dlsym can't use handle returned by dlopen?
Jeremy Chadwick
koitsu at FreeBSD.org
Thu Nov 13 00:14:57 PST 2008
On Thu, Nov 13, 2008 at 09:00:21AM +0100, Markus Hoenicka wrote:
> Quoting Jeremy Chadwick <koitsu at FreeBSD.org>:
>
>> I know that the .so's you're loading with dlopen() need to be built a
>> specific way/with certain arguments, otherwise they won't work (I
>> believe what I saw was dlsym() returning NULL). My symbol names were
>> getting stomped on, and there was a compiler flag that addressed that.
>
> Is that a BSD-specific problem? As mentioned previously, I don't run
> into trouble on other platforms. Is there any documentation available
> which tells me how to build a dlopen()'able object in a portable way?
I wouldn't classify is as a problem in any way, and I cannot imagine
it's specific to BSD; I'm much more inclined to believe it's specific
to gcc.
When I looked at the resulting symbol names using nm or objdump, certain
characters were prepended to them. There's a gcc or ld flag which
disables this behaviour. I'll have to dig around to remind myself what
it is. Once I read about it, it made perfect sense.
Again, if you want me to write some code and provide some output of
what I'm talking about, I can do so.
>>> function_pointer = dlsym(RTLD_DEFAULT, "function_name");
>>>
>>> Why is that? Or rather: what am I doing wrong?
>>
>> This code right here is *completely* wrong. RTLD_DEFAULT is a mode bit
>> for dlopen(). I'm willing to bet a strict set of warnings would
>
> Citing the FreeBSD dlsym(3) man page:
>
> "If dlsym is called with the special handle RTLD_DEFAULT, the search
> for the symbol follows the algorithm used for resolving undefined symbols
> when objects are loaded."
>
> You probably had RTLD_LAZY and RTLD_NOW in mind which are dlopen()
> flags. BTW RTLD_NEXT works just as well instead of RTLD_DEFAULT.
You are right -- I missed that part of the man page, and I was most
definitely thinking of RTLD_LAZY and RTLD_NOW.
I cannot explain the behaviour using dlsym(RTLD_DEFAULT, ...).
--
| Jeremy Chadwick jdc at parodius.com |
| Parodius Networking http://www.parodius.com/ |
| UNIX Systems Administrator Mountain View, CA, USA |
| Making life hard for others since 1977. PGP: 4BD6C0CB |
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