[Haskell] Re: ANN: HDBC (Haskell Database Connectivity)
simonmar at microsoft.com
Thu Jan 5 05:56:50 EST 2006
John Goerzen wrote:
> I think you have that backwards, but I'm unsure. According to the FFI
> spec, section 3.3:
> "Optionally, an import declaration can specify, after the calling
> convention, the safety level that should be used when invoking an
> external entity. A safe call is less efficient, but guarantees to
> leave the Haskell system in a state that allows callbacks from the
> external code. In contrast, an unsafe call, while carrying less
> overhead, must not trigger a callback into the Haskell system."
> There is no reason for any of these calls to trigger a callback into
> Haskell, so they can all be imported unsafe for greater efficiency.
> But it doesn't directly address threads, so I don't know what to make of
> that. Do you have a reference?
In GHC, other threads cannot run while an unsafe foreign call is in
progress. Our documentation is slightly lacking here, but this paper
describes it all:
If you have foreign calls which might block or just take a long time, it
is good practice not to mark them unsafe.
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