Re: Libtool mishandling of C++ libraries requiring -pthread

[Bob Friesenhahn]

On Wed, 1 Feb 2012, Russ Allbery wrote:
> I think the primary question is why Libtool is using -nostdlib when
> linking C++ libraries.  The bug discussion at:
>
>    http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25460
>
> says that it's because of some other issue with libstdc++ linkage with
> dlopened modules depending on C++ libraries, but this dates from 2005, and
> breaking C++ libraries that need threading seems like a bad outcome.  Does
> this problem still exist?  Can this workaround just be dropped?

My memory is fading, but I do know that libtool used to use the linker 
directly to perform linking, but then gradually transitioned to using 
the compiler for linking as much as possible.  Adding -nostdlib causes 
the compiler to behave more like the linker, while retaining some 
secret nuances (e.g. things that allow C++ exceptions to work) that 
the compiler adds.

Libtool queries GCC to determine where its libraries reside (might not 
be in a system location), and the libraries and options which will be 
used.  Unfortunately, GCC has bugs which cause it to lie about what it 
will do and not adjust its response based on options like -m64 and 
-pthread.  Libtool wants to know about all the libraries which will be 
involved in the link since it adds such information to the .la files. 
If the standard C++ library is a static library, then libtool needs to 
know about that and postpone linking with it, or build dependent code 
as static, since it should not link a static library into a shared 
library or DLL.

The libltdl module loader does need to load the dependency libraries 
since the system might not do this at all, or might not do it fully or 
correctly.  It can't do this without knowing the libraries used.  This 
has been known to be a definite factor for C-language modules which 
link with C++ libraries.

If Gary is reading this, I assume that his memory would be better than 
mine and perhaps he can add some wisdom.

> Note that the GCC maintainters closed the bug requesting -pthread still
> add -lpthread even under -nostdlib as invalid.

I would have done the same.

The easiest fix is likely for libtool to know that if -pthread was 
specified to the compiler that it should add -lthread while linking.

Bob
--
Bob Friesenhahn
address@hidden, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/