[Haskell-cafe] Redirecting output

[Louis Wasserman]

Okay, okay, I'm convinced that trying to mess with it in C is easier than in
Haskell.  =P

Louis Wasserman
wasserman.louis at gmail.com
http://profiles.google.com/wasserman.louis


On Fri, Feb 26, 2010 at 2:14 PM, Brandon S. Allbery KF8NH <
allbery at ece.cmu.edu> wrote:

> On Feb 26, 2010, at 14:44 , Louis Wasserman wrote:
>
> Is there any way I can temporarily reassign stdout?  In particular, will
> this be handled well by the FFI?  I'm hoping there's maybe something like
> PHP's output buffering thingy<http://php.net/manual/en/book.outcontrol.php>.
> I'm *binding* (not doing a process call) to an external library which
> outputs directly to stdout, but whose output I'd like to use...
>
>
> That way lies pain.  It can be done, but if it breaks it will break rather
> thoroughly and leave you stranded.  Untested (aside from compilation), no
> warranty, etc.:
>
>   module Redirect (withRedirectedStdout) where
>
>   import System.IO
>   import System.Posix.IO
>
>   -- usage: withRedirectedStdout destinationFileName $ yourIOActionHere
>   -- this might work with Haskell I/O, if you're lucky; don't bet on it
>   -- stick to FFI calls that use raw I/O
>   withRedirectedStdout :: FilePath -> IO a -> IO a
>   withRedirectedStdout f io= do
>     hFlush stdout
>     -- style note: should be stdOutput but that doesn't work for the binds
>     -- and I'm not sure it's worth doing those "right".  maybe if someone
> wants this
>     -- for the library I'll clean it up
>     old <- dup 1
>     closeFd 1
>     -- this will pattern-match fail if we don't get stdout back; if you
> manage to trigger it,
>     -- you get to keep both pieces.  (you probably closed stdin, which is
> usually a
>     -- recipe for disaster, or at minimum extreme confusion)
>     1 <- openFd f WriteOnly (Just 0600) (defaultFileFlags {noctty = True,
> trunc = True, nonBlock = False})
>     res <- io
>     closeFd 1
>     1 <- dup old
>     closeFd old
>     return res
>
> Actually, I bet this fails because your FFI function uses C/C++ stdio and
> doesn't flush it.  That will probably require bringing in C stdio's
> fflush(stdout), which will be even more painful.
>
>  --
> brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery at kf8nh.com
> system administrator [openafs,heimdal,too many hats] allbery at ece.cmu.edu
> electrical and computer engineering, carnegie mellon university    KF8NH
>
>
>
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