Ok, I used this advice: http://www.haskell.org/haskellwiki/GHC/Using_the_FFI#Importing_C_functions_that_turn_out_to_be_CPP_macros to make 'xmlFree' callable from Haskell. I was completly wrong in my first post... I do apologize.
On Sun, Nov 29, 2009 at 8:56 PM, El Barto <elbart...@gmail.com> wrote: > Thanks for your help & time, > > I checked the C API documentation: > http://xmlsoft.org/html/libxml-xmlreader.html#xmlTextReaderValue > "The result must be deallocated with xmlFree()" > > I pushed the sources here: > http://github.com/gwenn/libxml-reader > > My problem is with the function at line 249 in Text.XML.LibXML.XmlReader > To make it work, I have to comment out the line where xmlFree is called. > > -- xmlChar * xmlTextReaderValue(xmlTextReaderPtr reader) > -- Returns:the string or NULL if not available. The result must be > deallocated with xmlFree() > foreign import ccall unsafe "xmlreader.h xmlTextReaderValue" > c_xmlTextReaderValue :: Ptr XmlTextReader -> IO (CString) > value :: (MonadIO m) => XmlReader -> m (Maybe B.ByteString) > value (XmlReader reader_fp) = liftIO $ > withForeignPtr reader_fp $ \reader_ptr -> do > cstr <- c_xmlTextReaderValue reader_ptr > fromPtr B.packCString cstr c_xmlFree > > fromPtr :: (Ptr a -> IO b) -> Ptr a -> (Ptr a -> IO ()) -> IO (Maybe b) > fromPtr c2h ptr free | ptr == nullPtr = return Nothing > | otherwise = do > r <- c2h ptr > free ptr -- FIXME > return (Just r) > > May be the binding to xmlFree is wrong? > foreign import ccall unsafe "xmlreader.h xmlFree" > c_xmlFree :: Ptr a -> IO () > > To test, you will need libxml2-dev: > $ runhaskell Setup configure --extra-include-dirs=/usr/include/libxml2/ > $ runhaskell Setup build > $ ./dist/build/Test/Test > > C functions are described here: > http://xmlsoft.org/html/libxml-xmlreader.html > And here: > http://xmlsoft.org/html/libxml-globals.html > > Regards. > > > On Sun, Nov 29, 2009 at 7:02 PM, Duncan Coutts < > duncan.cou...@googlemail.com> wrote: > >> On Sun, 2009-11-29 at 18:00 +0100, El Barto wrote: >> > Hello, >> > I get a segfault when I do >> > str <- peekCString ptr >> > free ptr >> > return (Just str) >> >> As Thomas says, you've not really given us enough info here. >> >> I'll make some guesses and observations anyway :-) >> >> > But not when I don't free the C pointer. >> >> Are you sure the C code does not also free the memory? That would lead >> to a double-free which can easily manifest as a segfault. >> >> > I also get the same behaviour with ByteString.packCString... >> >> That's pretty odd since packCString does not free the C string, it makes >> a copy. I suppose it could possibly segfault if your C string was not >> actually null terminated (as C strings must be). >> >> There are also variants that do not copy, such as >> unsafePackMallocCString. See the docs for a description. >> >> > Could you please tell me if the memory is correctly freed by GHC when >> > I don't do it myself? >> > And how can I specify a custom free function (i.e. xmlFree function in >> > libxml2)? >> >> See the documentation for ForeignPtr. >> >> > Maybe I should use a data type with two fields : the String/ByteString >> > and the ForeignPtr to the CString? >> >> Is your C String supposed to be freed using the C free() function, or is >> it supposed to be freed using xmlFree() or something? >> >> Duncan >> >> >
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