Ports input to Processes

13 Apr 2002

      Hi Manuel,

Trying again now with ghc-5.02.3:

"Manuel M. T. Chakravarty"  writes:
...
Jens Petersen  wrote,
...
http://www.cse.unsw.edu.au/~chak/haskell/ports/
It compiles fine under ghc-5.02.2, and using the BufferMode
patch included at the end output seems to be ok,
Why LineBuffering?  Doesn't really change anything for me
(same GHC version).
You're right, I think.
...
...
but input
of more than 2048 bytes doesn't seem to be being handled
reliably.
[..]
mostly gives no output, but occasionally I see
Warning: Ports.listenToPort: Attempted to listen to a closed port!
Needs some debugging I guess. :)
I guess, it needs more documentation.  Your test program is
wrong in two places.
The function `listenToPort' starts to listens at the given
port exactly when `listenToPort' is executed; ie, any data
that goes over the port earlier will not be received in the
stream that this call to `listenToPort' returns.
You wrote
...
outpt <- newPort ' '
       errpt <- newPort ' '
       let p = proc cmd args
       p inpt outpt errpt
       putStrLn "output:"
       out <- listenToPort outpt
As the processes `cmd' is fork()ed in a separate thread, it
may already have finshed its business until you get to
`listenToPort outpt', which means you have missed all the
data.  For similar reasons
Ok.
...
...
errclosed <- isClosedPort errpt
       unless errclosed $
   do
   err <- listenToPort errpt
isn't recommended.
Fair enough.
...
I changed your program to what I have appended.  It then
works for me for the
% cat 4096 | test-processes cat
test...except that I sometimes get a "Broken Pipe".  So, I
guess, I have to do some debugging after all (plus improve
the documentation).
With your program (just modified to output the length of the
output string instead of the output itself), for small
amounts of input (ie <= 4KiB), on the first invocation I see:

% cat small-file | ./ports cat  
output:
0
error:
cat: -: Resource temporarily unavailable
test finished

and then subsequent invocation perform as expected:

% cat small-file | ./ports cat 
output:
340
error:
test finished

For input larger than 4KiB the program never gives any
output for me, and always seems to terminate prematurely:

% cat large-file | ./ports cat 
output:
%
...
PS: I'll add your program to the ports/tests/ directory if
    you don't mind.
Sure, please do.

Jens