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ocaml / flexdll

a dlopen-like API for Windows
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Stop passing --image-base on Cygwin64 #89

Closed dra27 closed 3 years ago

dra27 commented 3 years ago

On Cygwin64, --image-base 0x10000 is always passed to the linker. This generates invalid DLLs for Cygwin64. This has never worked reliably, as DLLs need unique base addresses on Cygwin to survive fork. However, since version 4.4.4 Cygwin's rebase forces DLLs to the correct base address in 0x2:00000000-0x4:00000000. Since Cygwin64 executables load in 0x1:00000000-0x1:80000000, DLLs are always too far RELOC_REL32.

Rather than dynamically generating branch islands, the new approach on the OCaml-side is to return to using __declspec(dllimport) only, which will cause the linker to do that work for us, as it should.

Regardless of the solution, --image-base should cease being passed to the linker on Cygwin64.

dra27 commented 3 years ago

@alainfrisch - what do you think about this one? It will completely break shared library compilation of OCaml < 4.12 for Cygwin64, but equally it is broken at the moment, just more subtly (fork doesn't work). Equally, we should not be compiling DLLs for Cygwin64 at all like this. Cygwin's rebaseall can "corrupt" them at any moment by changing their base address - I think it only works at the moment because Cygwin Setup's rebase operation looks only at new DLLs.

An alternative would be to add a new flag which disables this automatic passing of --image-base, but it feels odd to have an explicit flag in order to select correct behaviour over incorrect behaviour?!

alainfrisch commented 3 years ago

Yes, I agree with your plan. Breaking "more" OCaml < 4.12 for Cygwin64 is ok.

dra27 commented 3 years ago

Ta - I'll cut a release in the next couple of days and ping the Cygwin maintainer (although I think that may involve offering to become the Cygwin maintainer......)

xavierleroy commented 3 years ago

With my CompCert hat on, I would like to use OCaml under Cygwin64. How could we make progress with the new release of FlexDLL and the updated Cygwin64 package?

dra27 commented 3 years ago

I'm just testing 0.39 on my fork - assuming that's fine, I'll do the release later today.

dra27 commented 3 years ago

It's released, and I've proposed adopting the Cygwin package to push the release downstream as well.