Rafael Martinez Torres wrote:
HI:
Reading Haskell 98 FFI report, one question arised:
Imported and declared a foreign function , via
foreign import ccall foo:: IO(CInt)
Is it guaranteed to be executed in atomic way ? I mean , should it block the whole STG system ?
No, it's not guaranteed to be executed atomically.
With current implementations, it does block the whole STG system, but only until a callback is made, and when multiple (forkIO'ed) Concurrent Haskell threads call foreign imported functions that might call back into Haskell, things become almost unpredictable.
GHC supports an optional feature (pass --enable-threaded-rts to configure when compiling GHC) that makes GHC guarantee that the STG system is never blocked by foreign calls. It will probably become the default in the future (after the remaining bugs are fixed).
Idea: To implement a monitor , in a "external context" , and threads around him to access it, bypassing the Mvar paramter-pass among them...
Well, similar ideas have been brought up before, and I'm a bit sceptical:
*) Why should we introduce a feature at the language level for solving a problem that is already solved by MVars? You can write your own combinators to reduce the hassle of using MVars to a minimum.
*) Why would we want all external calls to be serialized? In most circumstances, I'd like more granularity than that.
What exactly do you have in mind?
Cheers,
Wolfgang
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