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Today's Topics:
1. Re: FFI Problems (Sylvain Henry)
2. Re: FFI Problems (Sylvain Henry)
----------------------------------------------------------------------
Message: 1
Date: Mon, 2 Mar 2015 16:20:00 +0100
From: Sylvain Henry <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] FFI Problems
Message-ID:
<CAPmptcWKW0-nyttQnu_-n-WsWdmndE_Z1RcBKwn=nvhcwav...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
Hi,
1) You have a copy-paste error on the following line (s/bitrate/track)
track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
2) By looking at the values you get in hexadecimal, you are reading 64bits
values when your C struct contains 32 bits values. If you use: peek ... ::
IO Int32, it should be ok.
3) From a style point of view, you could use Applicative operators (from
Control.Applicative) to avoid temporary names:
peek a = MusicMetaData
<$> liftM constantToCodec $ (((#peek struct music_metadata, codec)
a) :: IO Int32)
<*> fmap fromIntegral (((#peek struct music_metadata, length) a) ::
IO Int32)
<*> fmap fromIntegral (((#peek struct music_metadata, bitrate) a)
:: IO Int32)
<*> fmap fromIntegral (((#peek struct music_metadata, channels) a)
:: IO Int32)
<*> fmap fromIntegral (((#peek struct music_metadata, track) a) ::
IO Int32)
<*> (peekCString =<< (#peek struct music_metadata, title) a)
...
4) In your constantToCodec function, you don't need the temporary names for
all the constants.
Best regards,
Sylvain
PS : recently I have been working on rewriting the FFI page on the wiki to
make it more beginner-friendly ;-). It is not totally finished but I'm open
to any comment. https://www.haskell.org/wiki/Foreign_Function_Interface
2015-02-28 14:20 GMT+01:00 Thomas Jakway <[email protected]>:
> I'm very new to Haskell and am trying to write a "real" program to
> motivate myself to learn it better (so far I've only gotten through Project
> Euler problems after reading LYAH and most of RWH). I'm using Taglib (
> https://github.com/taglib/taglib) to read the metadata from a music file
> and print it. I have a struct C-side (with C linkage) serving as the
> bridge between Taglib's C++ and Haskell's FFI. A small demo program
> (compiled with gcc and linked against the C++ object files) gives the
> correct results, but Haskell is weirdly only getting *some *of it right.
> Specifically, the C string fields are working but ints are not.
>
> The output from the C demo (what Haskell should be printing):
>
> music_metadata
> title: It's My Life, artist: Bon Jovi, album: Bon Jovi Greatest Hits
> - The Ultimate Collection
> comment: , genre: Rock, track: 3,
> length: 224, bitrate: 256, channels: 2,
> codec: 768
>
> The output from Haskell:
>
> MusicMetadata {codec = UNKNOWN, length = 1099511628000, bitrate =
> 8589934848, channels = 12884901890, track = 8589934848, title = "It's My
> Life", artist = "Bon Jovi", album = "Bon Jovi Greatest Hits - The Ultimate
> Collection", comment = "", genre = "Rock"}
>
> I would have expected it to work or not work at all, but did not
> anticipate getting only some of it right.
>
> I was going to include snippets from my hsc file but given how new I am to
> Haskell I don't trust myself to assume where the problem is, so sorry if
> this is way too long:
>
> {-# LANGUAGE CPP, ForeignFunctionInterface #-}
>
> module MusicReader
> ( Codec,
> MusicMetadata,
> readMusicMetadata
> ) where
>
> import Control.Monad
> import Foreign
> import Foreign.C.Types
> import Foreign.C.String
> import System.IO.Unsafe as Unsafe
>
> #include "CodecDefines.h"
> #include "MusicReader.h"
>
> constantToCodec code
> | code == mp3 = MP3
> | code == flac = FLAC
> | code == ogg = OGG_VORBIS
> | code == mp4 = MP4
> | code == mpeg = MPEG
> | code == none = NONE
> | code == unknown = UNKNOWN
> | otherwise = UNKNOWN
> where mp3 = #const MP3_CODEC
> flac = #const FLAC_CODEC
> ogg = #const OGG_VORBIS_CODEC
> mp4 = #const MP4_CODEC
> mpeg = #const MPEG_CODEC
> none = #const NO_EXTENSION
> unknown = #const UNKNOWN_EXTENSION
>
> data Codec = MP3 | FLAC | OGG_VORBIS | MP4 | MPEG | NONE | UNKNOWN
> deriving (Show)
>
> data MusicMetadata = MusicMetadata { codec :: Codec,
> length :: Int,
> bitrate :: Int,
> channels :: Int,
> track :: Int,
> title :: String,
> artist :: String,
> album :: String,
> comment :: String,
> genre :: String } deriving (Show)
>
> instance Storable MusicMetadata where
> sizeOf _ = (#size struct music_metadata)
> alignment _ = alignment (undefined::CDouble)
> peek a = do
> codec <- liftM constantToCodec $ (((#peek struct music_metadata,
> codec) a) :: IO Int)
> length <- ((#peek struct music_metadata, length) a) :: IO Int
> bitrate <- ((#peek struct music_metadata, bitrate) a) :: IO Int
> channels <- ((#peek struct music_metadata, channels) a) :: IO Int
> track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
> title <- ((#peek struct music_metadata, title) a) :: IO CString
> artist <- ((#peek struct music_metadata, artist) a) :: IO CString
> album <- ((#peek struct music_metadata, album) a) :: IO CString
> comment <- ((#peek struct music_metadata, comment) a) :: IO CString
> genre <- ((#peek struct music_metadata, genre) a) :: IO CString
> --FIXME: find replacement for temporary names
> marshalledTitle <- peekCString title
> marshalledArtist <- peekCString artist
> marshalledAlbum <- peekCString album
> marshalledComment <- peekCString comment
> marshalledGenre <- peekCString genre
> return (MusicMetadata codec length bitrate channels track
> marshalledTitle marshalledArtist marshalledAlbum marshalledComment
> marshalledGenre)
> poke a = undefined
>
> --This is the "primitive" FFI call--calls the C function and gets a pointer
> --in return
> --TODO: write a higher level function this module should export that calls
> --primReadMusicMetadata and converts the C Pointer into the Haskell data
> --MusicMetadata
> foreign import ccall unsafe "read_metadata" primReadMusicMetadata ::
> CString -> IO (Ptr MusicMetadata)
>
> --convert the Haskell string to a CString, call into the FFI then
> --dereference the resulting pointer
> readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek) $
> primReadMusicMetadata cs)
>
>
>
> Here's the struct in MusicReader.h (in an extern C block):
> struct music_metadata
> {
> int codec;
> int length,
> bitrate,
> channels;
> int track;
> char *title,
> *artist,
> *album,
> *comment,
> *genre;
> };
>
>
> with the corresponding call:
> struct music_metadata* read_metadata(char*);
>
>
> I've tried playing around with the alignment but it didn't do anything. I
> also tried declaring the struct's fields as int32_t which also did nothing.
>
> The C demo in question is a very simple:
>
> #include <stdio.h>
> #include "MusicReader.h"
>
> #define FILENAME "Its_My_Life.m4a"
>
> int main()
> {
> struct music_metadata* metadata = read_metadata(FILENAME);
> printf("music_metadata\ntitle: %s,\tartist: %s,\talbum: %s\n",
> metadata->title, metadata->artist, metadata->album);
> printf("comment: %s,\tgenre: %s,\ttrack: %d,\n",
> metadata->comment, metadata->genre, metadata->track);
> printf("length: %d,\tbitrate: %d,\tchannels: %d,\n",
> metadata->length, metadata->bitrate, metadata->channels);
> printf("codec: %d\n");
>
> }
>
> It just reads the metadata into the struct and prints the fields.
>
> I've gotten the impression of the Haskell FFI being very
> beginner-unfriendly, which isn't surprising but still disappointing because
> it would be a great opportunity to replace some projects with Haskell.
>
> Any help is appreciated, including general feedback on my code!
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
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------------------------------
Message: 2
Date: Mon, 2 Mar 2015 16:30:27 +0100
From: Sylvain Henry <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] FFI Problems
Message-ID:
<CAPmptcU6Ye_k+H8d_=eehhoj0c1vz7kpdpcgpn1s5yktdb+...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
I forgot to comment this line from your code:
readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek) $
primReadMusicMetadata cs)
I would write:
readMusicMetadata a = withCString a $ \cs -> do
p <- primReadMusicMetadata cs
peek p
Or:
readMusicMetadata a = withCString a $ \cs -> (peek =<<
primReadMusicMetadata cs)
Or (with (<=<) from Control.Monad):
readMusicMetadata a = withCString a (peek <=< primReadMusicMetadata)
Best regards,
Sylvain
2015-03-02 16:20 GMT+01:00 Sylvain Henry <[email protected]>:
> Hi,
>
> 1) You have a copy-paste error on the following line (s/bitrate/track)
> track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>
> 2) By looking at the values you get in hexadecimal, you are reading 64bits
> values when your C struct contains 32 bits values. If you use: peek ... ::
> IO Int32, it should be ok.
>
> 3) From a style point of view, you could use Applicative operators (from
> Control.Applicative) to avoid temporary names:
> peek a = MusicMetaData
> <$> liftM constantToCodec $ (((#peek struct music_metadata, codec)
> a) :: IO Int32)
> <*> fmap fromIntegral (((#peek struct music_metadata, length) a)
> :: IO Int32)
> <*> fmap fromIntegral (((#peek struct music_metadata, bitrate) a)
> :: IO Int32)
> <*> fmap fromIntegral (((#peek struct music_metadata, channels) a)
> :: IO Int32)
> <*> fmap fromIntegral (((#peek struct music_metadata, track) a) ::
> IO Int32)
> <*> (peekCString =<< (#peek struct music_metadata, title) a)
> ...
>
> 4) In your constantToCodec function, you don't need the temporary names
> for all the constants.
>
> Best regards,
> Sylvain
>
> PS : recently I have been working on rewriting the FFI page on the wiki to
> make it more beginner-friendly ;-). It is not totally finished but I'm open
> to any comment. https://www.haskell.org/wiki/Foreign_Function_Interface
>
> 2015-02-28 14:20 GMT+01:00 Thomas Jakway <[email protected]>:
>
>> I'm very new to Haskell and am trying to write a "real" program to
>> motivate myself to learn it better (so far I've only gotten through Project
>> Euler problems after reading LYAH and most of RWH). I'm using Taglib (
>> https://github.com/taglib/taglib) to read the metadata from a music file
>> and print it. I have a struct C-side (with C linkage) serving as the
>> bridge between Taglib's C++ and Haskell's FFI. A small demo program
>> (compiled with gcc and linked against the C++ object files) gives the
>> correct results, but Haskell is weirdly only getting *some *of it
>> right. Specifically, the C string fields are working but ints are not.
>>
>> The output from the C demo (what Haskell should be printing):
>>
>> music_metadata
>> title: It's My Life, artist: Bon Jovi, album: Bon Jovi Greatest
>> Hits - The Ultimate Collection
>> comment: , genre: Rock, track: 3,
>> length: 224, bitrate: 256, channels: 2,
>> codec: 768
>>
>> The output from Haskell:
>>
>> MusicMetadata {codec = UNKNOWN, length = 1099511628000, bitrate =
>> 8589934848, channels = 12884901890, track = 8589934848, title = "It's My
>> Life", artist = "Bon Jovi", album = "Bon Jovi Greatest Hits - The Ultimate
>> Collection", comment = "", genre = "Rock"}
>>
>> I would have expected it to work or not work at all, but did not
>> anticipate getting only some of it right.
>>
>> I was going to include snippets from my hsc file but given how new I am
>> to Haskell I don't trust myself to assume where the problem is, so sorry if
>> this is way too long:
>>
>> {-# LANGUAGE CPP, ForeignFunctionInterface #-}
>>
>> module MusicReader
>> ( Codec,
>> MusicMetadata,
>> readMusicMetadata
>> ) where
>>
>> import Control.Monad
>> import Foreign
>> import Foreign.C.Types
>> import Foreign.C.String
>> import System.IO.Unsafe as Unsafe
>>
>> #include "CodecDefines.h"
>> #include "MusicReader.h"
>>
>> constantToCodec code
>> | code == mp3 = MP3
>> | code == flac = FLAC
>> | code == ogg = OGG_VORBIS
>> | code == mp4 = MP4
>> | code == mpeg = MPEG
>> | code == none = NONE
>> | code == unknown = UNKNOWN
>> | otherwise = UNKNOWN
>> where mp3 = #const MP3_CODEC
>> flac = #const FLAC_CODEC
>> ogg = #const OGG_VORBIS_CODEC
>> mp4 = #const MP4_CODEC
>> mpeg = #const MPEG_CODEC
>> none = #const NO_EXTENSION
>> unknown = #const UNKNOWN_EXTENSION
>>
>> data Codec = MP3 | FLAC | OGG_VORBIS | MP4 | MPEG | NONE | UNKNOWN
>> deriving (Show)
>>
>> data MusicMetadata = MusicMetadata { codec :: Codec,
>> length :: Int,
>> bitrate :: Int,
>> channels :: Int,
>> track :: Int,
>> title :: String,
>> artist :: String,
>> album :: String,
>> comment :: String,
>> genre :: String } deriving (Show)
>>
>> instance Storable MusicMetadata where
>> sizeOf _ = (#size struct music_metadata)
>> alignment _ = alignment (undefined::CDouble)
>> peek a = do
>> codec <- liftM constantToCodec $ (((#peek struct music_metadata,
>> codec) a) :: IO Int)
>> length <- ((#peek struct music_metadata, length) a) :: IO Int
>> bitrate <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>> channels <- ((#peek struct music_metadata, channels) a) :: IO Int
>> track <- ((#peek struct music_metadata, bitrate) a) :: IO Int
>> title <- ((#peek struct music_metadata, title) a) :: IO CString
>> artist <- ((#peek struct music_metadata, artist) a) :: IO CString
>> album <- ((#peek struct music_metadata, album) a) :: IO CString
>> comment <- ((#peek struct music_metadata, comment) a) :: IO
>> CString
>> genre <- ((#peek struct music_metadata, genre) a) :: IO CString
>> --FIXME: find replacement for temporary names
>> marshalledTitle <- peekCString title
>> marshalledArtist <- peekCString artist
>> marshalledAlbum <- peekCString album
>> marshalledComment <- peekCString comment
>> marshalledGenre <- peekCString genre
>> return (MusicMetadata codec length bitrate channels track
>> marshalledTitle marshalledArtist marshalledAlbum marshalledComment
>> marshalledGenre)
>> poke a = undefined
>>
>> --This is the "primitive" FFI call--calls the C function and gets a
>> pointer
>> --in return
>> --TODO: write a higher level function this module should export that calls
>> --primReadMusicMetadata and converts the C Pointer into the Haskell data
>> --MusicMetadata
>> foreign import ccall unsafe "read_metadata" primReadMusicMetadata ::
>> CString -> IO (Ptr MusicMetadata)
>>
>> --convert the Haskell string to a CString, call into the FFI then
>> --dereference the resulting pointer
>> readMusicMetadata a = join $ withCString a $ \cs -> ((liftM peek) $
>> primReadMusicMetadata cs)
>>
>>
>>
>> Here's the struct in MusicReader.h (in an extern C block):
>> struct music_metadata
>> {
>> int codec;
>> int length,
>> bitrate,
>> channels;
>> int track;
>> char *title,
>> *artist,
>> *album,
>> *comment,
>> *genre;
>> };
>>
>>
>> with the corresponding call:
>> struct music_metadata* read_metadata(char*);
>>
>>
>> I've tried playing around with the alignment but it didn't do anything.
>> I also tried declaring the struct's fields as int32_t which also did
>> nothing.
>>
>> The C demo in question is a very simple:
>>
>> #include <stdio.h>
>> #include "MusicReader.h"
>>
>> #define FILENAME "Its_My_Life.m4a"
>>
>> int main()
>> {
>> struct music_metadata* metadata = read_metadata(FILENAME);
>> printf("music_metadata\ntitle: %s,\tartist: %s,\talbum: %s\n",
>> metadata->title, metadata->artist, metadata->album);
>> printf("comment: %s,\tgenre: %s,\ttrack: %d,\n",
>> metadata->comment, metadata->genre, metadata->track);
>> printf("length: %d,\tbitrate: %d,\tchannels: %d,\n",
>> metadata->length, metadata->bitrate, metadata->channels);
>> printf("codec: %d\n");
>>
>> }
>>
>> It just reads the metadata into the struct and prints the fields.
>>
>> I've gotten the impression of the Haskell FFI being very
>> beginner-unfriendly, which isn't surprising but still disappointing because
>> it would be a great opportunity to replace some projects with Haskell.
>>
>> Any help is appreciated, including general feedback on my code!
>>
>> _______________________________________________
>> Beginners mailing list
>> [email protected]
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>>
>>
>
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