Hello, Below is a snippet of my code .... please look for the "poke" call with "<<<<<<<<<<<<<". This causes a "hang" when I run a test case. If I comment out the "poke" call then test case runs. Vasili -- |Correspond to some of the int flags from C's fcntl.h. data MQFlags = MQFlags { append1 :: Bool, -- ^ O_APPEND exclusive1 :: Bool, -- ^ O_EXCL noctty1 :: Bool, -- ^ O_NOCTTY nonBlock1 :: Bool, -- ^ O_NONBLOCK trunc1 :: Bool -- ^ O_TRUNC } -- |Default values for the 'MQFlags' type. False for each of -- append, exclusive, noctty, nonBlock, and trunc. -- TBD WNH MQdefaultFileFlags :: MQFlags defaultMQFlags = MQFlags { append1 = False, exclusive1 = False, noctty1 = False, nonBlock1 = False, trunc1 = False } data MQAttributes = MQAttributes { flags :: MQFlags, maxMsgNum :: Int, maxMsgSize :: Int, curNumMsgs :: Int } instance Storable MQAttributes where sizeOf (MQAttributes flags maxMsgNum maxMsgSize curNumMsgs) = 4 instance Storable MQFlags where sizeOf (MQFlags append1 exclusive1 noctty1 nonBlock1 trunc1) = 1 -- |Open and optionally create this message queue. See 'System.Posix.Files' -- for information on how to use the 'FileMode' type. mqOpen :: String -> MQOpenMode -> Maybe FileMode -- ^Just x => creates the file with the given modes, Nothing => the file must exist. -> MQFlags -> MQAttributes -> IO Fd mqOpen name how maybe_mode (MQFlags append1 exclusive1 noctty1 nonBlock1 truncate1) (MQAttributes flags maxMsgNum maxMsgSize curNumMsgs) = do withCString name $ \ p_name -> do putStrLn ("name -> " ++ name) allocaBytes (#const sizeof(struct mq_attr)) $ \ p_attrs -> do (#poke struct mq_attr, mq_flags) p_attrs flags <<<<<<<<<<<<<<<<<<< On Tue, Apr 22, 2008 at 5:05 PM, Bulat Ziganshin <bulat.ziganshin@gmail.com> wrote:
Hello Evan,
Wednesday, April 23, 2008, 1:48:30 AM, you wrote:
The FFI doc doesn't really talk about the alignment method at all, so I don't really understand how to write one or how it's used.
write: easy. just specify how much data shoulkd be aligned. for primitive datatypes this usually equals to datasize, for complex structures this should be the same as maximum alignment of elements involved:
instance Storable Float alignment = 4
instance Storable (a,b) alignment = maximum [alignment a, alignment b]
use: just align memory blocks allocated to store this datatype. usual alignment technique is:
alloc a = (allocBytes (sizeOf a + alignment a - 1) + (alignment a - 1)) .&. (alignment a - 1)
well, many standard allocators such as C malloc, actually provide you blocks with a maximum alignment required to store any (primitive) type, so you don't need to worry about it
-- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com