Library unloading (Re: [PATCH] auto-completing URLs on Ctrl-Enter)

[Thiago Macieira]

Martijn Klingens wrote:
>On Saturday 04 February 2006 13:23, Thiago Macieira wrote:
>> - you have no objects created there that could be using a non-global
>> symbol (libraries are loaded without global binding)
>>
>I'm not sure what you mean with the last of your 3 points. To take the
> example of Kopete back when I wanted this (about 3 years ago), there
> was libkopete and there was the MSN plugin, and of course the main app
> with kdelibs and friends.

This is mostly a corner case, and a twist of the second point (no objects 
of classes defined on the module).

Imagine the KAllInline class, where everything is inline. Imagine that 
this class has at least one virtual function (which means it has a 
vtable). And imagine that this class's .h file is from kdecore, so it's 
an installed header.

If the library was compiled with -fvisibility-inline-hidden, the vtable 
and virtual calls will resolve to within the library. So if you have an 
object of this class, their virtual tables will differ, depending on 
which library the object was created on.

Now, note that this kind of object is special because you can't even 
dynamic_cast<> it or throw it across libraries, or your program will 
crash.

>But now your last item. You said "you have no objects created there
> [...]", but the way I interpret that this cannot be true anyway by the
> time the dlclose happens, since with proper memory management all
> objects are deleted by then. Perhaps not really needed for the normal
> app termination case, but absolutely required if you want to design
> plugins for hot unloading and reloading, so I'm not sure I see the
> relation with symbols being global or non-global. Could you explain?

The point is that modules aren't loaded RTLD_GLOBAL. That means they 
resolve names first locally, then globally. So if the same name is 
defined twice (such as the case I illustrated above), we violate the rule 
of C++ that there is only one instance of any given one symbol.

Then again, everything is a twist of the first point: you cannot keep a 
pointer to any memory address being unmapped. This includes virtual table 
addresses.

-- 
  Thiago Macieira  -  thiago (AT) macieira.info - thiago (AT) kde.org
    PGP/GPG: 0x6EF45358; fingerprint:
    E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358

4. And æfter se scieppend ingelogode, he wrát "cenn", ac eala! se 
rihtendgesamnung andswarode "cenn: ne wát hú cennan 'eall'. Ástynt."
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