diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Revision history for sport
 
+## 3.1.0.0 -- 7-25-2025
+
+* Fix exclusive flag, was not being set
+
 ## 3.0.0.0 -- 6-7-2026
 
 * Update `withSport`, add `withNewSport`
diff --git a/lib/Sport/Serial.hs b/lib/Sport/Serial.hs
deleted file mode 100644
--- a/lib/Sport/Serial.hs
+++ /dev/null
@@ -1,103 +0,0 @@
--- | Low-level serial IO
-module Sport.Serial
-  ( withSerial
-  , openSerial
-  , SerialCfg(..)
-  , Parity(..)
-  , StopBits(..)
-  , defSerialCfg
-  ) where
-
-import Control.Exception
-import System.IO
-import System.Posix
-
--- | Configuration
-data SerialCfg = SerialCfg
-  { path :: FilePath
-  , speed :: BaudRate
-  , byteSize :: Int -- ^ number of bits per byte
-  , parity :: Maybe Parity
-  , stopBits :: StopBits
-  , rtimeout :: Maybe Int
-  , excl :: Bool -- ^ exclusive
-  } deriving (Eq, Show)
-
--- | Default configuration
-defSerialCfg :: SerialCfg
-defSerialCfg = SerialCfg
-  { path = "/dev/ttyUSB0"
-  , speed = B115200
-  , byteSize = 8
-  , parity = Nothing
-  , stopBits = One
-  , rtimeout = Nothing
-  , excl = True
-  }
-
-data Parity = Even | Odd
-  deriving (Eq, Read, Show)
-
-data StopBits = One | Two
-  deriving (Eq, Read, Show)
-
--- | Acquire bracketed serial handle
-withSerial :: SerialCfg -> (Handle -> IO a) -> IO a
-withSerial cfg = bracket (openSerial cfg) hClose
-
--- | Open serial sport handle
-openSerial :: SerialCfg -> IO Handle
-openSerial cfg =
-  bracketOnError (openFd (path cfg) ReadWrite flags) closeFd $ \fd -> do
-    configAttrs fd cfg
-    fdToHandle fd
-  where
-    flags =
-      defaultFileFlags
-        { noctty    = True
-        , nonBlock  = True
-        , exclusive = excl cfg
-        }
-
--- | configure FD terminal attributes
-configAttrs :: Fd -> SerialCfg -> IO ()
-configAttrs fd cfg = do
-  attrs <- getTerminalAttributes fd
-  setTerminalAttributes fd (config attrs) Immediately
-  where
-    config attrs = attrs
-      `withInputSpeed` speed cfg
-      `withOutputSpeed` speed cfg
-      `withBits` byteSize cfg
-      `withParity` parity cfg
-      `withStopBits` stopBits cfg
-      `withoutMode` StartStopInput
-      `withoutMode` StartStopOutput
-      `withoutMode` EnableEcho
-      `withoutMode` EchoErase
-      `withoutMode` EchoKill
-      `withoutMode` ProcessInput
-      `withoutMode` ProcessOutput
-      `withoutMode` MapCRtoLF
-      `withoutMode` EchoLF
-      `withoutMode` HangupOnClose
-      `withoutMode` KeyboardInterrupts
-      `withoutMode` ExtendedFunctions
-      `withMode` LocalMode
-      `withMode` ReadEnable
-      `withReadTimeout` rtimeout cfg
-      `withMinInput` 0
-
-withParity :: TerminalAttributes -> Maybe Parity -> TerminalAttributes
-withParity attrs Nothing  = attrs `withoutMode` EnableParity
-withParity attrs (Just p) = case p of
-  Even -> attrs `withoutMode` OddParity
-  Odd  -> attrs `withMode`    OddParity
-
-withStopBits :: TerminalAttributes -> StopBits -> TerminalAttributes
-withStopBits attrs One = attrs `withoutMode` TwoStopBits
-withStopBits attrs Two = attrs `withMode`    TwoStopBits
-
-withReadTimeout :: TerminalAttributes -> Maybe Int -> TerminalAttributes
-withReadTimeout attrs Nothing  = attrs
-withReadTimeout attrs (Just t) = attrs `withTime` t
diff --git a/lib/Sport/Serial.hsc b/lib/Sport/Serial.hsc
new file mode 100644
--- /dev/null
+++ b/lib/Sport/Serial.hsc
@@ -0,0 +1,121 @@
+{-# LANGUAGE ForeignFunctionInterface #-}
+
+-- | Low-level serial IO
+module Sport.Serial
+  ( withSerial
+  , openSerial
+  , SerialCfg(..)
+  , Parity(..)
+  , StopBits(..)
+  , defSerialCfg
+  ) where
+
+import Control.Exception
+import Control.Monad
+import Foreign
+import Foreign.C
+import System.IO
+import System.Posix
+
+-- | Configuration
+data SerialCfg = SerialCfg
+  { path :: FilePath
+  , speed :: BaudRate
+  , byteSize :: Int -- ^ number of bits per byte
+  , parity :: Maybe Parity
+  , stopBits :: StopBits
+  , rtimeout :: Maybe Int
+  , excl :: Bool -- ^ exclusive
+  } deriving (Eq, Show)
+
+-- | Default configuration
+defSerialCfg :: SerialCfg
+defSerialCfg = SerialCfg
+  { path = "/dev/ttyUSB0"
+  , speed = B115200
+  , byteSize = 8
+  , parity = Nothing
+  , stopBits = One
+  , rtimeout = Nothing
+  , excl = True
+  }
+
+data Parity = Even | Odd
+  deriving (Eq, Read, Show)
+
+data StopBits = One | Two
+  deriving (Eq, Read, Show)
+
+-- | Acquire bracketed serial handle
+withSerial :: SerialCfg -> (Handle -> IO a) -> IO a
+withSerial cfg = bracket (openSerial cfg) hClose
+
+-- | Open serial sport handle
+openSerial :: SerialCfg -> IO Handle
+openSerial cfg =
+  bracketOnError (openFd (path cfg) ReadWrite flags) closeFd $ \fd -> do
+    when (excl cfg) $ setExcl fd
+    configAttrs fd cfg
+    fdToHandle fd
+  where
+    flags =
+      defaultFileFlags
+        { noctty    = True
+        , nonBlock  = True
+        , exclusive = excl cfg
+        }
+
+-- | configure FD terminal attributes
+configAttrs :: Fd -> SerialCfg -> IO ()
+configAttrs fd cfg = do
+  attrs <- getTerminalAttributes fd
+  setTerminalAttributes fd (config attrs) Immediately
+  where
+    config attrs = attrs
+      `withInputSpeed` speed cfg
+      `withOutputSpeed` speed cfg
+      `withBits` byteSize cfg
+      `withParity` parity cfg
+      `withStopBits` stopBits cfg
+      `withoutMode` StartStopInput
+      `withoutMode` StartStopOutput
+      `withoutMode` EnableEcho
+      `withoutMode` EchoErase
+      `withoutMode` EchoKill
+      `withoutMode` ProcessInput
+      `withoutMode` ProcessOutput
+      `withoutMode` MapCRtoLF
+      `withoutMode` EchoLF
+      `withoutMode` HangupOnClose
+      `withoutMode` KeyboardInterrupts
+      `withoutMode` ExtendedFunctions
+      `withMode` LocalMode
+      `withMode` ReadEnable
+      `withReadTimeout` rtimeout cfg
+      `withMinInput` 0
+
+withParity :: TerminalAttributes -> Maybe Parity -> TerminalAttributes
+withParity attrs Nothing  = attrs `withoutMode` EnableParity
+withParity attrs (Just p) = case p of
+  Even -> attrs `withoutMode` OddParity
+  Odd  -> attrs `withMode`    OddParity
+
+withStopBits :: TerminalAttributes -> StopBits -> TerminalAttributes
+withStopBits attrs One = attrs `withoutMode` TwoStopBits
+withStopBits attrs Two = attrs `withMode`    TwoStopBits
+
+withReadTimeout :: TerminalAttributes -> Maybe Int -> TerminalAttributes
+withReadTimeout attrs Nothing  = attrs
+withReadTimeout attrs (Just t) = attrs `withTime` t
+
+setExcl :: Fd -> IO ()
+setExcl fd = ioctl fd #{const TIOCEXCL} nullPtr
+
+ioctl :: Fd -> Int -> Ptr d -> IO ()
+ioctl f req =
+  throwErrnoIfMinus1_ "ioctl"
+    . c_ioctl (fromIntegral f) (fromIntegral req)
+    . castPtr
+
+#include <sys/ioctl.h>
+foreign import ccall "ioctl" c_ioctl :: CInt -> CInt -> Ptr () -> IO  CInt
diff --git a/sport.cabal b/sport.cabal
--- a/sport.cabal
+++ b/sport.cabal
@@ -1,6 +1,6 @@
 cabal-version:      3.0
 name:               sport
-version:            3.0.0.0
+version:            3.1.0.0
 synopsis:           UNIX serial port
 description:        Concurrent serial IO
 license:            MIT