[Haskell-cafe] errno handling in concurrent haskell

[Manlio Perillo]

Hi.

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?


P.S.:
I have found this is C.Foreign.Error.hs, in base package (not the latest 
version):

throwErrnoIfRetry            :: (a -> Bool) -> String -> IO a -> IO a
throwErrnoIfRetry pred loc f  =
   do
     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?



Thanks  Manlio Perillo