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
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