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:
http://www.haskell.org/~simonmar/papers/conc-ffi.pdf
If you have foreign calls which might block or just take a long time, it
is good practice not to mark them unsafe.
Cheers,
Simon
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