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Subject: Re: [boost] [all] Request for out of the box visibility support
From: degski (degski_at_[hidden])
Date: 2018-08-21 03:35:21

On Tue, 21 Aug 2018 at 02:59, Gavin Lambert via Boost <boost_at_[hidden]>

> On 19/08/2018 08:17, Andrey Semashev wrote:
> > There is no problem linking static libraries into shared libraries, I
> > do it every day.
> This is not entirely true -- while you can do that, and it can work, it
> comes with a very big caveat that will trap the unwary.

I'm happy you are commenting on this problem again.

Let's say you have a library A and another library B, and an executable C.
> Compile everything as shared, now C will dynamically load B and A, and
> everything will Just Workâ„¢.
> Compile everything as static, now C will statically contain B and A, and
> everything will Just Workâ„¢.
> Compile A as static and B as shared. Now things *might* work, or you
> might have a problem -- it depends on how the libraries are actually
> used; in particular whether A is used by B or C or both.
> If it's used only by one or the other (and in particular is *not*
> exposed in the public API of B at all), then everything is fine.
> If B does a poor job of hiding its use of A (ie. it's included in public
> header files, even if not part of public API), and if C uses A as well,
> you now have an ODR problem. And the problem is worse if A's objects
> are used in the public API of B.
> The key thing to realise is that "A as statically linked into B" and "A
> as statically linked into C" are technically separate copies of the
> library. And passing objects around between separate copies of the same
> library is highly perilous -- a lot of the time you can get away with it
> (provided that the same compiler settings were used in both cases, so
> you don't get different memory layouts) -- but some things will be
> "wrong" such as separate copies of static variables and the like, and
> this can cause misbehaviour.
> This is why you're always encouraged to link to the runtime library as
> shared -- because since it's a dependency of every other library, the
> only time it's safe to use it statically is if you don't use *any*
> shared libraries at all. Otherwise you end up with multiple copies of
> the runtime, and thus multiple separate heaps, and hilarity ensues.

This is I think not what happens in practice, in vcpkg f.e. creation of a
static library will by default also imply that that library is statically
linked to the run-time. I also do this when building Boost, while you are
actually saying that that option/possibility should not even exist as I
presume that the combination of a dynamic library statically linked to the
run-time crt makes even less sense.

Having multiple copies of other libraries is perhaps less dramatic than
> that, and you can make it "safe" if you're very careful about
> segregation, but it's a highly effective source of potential bugs.

You can only find out that you weren't careful when you are finding it out,
which might be a case of Seems To Workâ„¢, until it doesn't, as it's a full


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*“No, it isn’t truth. Truth isn’t truth" - Rudolph W. L. Giuliani*

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