A workaround for the “Template Haskell + C” bug?

Question

I've got the following situation:

• Library X is a wrapper over some code in C.
• Library A depends on library X.
• Library B uses Template Haskell and depends on library A.

GHC bug #9010 makes it impossible to install library B using GHC 7.6. When TH is processed, GHCi fires up and tries to load library X, which fails with a message like

Loading package charsetdetect-ae-1.0 ... linking ... ghc:
~/.cabal/lib/x86_64-linux-ghc-7.6.3/charsetdetect-ae-1.0/
libHScharsetdetect-ae-1.0.a: unknown symbol `_ZTV15nsCharSetProber'

(the actual name of the “unknown symbol” differs from machine to machine).

Are there any workarounds for this problem (apart from “don't use Template Haskell”, of course)? Maybe library X has to be compiled differently, or there's some way to stop it from loading (as it shouldn't be called during code generation anyway)?

Answer 1

This is really one of the main reasons that 7.8 switched to dynamic GHCi by default. Rather than try to support every feature of every object file format, it builds dynamic libraries and lets the system dynamic loader handle them.

Try building with the g++ option -fno-weak. From the g++ man page:

-fno-weak

Do not use weak symbol support, even if it is provided by the linker. By default, G++ will use weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it will result in inferior code and has no benefits. This option may be removed in a future release of G++.

There is another issue with __dso_handle. I found that you can at least get the library to load and apparently work by linking in a file which defines that symbol. I don't know whether this hack will cause anything to go wrong.

So in X.cabal add

if impl(ghc < 7.8)
    cc-option: -fno-weak
    c-sources: cbits/dso_handle.c

where cbits/dso_handle.c contains

void *__dso_handle;