[Haskell-cafe] cabal repl fails with GHC 7.8.2

Omari Norman omari at smileystation.com
Sun May 4 02:32:29 UTC 2014

On Sat, May 3, 2014 at 9:34 PM, Omari Norman <omari at smileystation.com> wrote:
>> I had a look at deka. It seems you need to list the decContext.c and
>> decQuad.c in the other order in the cabal file to make the .o files
>> show up in the right order in the call to ghc (you can see it with
>> cabal repl -v).
> Indeed, flipping the two and adding -fPIC to cc-options: in the cabal
> file made cabal repl work.  Thanks a bunch.
> The version I'm working on now links in several more C object files
> and I'm having the same problem there; I am reading up on linkers and
> fiddling around with it.

What I figured out is that cabal and/or ghci need to have the C object
files listed so that the earlier object files provide the symbols that
are needed by the later object files.  This is the opposite order one
would expect with a linker, where it is generally best to list object
files with undefined symbols first, and then list the libraries that
provide those objects later.

This sort of cycle fixes the problem:

1. Get error message
2. Use grep and nm to rummage around in dist/build/**/*.o to see what
object file provides the missing symbol
3. Rearrange your c-sources to put the needed file earlier
4. Repeat

The library I'm using has no circular dependencies, so I got it
working this way.  No -fPIC is needed.

It seems the C object files are loaded statically and GHCi will not
keep track of undefined symbols and wait to find them later.  Why
earlier versions did this is unclear; as the error message suggests I
will file a bug.

Thanks everybody for your help! --Omari

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