[Caml-list] Pipes in 3.01

[Caml-list] Pipes in 3.01

[Laszlo Nemeth]

Hi,

I'm trying to implement an api for daVinci (my first ocaml program!)
and pipes don't seem to work:

module DaVinciAPI : DAVINCIAPI =
  struct
    exception DaVinciInitError
    let daVinci () =
      begin try
        let (dIn, dOut) = Unix.pipe () in
        let dProcess = Unix.create_process "daVinci"
                                           [| "-pipe" |]
                                           ~stdin:dIn
                                           ~stdout:dOut
                                           ~stderr:Unix.stderr
        and buf = String.create 100 in
        let len = Unix.read dIn ~buf ~pos:0 100 in
        if len > 0 then () else raise DaVinciInitError
      with Unix.Unix_error _ -> raise DaVinciInitError
      end;

The application starts up nicely, but I can't communicate with it. The
read blocks. On the other hand, if I start up daVinci in a shell (with
-pipe) it immediately sends back the string ok. I also tried to write
to the pipe, but to no avail (while doing the same from a shell works
perfectly).

Sorry for the probably trivial question but I haven't been able to
find (the new examples) examples of programming with pipes. 

Any help, hints, pointers are appreciated.

Thanks, Laszlo

PS. I'm using the interactive environment (after linking in the unix
library) under Solaris.
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Re: [Caml-list] Pipes in 3.01

[Laszlo Nemeth]

Thanks for all those (Damien, Francois, Fabrice) who replied and
pointed out my misunderstanding of IPC. Nevertheless, I did not manage
to make it work with 'create_process'. Instead, I started using
'open_process' which works wonderfully.

Thanks again. Laszlo


--- the working snippet:
module DaVinciAPI : DAVINCIAPI =
  struct
    exception DaVinciInitError
    let daVinci () =
      begin try
        let (dIn, dOut) = Unix.open_process "daVinci -pipe" 
        and buf = String.create 10 in
        let len = Unix.read (Unix.descr_of_in_channel dIn) ~buf ~pos:0 10
        in
          buf
      with Unix.Unix_error _ -> raise DaVinciInitError
  end

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Re: [Caml-list] Pipes in 3.01

[Damien Doligez]

>From: Laszlo Nemeth <laszlo@ropas.kaist.ac.kr>

>        let (dIn, dOut) = Unix.pipe () in
You're creating one pipe.

>        let dProcess = Unix.create_process "daVinci"
>                                           [| "-pipe" |]
>                                           ~stdin:dIn
>                                           ~stdout:dOut
>                                           ~stderr:Unix.stderr

You're piping the output of daVinci into its own input.

>        let len = Unix.read dIn ~buf ~pos:0 100 in

You're competing with daVinci on reading its output

>On the other hand, if I start up daVinci in a shell (with
>-pipe) it immediately sends back the string ok.

I guess it does the same here, but it reads back the "ok" before you
do, then it waits for some more input.


If you want to send some input to daVinci AND get back its output, you
will need to create two pipes.

-- Damien
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