The Errno Story

[John Meacham]

I was thinking about this too but had a different idea, an alternate
calling form which grabbed the result of errno and returned it to
haskell land. that way the cost of geting errno is only paid for those
foreign imports which care about it. like

foreign import ccall_errno "unistd.h chdir" :: Ptr CChar -> IO (CInt,CInt)

where the second CInt returned will be the value of errno after the call
to chdir.

another solution is to do everything in C wrappers which return errno as
well as the return value.

another is some sort of blocking function which blocks context switching
within the same OS thread temporarily..

or your idea below. just thought I'd throw some alternate ideas into the
mix. I am not a fan of having to stow errno for every ffi call..
        John

On Thu, Jul 24, 2003 at 11:48:02PM +0200, Wolfgang Thaller wrote:
> First, the "bug" part:
> 
> None of the libraries that use errno are compatible with concurrency.
> When a (lightweight) thread-switch occurs between the call to some 
> foreign function and the corresponding call to 
> Foreign.C.Error.getErrno, we have a problem:
> 
> a) the RTS trashes the value of errno
> b) another Haskell thread might call a function that sets errno
> c) on most platforms, errno is thread-local state, with all the 
> consequences that has for the threaded RTS (bound threads required....)
> 
> Now, the "ffi" part:
> 
> b) and c) could be solved by using bound threads, but that's probably 
> too inefficient for something that's used as frequently as errno.
> Bound threads wouldn't solve a).
> 
> I therefore propose that we make the RTS save & restore the value of 
> errno in the TSO, thus making errno "Haskell-Thread-Local-State", and 
> solving all of the above problems.
> I also propose adding a note to the FFI spec (or the bound threads 
> document, I'm not sure which is the right place) that says that 
> implementations have to make sure that "Errno" is local to each 
> *Haskell* thread.
> 
> 
> Cheers,
> 
> Wolfgang
> 
> _______________________________________________
> FFI mailing list
> FFI at haskell.org
> http://www.haskell.org/mailman/listinfo/ffi

-- 
---------------------------------------------------------------------------
John Meacham - California Institute of Technology, Alum. - john at foo.net
---------------------------------------------------------------------------