Re: [Caml-list] Unix.file_descr -> int ???

[Bruno.Verlyck@inria.fr]

   Date: Wed, 12 Jun 2002 09:42:04 +0200
   From: Basile STARYNKEVITCH <basile.starynkevitch@cea.fr>

   For example, I will very soon add in my program (opensource GPL-ed, see
   www.poesia-filter.org and its CVS repository PoesiaSoft/PoesiaMonIcap) some
   code which
   1. Create three pipes (using the pipe(2) system call) I O and C
   2. Fork(2) a child process
   3. in the child process execve(2) a program passing it the file descriptors
      of I (read-end, i.e. the first file descriptor returned by pipe) O
      (write-end, i.e. the second file descriptor returned by pipe) and C
      (read-end). For instance, it may execve a program
      /usr/libexec/poesiafilter/nlp_italian and passes it as arguments
	    arg[0] = "nlp_italian"
	    arg[1] = "-i"
	    arg[2] = "5"
	    arg[3] = "-o"
	    arg[4] = "7"
	    arg[5] = "-c"
	    arg[6] = "8"
	    arg[7] = "-C"
	    arg[8] = "/etc/poesia/nlp_italian_config"
	    arg[9] is the null pointer
   and the 5, 7, and 8 are file descriptors number.  This seems a legitimate
   use of file descriptors (there are some Unix programs which does that).

   The program (which I call the Poesia monitor) which does this Unix-pecular
   stuff is coded in Ocaml.  Obviously it requires a conversion from
   Unix.file_descr to either integer or strings.  If the child program where
   coded in Ocaml (and some might be), they will need a mean to convert a
   numerical string arguments (like above arg[4] = "7") to a Unix.file_descr
Basile,

you should definitely have a look at Cash.  Your programs are exactly of the
kind Cash was made for.  You can easily do everything you told us above, and
more (e.g. here, you use any file_descr attached to your pipe, but don't try to
force it on file descriptor 3), ..

   We really need something like
     val ugly_filedescr_of_int: int -> Unix.file_descr
     val ugly_int_of_filedescr: Unix.file_descr -> int
   and I'll guess that in practice they both could be (in that very precise
   case, for a Unix plateform) defined as Obj.magic suitably "casted".
.. and with a much less ugly interface than this.  Check around `Channel
mapping machinery' in the documentation for detailed explanations.

   The most important debate is: should Ocaml give access to all Unix ugliness
   (and dirty tricks)?
I don't believe the interface has to be ugly.  But then, it can't be as simple
as the one you propose :-).  There's a real abstraction clash between those
little file descriptors and channels, and not only at the typing level.  Cash
brings you a clean bridge between both.

Bruno.
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