[9fans] How to do a generic filter for files?

[9fans] How to do a generic filter for files?

[Jean Mehat]

I would like to do a program, that I would interpose between
a Plan 9 file and processes accessing this file. My first idea was
to have a program post a pipe in /srv and mount this pipe end
in place of the interesting file (say /dev/draw/1/data, for example).
Then when anything is written by a program in /dev/draw/1/data,
the program receives the data on the other end of the pipe, writes
it into the real file and dumps it into a file for further examination,
for example.

The problem is with files that can be read, like /net/arp; the program
should open and read this file only when a process is trying to read
the file; so the question is: is there a simple way for the program to
know that a process opened the pipe and is trying to read its contents?
Or, is there no simpler way than to build a user level file server to do
that?

--
Jean Mehat, universite de Paris 8 Vincennes a Saint Denis,
jm@ai.univ-paris8.fr, (33) 1 4940 6403, (33) 1 4940 6783 (fax)

Re: [9fans] How to do a generic filter for files?

[rog]

i wrote:
> somewhat more trivial than it used to be.

that is, somewhat more trivial than a not-very-trivial thing... :-)
ahem.

Re: [9fans] How to do a generic filter for files?

[rog]

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a pipe is kept open by virtue of being placed in /srv. there are no
indications of further opens.

pipefile(1) takes this approach as far as is reasonable, i think.

if you want to know when a file is opened, you will indeed need to
write a user-level fileserver.  the 9p(2) library makes that somewhat
more trivial than it used to be.

  cheers,
    rog.


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To: cse.psu.edu!9fans
Subject: [9fans] How to do a generic filter for files?
Date: Wed, 10 Jan 2001 17:20:26 +0100 (MET)
Message-ID: <200101101620.RAA08051@colombie.ai.univ-paris8.fr>

I would like to do a program, that I would interpose between
a Plan 9 file and processes accessing this file. My first idea was
to have a program post a pipe in /srv and mount this pipe end
in place of the interesting file (say /dev/draw/1/data, for example).
Then when anything is written by a program in /dev/draw/1/data,
the program receives the data on the other end of the pipe, writes
it into the real file and dumps it into a file for further examination,
for example.

The problem is with files that can be read, like /net/arp; the program
should open and read this file only when a process is trying to read
the file; so the question is: is there a simple way for the program to
know that a process opened the pipe and is trying to read its contents?
Or, is there no simpler way than to build a user level file server to do
that?

--
Jean Mehat, universite de Paris 8 Vincennes a Saint Denis,
jm@ai.univ-paris8.fr, (33) 1 4940 6403, (33) 1 4940 6783 (fax)

Re: [9fans] How to do a generic filter for files?

[Boyd Roberts]

> that is, somewhat more trivial than a not-very-trivial thing... :-)

so you're saying it's slightly more trivial than a trivial
thing that you've never seen?  :-)