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
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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