test.hs

import Text.Printf
import Control.Concurrent
import Control.Monad
import Control.Exception

foreign import ccall unsafe "test_c.h read_errno" get_errno :: IO Int
foreign import ccall unsafe "test_c.h write_errno" set_errno :: Int -> IO ()

main :: IO ()
main = do
  forM_ [1..10000] $ \i -> do
    forkIO $ forever $ mask_ $ do
      set_errno i
      r <- get_errno
      when (i /= r) $ putStrLn $ printf "Awaited %d, got %d" i r
  threadDelay maxBound

test_c.c

#include "errno.h"
#include "test_c.h"

#define ERRNO my_errno

__thread int my_errno;

int read_errno(void)
{
  return ERRNO;
}

void write_errno(const int i)
{
  ERRNO = i;
}

test_c.h

#ifndef TEST_H
#define TEST_H

int read_errno(void);
void write_errno(const int);

#endif // TEST_H