[Haskell-cafe] Storable class?
vigalchin at gmail.com
Tue Apr 22 23:26:37 EDT 2008
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.
-- |Correspond to some of the int flags from C's fcntl.h.
data 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
append1 = False,
exclusive1 = False,
noctty1 = False,
nonBlock1 = False,
trunc1 = False
data 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
-> Maybe FileMode -- ^Just x => creates the file with the given
modes, Nothing => the file must exist.
-> 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 at gmail.com>
> 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
> 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 at gmail.com
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