[Haskell-cafe] Re: errno handling in concurrent haskell

[Simon Marlow]

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 
GetLastError().

> 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?

yes

> Why the throwErrno function does not accept errno as parameter?

because it reads the global errno.

Cheers,
	Simon