dlsym can't use handle returned by dlopen?

[Jeremy Chadwick]

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 |