netdev - Re: iproute2: iplink

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Message-ID: <CAGm1_kvbqOtArw4dUpnzTRB1JtfBCaAAeyFYCPrcaGYUF10NSg@mail.gmail.com>
Date:	Mon, 19 Mar 2012 23:22:59 +0100
From:	Yegor Yefremov <yegorslists@...glemail.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	netdev@...r.kernel.org
Subject: Re: iproute2: iplink_ stuff cleanup

>> 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?

Regards,
Yegor
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