 bytes ?
> > >
> > > Using 8192 bytes on stack would avoid MSG_TRUNK mess.
> > 
> > Yes it would, but that doesn't answer my question :)
> 
> Your question might be wrong ? :=)
> 
> > I would like to use a buffer big enough, but not a whole 8k in size.
> > dhcpcd has quite a small runtime and I'd like to keep it that way.
> 
> 8192 bytes on stack is too much for you ?
> 
> Then you should automatically resize your buffer, and not using
> MSG_TRUNK at all (there is no guarantee the information you need will be
> part of the truncated part)
> 
> 

On < 2.6.22 kernels, recv() returns the length of your buffer, not size
of netlink frame.

You'll need something like :

size_t sz = 256;
char *buf = malloc(sz);
while (1) {
	if (!buf) error();
	len = recv(fd, buf, sz, MSG_PEEK | MSG_TRUNC);
	if (len < sz)
		break;
	if (len == sz)
		sz *= 2; // old kernel, try to double size
	else
		sz = len; // recent kernel is nice with us
	buf = realloc(buf, sz);
}
len = recv(fd, buf, sz, 0);




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