[Haskell-cafe] accessing a ByteArray from FFI

[Johan Tibell]

On Wed, Jun 25, 2014 at 2:54 PM, Mihaly Barasz <klao at nilcons.com> wrote:

> Well, I don't know enough about how GC is specified in GHC, but _in
>  practice_ calls to GC could happen only on entry to the exampleFn
> closure.  Not between the address computation and the foreign call. (I
> simply looked at the generated code, I don't know if there is any
> guarantee for that.)
>

There can be additional heap checks at the start of any basic block in the
generated assembly for the function. Right, in practice there's probably
not an issue.


> Thanks, I'll look into that. Are there any pointers/examples?
>

There's some code out there on the web that uses the extension. Here's an
example:
https://github.com/tibbe/hashable/blob/master/Data/Hashable/Class.hs#L470


> I don't know why, but the constructor for Vector is not exported, only
> for the MVector. But, this use of unsafeThaw is completely benign.
> (Actually, it fully disappears in the generated code. :))
>

Probably because it's a ByteArray#, not an Array#. For the latter unsafe
thawing results in the object being put on the GC mutable list (in practice
that means that the info table ptr changes).
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