[Haskell-cafe] Re: errno handling in concurrent haskell
marlowsd at gmail.com
Tue Jan 13 07:49:32 EST 2009
Manlio Perillo wrote:
> I have some doubts about errno handling in a Concurrent Haskell program.
> Let's suppose that GHC non threaded runtime is used, so that each
> Haskell thread is bound to an OS thread.
> Let's suppose there are two threads running (`A` and `B`).
> Thread `A` calls a function `f`, that, in turn, calls via FFI a C
> function `c_f`.
> Function `c_f` fails, settings errno; however the GHC scheduler suspends
> execution of thread `A` and switch to thread `B`, before the current
> value of errno is read.
> Now, let's suppose thread `B` calls a function `g`, that, in turn, calls
> via FFI a C function `c_g`.
> Function `c_g`, too, fails, setting errno.
> Is this possible?
It's safe, we save the value of errno when a Haskell thread is descheduled,
and restore it when it is scheduled again. We do the same on Windows for
> I have found this is C.Foreign.Error.hs, in base package (not the latest
> throwErrnoIfRetry :: (a -> Bool) -> String -> IO a -> IO a
> throwErrnoIfRetry pred loc f =
> res <- f
> if pred res
> then do
> err <- getErrno
> if err == eINTR
> then throwErrnoIfRetry pred loc f
> else throwErrno loc
> else return res
> This function calls getErrno two times.
> Is this safe?
> Why the throwErrno function does not accept errno as parameter?
because it reads the global errno.
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