Subject: Re: Updating arp(8) to use getifaddrs (cf. bin/8566)
To: Andrew Brown <>
From: None <>
List: tech-net
Date: 11/08/2002 11:11:26
>On Thu, Nov 07, 2002 at 09:17:35AM -0500, Andrew Brown wrote:
>  | >The problem with SIOCGIFCONF as used by most programs is that it
>  | >doesn't cater for when the number of interfaces * sizeof(struct ifreq)
>  | >(== 32) exceeds the buffer size passed in, so you might get truncated
>  | >results.  There is a way to detect and work around this, except non
>  | >of the programs in usr.sbin/* that use SIOCGIFCONF do this, and won't
>  | >work correctly if the buffer size is exceeded.
>  | >(I checked various programs in usr.sbin which use SIOCGIFCONF, and
>  | >they support between 10, 16, 32, 256 (or 1 case, 512) addresses.)
>  | 
>  | they're done incredibly wrong then.  my understanding of the
>  | "canonical" way to use SIOCGIFCONF was to call it once with an empty
>  | struct ifconf (ie, ifc_buf is null and ifc_len is zero), and then
>  | malloc a buffer of the size returned in ifc_len.  probably with a pad
>  | so that any *new* addresses that show up in between your first a
>  | second calls are also covered.  it's simple to check that you didn't
>  | "get back" more data than you expected if you save the value of
>  | ifc_len between the first and second calls.

	SIOCGIFCONF is way too hard to get right, that's the very reason why
	we now have getifaddrs(3).  i'm all for the change.