netdev - Re: iproute2: iplink

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Message-ID: <20120315091857.2114cb48@nehalam.linuxnetplumber.net>
Date:	Thu, 15 Mar 2012 09:18:57 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Yegor Yefremov <yegorslists@...glemail.com>
Cc:	netdev@...r.kernel.org
Subject: Re: iproute2: iplink_ stuff cleanup

On Thu, 15 Mar 2012 14:53:51 +0100
Yegor Yefremov <yegorslists@...glemail.com> wrote:

> Hello Stephen,
> 
> I'm still struggling to get ip/iplink_* routines to show up in
> Android's ip binary
> (https://groups.google.com/d/topic/android-building/yjV4iYnT1Zc/discussion).
> I've looked at the algorithm that is used to access those functions
> (like can_parse_opt, vlan_parse_opt etc.)
> 
>  snprintf(buf, sizeof(buf), LIBDIR "/ip/link_%s.so", id);
>         dlh = dlopen(buf, RTLD_LAZY);
>         if (dlh == NULL) {
>                 /* look in current binary, only open once */
>                 dlh = BODY;
>                 if (dlh == NULL) {
>                         dlh = BODY = dlopen(NULL, RTLD_LAZY);
>                         if (dlh == NULL)
>                                 return NULL;
>                 }
>         }
> 
>         snprintf(buf, sizeof(buf), "%s_link_util", id);
>         l = dlsym(dlh, buf);
>         if (l == NULL)
>                 return NULL;
> 
> as far as I can see from the ip/Makefile there are no dynamic libs
> like /ip/link_%s.so. Wouldn't it be simpler to let all iplink_*
> objects to export their interfaces via header files and just make a
> table in ip/iplink.c to hold protocol ID and pointer at link_utils
> struct.
> 
> struct link_util can_link_util = {
>         .id             = "can",
>         .maxattr        = IFLA_CAN_MAX,
>         .parse_opt      = can_parse_opt,
>         .print_opt      = can_print_opt,
>         .print_xstats   = can_print_xstats,
> };
> 
> Am I missing something?
> 
> Regards,
> Yegor

Ip utilities have dynamic extensibility, it is possible for someone
to add shared libraries for new functionality. This is a cool feature
but isn't used directly by the standard code. It does use it indirectly
by using dlopen() to find functions locally.

Think of it as introspection in C.

I won't take it out of the standard version, but you may have to find
another way to handle it on the Android universe.
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