[Haskell-cafe] Locking of threads in one OS thread

Carter Schonwald carter.schonwald at gmail.com
Wed Aug 20 01:02:33 UTC 2014

several ideas
a) you can use forkOS to run the ffi call and the the subsequent errno
value checking in a pinned OS thread so you can guarantee you can access
the thread local storage
b) you could have a CallManager that keeps a MVar or queue or something,
along with a handle on the forkOS thread id, and "intiialize" the forkOS
thread with a teeny worker that read from an input MVar and writes to a
result MVar (thus guarantee only one call a time)
c) something that uses these ideas together.

theres a lot of strategies you can use. but i hope this gives you a sketch
(i can spell it out more later this week if need be)

On Tue, Aug 19, 2014 at 5:36 PM, Nikolay Amiantov <nikoamia at gmail.com>

> Hello Cafe,
> I'm using FFI to interact with a library which calls, when fail, leave
> the reason in some kind of "errno"-like variable which is retrived via
> another call. AFAIU, this is not thread-safe in Haskell even if
> thread-local storage is used inside the library, because Haskell uses
> its own thread management and the Haskell thread in the same OS thread
> might be switched between the actual call and the retrival of errno
> value. This should be somehow handled already in Haskell (errno is
> widely used with syscalls in Linux, for example), but the source of
> Foreign.C.Error suggests that this is not handled in any way at all.
> For example, throwErrnoIf is implemented as such:
> throwErrno loc  = do
>     errno <- getErrno
>     ioError (errnoToIOError loc errno Nothing Nothing)
> throwErrnoIf pred loc f  = do
>     res <- f
>     if pred res then throwErrno loc else return res
> So, the question is: how is it ensured that this block is "atomic" in
> sense that at most one Haskell thread computes this whole function at
> every moment in every OS thread?
> Thanks for any explanations! (And sorry if my English is poor)
> Nikolay.
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