[Haskell-cafe] Re: Are handles garbage-collected?

[Remi Turk]

On Sun, Oct 24, 2004 at 02:16:50PM +0200, Peter Simons wrote:
> Tomasz Zielonka writes:
> 
>  > AFAIK, Handles have finalisers which close them, but I
>  > don't know if GHC triggers garbage collection when file
>  > descriptors run out. If not, you will have problems if
>  > you manage to run out of fds between GCs.
> 
> Thank you for answering.
> 
> Now there is only one problem: Assuming I could _not_ use
> 'bracket', 'withFile', 'finally' or any of the other usual
> scope-guarding techniques, what would I do? (The handle has
> to be passed up to the outside of the scope in which it was
> opened.)
Refactoring comes to the mind... ;)

> If I stored the handle in an 'MVar' and attached an
> MVar-finalizer to that, would that work better? Would the
> MVar's finalizer be run any sooner than the one attached to
> the handle anyway? 
Both are just finalizers, so it won't make any difference AFAICS.

> Or can I explicitly trigger garbage collection somehow? Say,
> in case I receive an exception telling me that file
> descriptors are running out? Which exception would I even
> get in this case?
> 
> Peter

You could try using something like this instead of the normal
openFile:
(or make it even less portable by importing GHC.IOBase and only
retrying on ResourceExhausted)


module Main where

import IO
import System.Mem

myOpenFile path mode
        = catch (openFile path mode) $ \_ -> do
                putStrLn "==> Open failed. Retrying <=="
                performGC
                openFile path mode

open    = openFile "/tmp/foo" ReadMode
myOpen  = myOpenFile "/tmp/foo" ReadMode


Example:

-- Let's eat all available filedescriptors.

*Main> sequence (repeat open)
Loading package haskell98 ... linking ... done.
*** Exception: /tmp/foo: openFile: resource exhausted (Too many open files)

-- They are indeed all gone.

*Main> h <- open
*** Exception: /tmp/foo: openFile: resource exhausted (Too many open files)

-- What about performing garbage collection?

*Main> h <- myOpen
==> Open failed. Retrying <==

*Main> h
{handle: /tmp/foo}

*Main> 


Groeten,
Remi

-- 
Nobody can be exactly like me. Even I have trouble doing it.