For recent versions of GHC at least, the runtime saves errno
in the Thread Storage Object (TSO
) and takes care of managing it when it migrates a runtime thread to another OS thread. That means it should be safe to rely on errno
in both unbound (forkIO
) and bound (forkOS
) threads. Here's a test for Linux:
err.hs
import Control.Concurrent
import Control.Exception
import Foreign.C
foreign import ccall safe get_ostid :: IO CUInt
foreign import ccall safe sleep :: CUInt -> IO CUInt
foreign import ccall unsafe get_errno :: IO CInt
foreign import ccall unsafe set_errno :: CInt -> IO ()
forkIO' f = do
done <- newEmptyMVar
forkIO (f `finally` putMVar done ())
return (takeMVar done)
prefix = do
id <- get_ostid
return $ show id ++ ": "
main = do
wait <- forkIO' $ do -- spawn a lightweight thread
errno <- get_errno
(putStr =<< prefix) >> putStrLn ("outer errno is " ++ show errno)
(putStr =<< prefix) >> putStrLn "Setting outer errno to 3"
set_errno 3
wait' <- forkIO' $ do -- spawn another lightweight thread
errno <- get_errno
(putStr =<< prefix) >> putStrLn ("inner errno is " ++ show errno)
(putStr =<< prefix) >> putStrLn "Setting inner errno to 2"
set_errno 2
sleep 2 -- force this lightweight thread to tie up the OS thread
errno <- get_errno
(putStr =<< prefix) >> putStrLn ("inner errno is " ++ show errno)
threadDelay 1000000 -- wait a second
-- By now, we should be in another OS thread.
errno <- get_errno
(putStr =<< prefix) >> putStrLn ("outer errno is " ++ show errno)
wait'
wait
err.c
#include <errno.h>
#include <sys/syscall.h>
#include <unistd.h>
int get_errno(void) { return errno; }
void set_errno(int e) { errno = e; }
unsigned get_ostid(void) {
return syscall(SYS_gettid);
}
Compile with:
ghc -o err -threaded err.hs err.c
And the result should look something like:
12282: outer errno is 0
12282: Setting outer errno to 3
12282: inner errno is 0
12282: Setting inner errno to 2
12283: outer errno is 3
12282: inner errno is 2
The OS thread IDs are printed at the beginning of each line. Note that the errno
of 3
was migrated to the second OS thread (12283
).