[Haskell-cafe] named pipe interface

[Jean-Marie Gaillourdet]

Hi, 

On 14.01.2012, at 12:11, Serge D. Mechveliani wrote:

> On Fri, Jan 13, 2012 at 12:19:34PM -0800, Donn Cave wrote:
>> Quoth "Serge D. Mechveliani" <mechvel at botik.ru>,
>> ...
>>> Initially, I did the example by the Foreign Function Interface for C.
>>> But then, I thought "But this is unnatural! Use plainly the standard
>>> Haskell IO, it has everything".
>>> 
>>> So, your advice is "return to FFI" ?
>> 
>> Well, it turns out that the I/O system functions in System.Posix.IO
>> may work for your purposes.  I was able to get your example to work
>> with these functions, which correspond to open(2), read(2), write(2).
>> 
>> I would also use these functions in C, as you did in your C program.
>> Haskell I/O functions like hGetLine are analogous to C library I/O
>> like fgets(3) - in particular, they're buffered, and I would guess
>> that's why they don't work for you here.
>> 
>> Specifically,
>>   openFile "toA" WriteOnly Nothing defaultFileFlags
>>   openFile "fromA" ReadOnly Nothing defaultFileFlags
>> 
>>   fdWrite toA str
>>   (str, len) <- fdRead fromA 64
>>   return str
> 
> 
> Great! Thank you very much.
> As I find,  Posix.IO  is not of the standard, but it is of GHC.
> Anyway, it fits my purpose.
> By  openFile  you, probably, mean  openFd.
> 
> Another point is the number of open files, for a long loop.
> I put
>  toA_IO   = openFd "toA"   WriteOnly Nothing defaultFileFlags
>  fromA_IO = openFd "fromA" ReadOnly  Nothing defaultFileFlags
> 
>  axiomIO :: String -> IO String
>  axiomIO str = do
>                toA   <- toA_IO   
>                fromA <- fromA_IO
>                fdWrite toA str
>                (str, _len) <- fdRead fromA 64
>                return str
> 
> When applying  axiomIO  in a loop of 9000 strings, it breaks:
> "too many open files".
> I do not understand why it is so, because  toA_IO and fromA_IO  are 
> global constants (I have not any experience with `do').
> 
> Anyway, I have changed this to
> 
>  toA   = unsafePerformIO toA_IO
>  fromA = unsafePerformIO fromA_IO
>  axiomIO :: String -> IO String
>  axiomIO str = do
>                fdWrite toA str
>                (str, _len) <- fdRead fromA 64
>                return str
> 
> And now, it works in loong loops too
> (I need to understand further whether my usage of  unsafePerformIO  
> really damages the project).

I'd say this use of unsafePerformIO is *not* safe. E.g. a Haskell compiler is allowed to evaluate the right hand side of toA and fromA multiple times. If you aren't 100% sure that it is ok to use unsafePerformIO, don't use it!

Try something like the following:

> initIO = do
>     to <- toA_IO
>     from <- fromA_IO
>     return (to, from)

> axiomIO (to,from) str = do
>     fdWrite to str
>     (str, _len) <- fdRead from 64
>     return str

> main = do
>     fds <- initIO
>     …
>     res <- axiomIO fds "…"

Obviously I didn't compile and test this code.

> Its performance is  9/10  of the  C <-> C  performance 
> (ghc -O, gcc -O, Linux Debian). 
> It is still slow:  120000 small strings/second on a 2 GHz machine.
> But this is something to start with.
> 
>> I was able to get your example to work
>> with these functions, which correspond to open(2), read(2), write(2).
>> 
>> I would also use these functions in C, as you did in your C program.
>> Haskell I/O functions like hGetLine are analogous to C library I/O
>> like fgets(3) - in particular, they're buffered, and I would guess
>> that's why they don't work for you here.
> 
> Indeed. Initially, I tried  C <-> C,  and used  fgets, fputs, fflush.
> And it did not work, it required to open/close files inside a loop;
> I failed with attempts. Again, do not understand, why (do they wait
> till the buffer is full?).
> 
> Then, I tried  read/write,  as it is in  fifoFromA.c  which I posted.
> And it works.
> Now,  Haskell <-> C  gives a hope. Nice.
> 
> Thanks,
> 
> ------
> Sergei
> mechvel at botik.ru
> 
> 
> 
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