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Date:	Mon, 19 Mar 2012 15:53:27 -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 Mon, 19 Mar 2012 23:22:59 +0100
Yegor Yefremov <yegorslists@...glemail.com> wrote:

> >> 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.
> 
> The solution turned out to be simple:
> https://android-review.googlesource.com/#/c/34240/.
> -Wl,--no-gc-sections did the job.
> 
> Can you suggest some more pro arguments for dynamic method of
> exporting API routines?

You could make it optional on your platform, but for mainline Linux
it has been around long enough that someone surely depends on it.

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