[Haskell-cafe] ccall vs capi

Brandon Allbery allbery.b at gmail.com
Sun Jun 13 21:47:10 UTC 2021

capi uses the C compiler to make the call, requiring ghc to write out and
compile a C stub file for each import. This guarantees C compatibility and
also works with cpp-defined symbols.

ccall attempts to emulate what the C compiler does with generated assembly
language, but may get it wrong on Darwin; I recall Moritz Angermann saying
someone would have to dump out the generated assembly from both cases to
see what's different and what needs to change for ccall to work on Darwin.

Possible differences include not zeroing the upper parts of registers with
32-bit values in them and not treating other registers properly (for
example, getting caller-saved registers wrong). It's also conceivable that
there's a cpp macro involved normally and there's a hidden bug in a
fallback libSystem version of a function. Someone will have to examine the
generated assembly as above to see what's actually wrong.

On Sun, Jun 13, 2021 at 5:21 PM Julian Ospald <hasufell at hasufell.de> wrote:

> Hi,
> recently during a ghcup release, we stumbled over yet another `ccall`
> bug on Darwin: https://github.com/biegunka/terminal-size/pull/16
> This took Ben some time to debug (thanks again) and it isn't always
> clear with such
> bugs whether something is wrong with GHC/RTS/GC or a library, so this
> can be a time-sink.
> It's also not the first time it happened:
> * https://github.com/haskell/unix/pull/110#issuecomment-613384845
> *
> https://github.com/hasufell/hpath/commit/74d686547e67da3eeae327c84c63f83833205890
> I can't say that I fully understood what the exact difference here is
> and why it seems to matter much more on Darwin than on other systems. So
> I think it would be helpful if we could come up with:
> 1. a clear description of the differences of ccall vs capi
> 2. an explanation why ccall can lead to terrible bugs on darwin
> 3. a guideline of when using one over the other is appropriate
> Not everyone deals with ffi on a regular basis, so I hope the experts
> here can shed some light!
> Cheers,
> Julian
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brandon s allbery kf8nh
allbery.b at gmail.com
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