[Haskell-cafe] Re: How to automatically free memory allocated by malloc? and how to reliably realloc such buffer?

[Simon Marlow]

Bulat Ziganshin wrote:

> Wednesday, May 24, 2006, 2:08:10 PM, you wrote:
> 
> 
>>>    fptr  <- newForeignPtr fin nullPtr
>>>    return (Mem bufRef ... fptr)
> 
>>I hope you surround each use of the actual Ptr with 'withForeignPtr'? 
>>If so, I imagine this is safe.
> 
> no, i hope that fptr's finalizers will be no executed as long as
> perform any operations on this Mem structure. i think that there
> should be no problems as long as bufRef not returned from functions
> working with Mem, like this
> 
> -- problematic code:
> vRequestBuf (Mem bufRef ... fptr) = do
>     readURef bufRef
> 
> makeProblems = do
>     mem <- newMem
>     buf <- vRequestBuf mem
>     poke buf 0  -- at this time buffer may be already deallocated
>                 -- because 'mem' is not further referenced

This isn't safe, I'm afraid.  Suppose GHC inlined vRequestBuf, and 
discovered that fptr isn't used so discarded the reference to it.  Then 
the ForeignPtr could be finalized too early.

You can either use withForeignPtr around each operation that accesses 
the Ptr, or you can use touchForeignPtr at the end of each operation.

> it seems not so easy hack as i thought. first and obvious is that
> wrapping all buffer pointers to the ForeignPtr will be rather slow on
> ghc 6.4. second and not so obvious is model of buffers' usage
> 
> now i've modified MemoryStream interface to the following:
> 
> class (Stream IO h) => MemoryStream h where
>     -- | Request access to memory buffer for READING or WRITING.
>     -- Operation returns 'pos' and 'end' - pointers to the start and after-end
>     -- part of buffer available for reading or writing.
>     -- It returns pos==end if there is no more data to read (vIsEOF)
>     -- or no more space to write to (for streams with limited size).
>     -- In other cases buffer received by this call must be released by call
>     -- to 'vReleaseBuf'
>     vRequestBuf :: h -> ReadWrite -> IO (Ptr a, Ptr b)
> 
>     -- | Release buffer that was received via call to 'vRequestBuf' and
>     -- tells new position after some number of bytes at the start of buffer was
>     -- read or written. After this call buffer is no more available for any
>     -- operation
>     vReleaseBuf :: h -> ReadWrite -> Ptr a -> IO ()

This is slightly off-topic, but the request/release design forces the 
upper layer to do the exception handling (eg. with 
Control.Exception.bracket), whereas a with-style design would include 
the exception handling and make the API less error-prone to use.

If you're inside a block it's easier, but you still have to worry about 
synchronous exceptions.

> as an example of it's usage is the following function that implements
> vPutChar for any MemoryStream:
> 
> fastPutChar s c = do
>     (pos,end) <- vRequestBuf s WRITING
>     if pos==end  then vThrow s fullErrorType  else do
>         writeByteAt pos $! (ord c)
>         vReleaseBuf s WRITING  $! (pos+:1)
 >
> it seems that scheme with ForeignPtr will be fast and reliable if
> both the following conditions are met:
> 
> 1. stream transformer got access to the buffer only through the
> 'vRequestBuf' and ALWAYS releases it after use with call to
> 'vReleaseBuf'.
> 
> 2. base Stream use ForeignPtr to hold finalizer. this ForeignPtr is
> touched in the 'vReleaseBuf' and after any buffer reallocation
> 
> 
> but the problem is what i can't use vRequestBuf/vReleaseBuf in highly
> optimized code, it's too slow. does 'touchForeignPtr' require any time
> to execute or it is no-op that have meaning only for program analysis?

Yes, it's a no-op.

> may be it's better just to require from user to explicitly execute
> 'vClose' operation...

I don't think that would be a good design.

>>>reallocBuffer (Mem bufRef ...) newsize = do
>>>        buf <- readURef bufRef
>>>        block $ do
>>>            newbuf <- reallocBytes buf newsize
>>>            writeURef bufRef newbuf
> 
>>You're missing a 'withForeignPtr'.  Something like this, I think:
> 
>>  reallocBuffer (Mem bufRef ... fp) newsize =
>>    withForeignPtr fp $ do
>>          buf <- readURef bufRef
>>          newbuf <- reallocBytes buf newsize
>>          writeURef bufRef newbuf
> 
> i agree that withForeignPtr required here to ensure that 'fp' will not run
> it's finalzer just at this moment. but it seems that 'block' ia ALSO
> required to ensure that 'bufRef' after 'realloc' will be updated with new
> value

Yes, that's true.

Cheers,
	Simon