linux-kernel - Re: recv(...,MSG

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Date:	Thu, 9 Aug 2012 01:17:11 -0700 (PDT)
From:	ibrahem.batta@...il.com
To:	fa.linux.kernel@...glegroups.com
Cc:	Alan.Cox@...ux.org, linux-net@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Samuel Thibault <samuel.thibault@...c.net>
Subject: Re: recv(...,MSG_TRUNC)

On Saturday, June 29, 2002 10:40:18 PM UTC+3, Samuel Thibault wrote:
> (Previous mail seemed to be garbaged by mime attachment)
> 
> Hello,
> 
> man recv says, about flags :
> 
>        MSG_TRUNC
>               Return  the real length of the packet, even when it
>               was longer than the passed buffer. Only  valid  for
>               packet sockets.
> 
> But it is neither implemented in ipv4/udp.c, nor in ipv6/udp.c, although
> it is in tcp.c, for instance !
> 
> By searching with google, I could read old manpages where it didn't exist,
> but I find it very useful, especially in conjunction with MSG_PEEK, for
> trying to read a packet with a little buffer, and then really get it with
> an appropriate buffer.
> 
> So here's a patch which cures 2.4.18, and also works on 2.5 kernels.
> 
> Best regards,
> 
> Samuel Thibault
> 
> diff -urN linux-2.4.18/net/ipv4/udp.c linux-2.4.18-cor/net/ipv4/udp.c
> --- linux-2.4.18/net/ipv4/udp.c	Mon Jun 10 23:34:59 2002
> +++ linux-2.4.18-cor/net/ipv4/udp.c	Mon Jun 10 23:35:31 2002
> @@ -680,7 +680,7 @@
>    	}
>  	if (sk->protinfo.af_inet.cmsg_flags)
>  		ip_cmsg_recv(msg, skb);
> -	err = copied;
> +	err = (flags&MSG_TRUNC) ? skb->len - sizeof(struct udphdr) : copied;
> 
>  out_free:
>    	skb_free_datagram(sk, skb);
> diff -urN linux-2.4.18/net/ipv6/udp.c linux-2.4.18-cor/net/ipv6/udp.c
> --- linux-2.4.18/net/ipv6/udp.c	Mon Jun 10 23:35:07 2002
> +++ linux-2.4.18-cor/net/ipv6/udp.c	Mon Jun 10 23:35:36 2002
> @@ -432,7 +432,7 @@
>  			}
>  		}
>    	}
> -	err = copied;
> +	err = (flags&MSG_TRUNC) ? skb->len - sizeof(struct udphdr) : copied;
> 
>  out_free:
>  	skb_free_datagram(sk, skb);
> 
> -
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man i want to see full example about receivefrom and MSG_TRUNC flag please 
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