diff --git a/snap-server.cabal b/snap-server.cabal
--- a/snap-server.cabal
+++ b/snap-server.cabal
@@ -1,5 +1,5 @@
 name:           snap-server
-version:        0.4.2
+version:        0.4.3
 synopsis:       A fast, iteratee-based, epoll-enabled web server for the Snap Framework
 description:
   Snap is a simple and fast web development framework and server written in
@@ -117,7 +117,7 @@
     murmur-hash >= 0.1 && < 0.2,
     network >= 2.3 && <2.4,
     old-locale,
-    snap-core >= 0.4.2 && <0.5,
+    snap-core >= 0.4.3 && <0.5,
     template-haskell,
     time,
     transformers,
diff --git a/src/Snap/Http/Server/Config.hs b/src/Snap/Http/Server/Config.hs
--- a/src/Snap/Http/Server/Config.hs
+++ b/src/Snap/Http/Server/Config.hs
@@ -31,6 +31,7 @@
 
   , setHostname
   , addListen
+  , setListen
   , setAccessLog
   , setErrorLog
   , setLocale
@@ -509,6 +510,11 @@
 ------------------------------------------------------------------------------
 addListen :: MonadSnap m => ConfigListen -> Config m a -> Config m a
 addListen a m = m {listen = a : listen m}
+
+
+------------------------------------------------------------------------------
+setListen :: MonadSnap m => [ConfigListen] -> Config m a -> Config m a
+setListen a m = m {listen = a}
 
 
 ------------------------------------------------------------------------------
diff --git a/src/Snap/Internal/Http/Server/LibevBackend.hs b/src/Snap/Internal/Http/Server/LibevBackend.hs
--- a/src/Snap/Internal/Http/Server/LibevBackend.hs
+++ b/src/Snap/Internal/Http/Server/LibevBackend.hs
@@ -662,7 +662,7 @@
     go off bytes fd
       | bytes == 0 = return ()
       | otherwise  = do
-            sent <- SF.sendFile sfd fd off bytes
+            sent <- SF.sendFile (waitForLock False c) sfd fd off bytes
             if sent < bytes
               then tickleTimeout c defaultTimeout >>
                    go (off+sent) (bytes-sent) fd
diff --git a/src/Snap/Internal/Http/Server/SimpleBackend.hs b/src/Snap/Internal/Http/Server/SimpleBackend.hs
--- a/src/Snap/Internal/Http/Server/SimpleBackend.hs
+++ b/src/Snap/Internal/Http/Server/SimpleBackend.hs
@@ -220,7 +220,8 @@
     go off bytes fd
       | bytes == 0 = return ()
       | otherwise  = do
-            sent <- SF.sendFile sfd fd off bytes
+            sent <- SF.sendFile (threadWaitWrite $ fromIntegral sock)
+                                sfd fd off bytes
             if sent < bytes
               then tickle >> go (off+sent) (bytes-sent) fd
               else return ()
diff --git a/src/System/SendFile/Darwin.hsc b/src/System/SendFile/Darwin.hsc
--- a/src/System/SendFile/Darwin.hsc
+++ b/src/System/SendFile/Darwin.hsc
@@ -10,23 +10,26 @@
 import Foreign.Storable (peek, poke)
 import System.Posix.Types (Fd, COff)
 
-sendFile :: Fd -> Fd -> Int64 -> Int64 -> IO Int64
-sendFile out_fd in_fd off count
+sendFile :: IO () -> Fd -> Fd -> Int64 -> Int64 -> IO Int64
+sendFile onBlock out_fd in_fd off count
   | count == 0 = return 0
   | otherwise  = alloca $ \pbytes -> do
         poke pbytes $ min maxBytes (fromIntegral count)
-        sbytes <- sendfile out_fd in_fd (fromIntegral off) pbytes
+        sbytes <- sendfile onBlock out_fd in_fd (fromIntegral off) pbytes
         return $ fromIntegral sbytes
 
-sendfile :: Fd -> Fd -> COff -> Ptr COff -> IO COff
-sendfile out_fd in_fd off pbytes = do
+sendfile :: IO () -> Fd -> Fd -> COff -> Ptr COff -> IO COff
+sendfile onBlock out_fd in_fd off pbytes = do
     status <- c_sendfile out_fd in_fd off pbytes
     nsent <- peek pbytes
     if status == 0
       then return nsent
       else do errno <- getErrno
               if (errno == eAGAIN) || (errno == eINTR)
-                then return nsent
+                then do
+                    if nsent == 0
+                      then onBlock >> sendfile onBlock out_fd in_fd off pbytes
+                      else return nsent
                 else throwErrno "System.SendFile.Darwin"
 
 -- max num of bytes in one send
@@ -38,4 +41,5 @@
     :: Fd -> Fd -> COff -> Ptr COff -> Ptr () -> CInt -> IO CInt
 
 c_sendfile :: Fd -> Fd -> COff -> Ptr COff -> IO CInt
-c_sendfile out_fd in_fd off pbytes = c_sendfile_darwin in_fd out_fd off pbytes nullPtr 0
+c_sendfile out_fd in_fd off pbytes =
+    c_sendfile_darwin in_fd out_fd off pbytes nullPtr 0
diff --git a/src/System/SendFile/FreeBSD.hsc b/src/System/SendFile/FreeBSD.hsc
--- a/src/System/SendFile/FreeBSD.hsc
+++ b/src/System/SendFile/FreeBSD.hsc
@@ -11,25 +11,31 @@
 import Foreign.Storable (peek)
 import System.Posix.Types (COff, Fd)
 
-sendFile :: Fd -> Fd -> Int64 -> Int64 -> IO Int64
-sendFile out_fd in_fd off count
+sendFile :: IO () -> Fd -> Fd -> Int64 -> Int64 -> IO Int64
+sendFile onBlock out_fd in_fd off count
   | count == 0 = return 0
   | otherwise  = alloca $ \pbytes -> do
-        sbytes <- sendfile out_fd in_fd (fromIntegral off)
-                                        (fromIntegral count) pbytes
+        sbytes <- sendfile onBlock out_fd in_fd
+                           (fromIntegral off)
+                           (fromIntegral count)
+                           pbytes
         return $ fromIntegral sbytes
 
-sendfile :: Fd -> Fd -> COff -> CSize -> Ptr COff -> IO COff
-sendfile out_fd in_fd off count pbytes =
-    do threadWaitWrite out_fd
-       res <- c_sendfile_freebsd in_fd out_fd off count nullPtr pbytes 0
-       nsent <- peek pbytes
-       if (res == 0)
-          then return nsent
-          else do errno <- getErrno
-                  if (errno == eAGAIN) || (errno == eINTR)
-                   then return nsent
-                   else throwErrno "System.SendFile.FreeBSD.sendfile"
+sendfile :: IO () -> Fd -> Fd -> COff -> CSize -> Ptr COff -> IO COff
+sendfile onBlock out_fd in_fd off count pbytes = do
+    res <- c_sendfile_freebsd in_fd out_fd off count nullPtr pbytes 0
+    nsent <- peek pbytes
+    if (res == 0)
+       then return nsent
+       else do
+           errno <- getErrno
+           if (errno == eAGAIN) || (errno == eINTR)
+             then if nsent == 0
+                    then do
+                        onBlock
+                        sendfile onBlock out_fd in_fd off count pbytes
+                    else return nsent
+             else throwErrno "System.SendFile.FreeBSD.sendfile"
 
 -- max num of bytes in one send
 maxBytes :: CSize
diff --git a/src/System/SendFile/Linux.hsc b/src/System/SendFile/Linux.hsc
--- a/src/System/SendFile/Linux.hsc
+++ b/src/System/SendFile/Linux.hsc
@@ -10,26 +10,26 @@
 import Foreign.Storable (poke)
 import System.Posix.Types (Fd, COff, CSsize)
 
-sendFile :: Fd -> Fd -> Int64 -> Int64 -> IO Int64
-sendFile out_fd in_fd off count
+sendFile :: IO () -> Fd -> Fd -> Int64 -> Int64 -> IO Int64
+sendFile onBlock out_fd in_fd off count
   | count == 0 = return 0
   | off == 0   = do
-        sbytes <- sendfile out_fd in_fd nullPtr bytes
+        sbytes <- sendfile onBlock out_fd in_fd nullPtr bytes
         return $ fromIntegral sbytes
   | otherwise  = alloca $ \poff -> do
         poke poff (fromIntegral off)
-        sbytes <- sendfile out_fd in_fd poff bytes
+        sbytes <- sendfile onBlock out_fd in_fd poff bytes
         return $ fromIntegral sbytes
     where
       bytes = min (fromIntegral count) maxBytes
 
-sendfile :: Fd -> Fd -> Ptr COff -> CSize -> IO CSsize
-sendfile out_fd in_fd poff bytes = do
+sendfile :: IO () -> Fd -> Fd -> Ptr COff -> CSize -> IO CSsize
+sendfile onBlock out_fd in_fd poff bytes = do
     nsent <- c_sendfile out_fd in_fd poff bytes
     if nsent <= -1
       then do errno <- getErrno
               if errno == eAGAIN
-                then sendfile out_fd in_fd poff bytes
+                then onBlock >> sendfile onBlock out_fd in_fd poff bytes
                 else throwErrno "System.SendFile.Linux"
       else return nsent
 
diff --git a/test/snap-server-testsuite.cabal b/test/snap-server-testsuite.cabal
--- a/test/snap-server-testsuite.cabal
+++ b/test/snap-server-testsuite.cabal
@@ -46,7 +46,7 @@
      old-locale,
      parallel > 2,
      process,
-     snap-core >= 0.4.1 && <0.5,
+     snap-core >= 0.4.3 && <0.5,
      template-haskell,
      test-framework >= 0.3.1 && <0.4,
      test-framework-hunit >= 0.2.5 && < 0.3,