GHC uses _stub.c files regardless of whether any 'foreign import' decls remain in a .hs file

[ncalexan@…]

It appears GHC links any _stub.o files it can find, which is not correct (although usually it only leads to duplicate symbols.) To duplicate, have two modules A and B, and a foreign import in A. A_stub.o will be built as usual. Copy A_stub.o to B_stub.o, relink and have duplicate symbols.

[simonmar]

I believe this only happens if you are using --make or GHCi, and GHC decides the file does not need compiling. In this case it picks up any _stub.o file it finds. We should really check the source file for foreign export / foreign import wrapper before using the _stub.o file.

[jcab@…]

The description of the bug is incorrect. It picks up the _stub.c files, not the _stub.o files, and it picks them up even if the file is recompiled (and didn't re-generate the _stub.c file).

Verified with GHC 6.6. not even -fforce-recomp works, only manually deleting the _stub.c files works around the problem.

Problem with 6.6 is that it won't generate _stub.c files for modules where previous versions did (for modules that _use_ other modules which have the FFI that causes _stub.c files), so after upgrading you may encounter this problem.

[jcab@…]

Repro for the issue with stub.c files

[jcab@…]

In order to repro, just install GHC in a Windows machine, unzip the attached file in some directory, and from that directory run the command:

ghc -fglasgow-exts -fallow-undecidable-instances -fallow-overlapping-instances -fno-monomorphism-restriction -threaded -lKernel32 -lUser32 -lGdi32 -lOle32 -lComctl32 -optc-D_WIN32_IE=0x700 -optc-D_WIN32_WINNT=0x600 Win32Test.hs -fforce-recomp

There's a stub.c (renamed as stubBugBug.c) in the Win32 directory. There's another stub.c file in the root directory (Win32Test_stub.c). Both (the one in Win32 directory needs to be renamed back to stub.c, of course) will cause compile errors if present. Deleting them (or renaming them) will work around the issue. Both came from GHC 6.4.2.

Let me know if you have any trouble reproing this.

[simonmar]

We should really fix, but it's not trivial, doesn't affect many people, and there's an easy workaround (just delete the file). So, _|_ for now.

[igloo]

Can't/shouldn't we just record whether or not there's a stub file in the .hi file?

[simonpj]

Simon and I looked briefly at this:

• The mg_foreign field of a ModGuts records the foreign-stub information
• It would be straightforward to add a Bool to ModIface to record whether there were stubs
• The only tricky bit is making use of that ModIface field to construct the right hm_linkable list of Linkables, recorded in HomeModInfo.

So this is probably not hard to fix; just a bit fiddly to poke around and see what needs to be done. Any volunteers?

Simon

[igloo]

See also #3687

[simonmar]

Fixed: changeset:7b0ff1792d699ff02a604163c9ccf4a98a1ca3eb