[Lazarus] Dynamic loading of a custom sqlite library

[michael.vancanneyt at wisa.be]

On Fri, 10 Jun 2011, Max Vlasov wrote:

> 2011/6/10 <michael.vancanneyt at wisa.be>
>
>>
>>
>> On Fri, 10 Jun 2011, Maxim Ganetsky wrote:
>>
>>  09.06.2011 13:17, Max Vlasov пишет:
>>>
>>>> Finally found that this was because sqlite3 was in the uses list, so
>>>> actually it was scenario when fpc linked sqlite dynamically and besides
>>>> I did my own dynamic loading with my own unit of another version/file of
>>>> sqlite. So probably there was a conflict with shared data. In my case it
>>>> looks like everything was ok if the linked library and loaded was the
>>>> same file (system libsqlite3.so). But if linked was system libsqlite3.so
>>>> and my loaded was a library with a different name/soname the faults took
>>>> place.
>>>>
>>>> Just curious, such things occasionally can happen from time to time. Is
>>>> this about the way sqlite library is compiled so no two independent
>>>> copies of the library coexist dynamically linked/loaded due to
>>>> intersetion of the shared data. Or this is about the way linux handles
>>>> them?
>>>>
>>>
>>> It is best to ask this question in FPC devel mailing list, this behavior
>>> may be a bug.
>>>
>>
>> It is not a FPC bug; you're not supposed to use the 2 units at the same
>> time.
>> There is no way the compiler can detect this. At best the linux linker
>> should detect it.
>>
>>
> Michael,
> actually what appeared was that two libraries with different filenames and
> different soname (although compiled from the same sources, one dynamically
> linked and another dynamically loaded) shared some global data during
> run-time. I thought that in this case they should look virtually different
> for the linux loader to separate everything, but actually they were not.
>
> What interesting is that a solution was found. It's a flag RTLD_DEEPBIND for
> dlopen, I discovered it by chance, wrongly assuming RTLD_LOCAL as having $8
> (netware libc) while RTLD_LOCAL=0 in linux. In Linux RTLD_DEEPBIND = $8
>
> RTLD_DEEPBIND is not present in any of lazarus/fpc sources, but after I
> added it to my dlopen call, dynamically loaded library started to work when
> there was still a dynamically linked sqlite. So they became independent from
> each other.
>
> Quote from http://linux.die.net/man/3/dlopen
>
>> Place the lookup scope of the symbols in this library ahead of the global
>> scope. This means that a self-contained library will use its own symbols in
>> preference to global symbols with the same name contained in libraries that
>> have already been loaded. This flag is not specified in POSIX.1-2001.
>>

Yes, I can see how this can help.

>
> I'm not sure this will be helpful for everyone, but maybe it is worth to add
> this constant to rtl\unix\dl.pp file?

It depends. Does it also exist in FreeBSD and Mac OS ? 
If not, then I think this is not such a good idea.

Michael.