Path: utzoo!attcan!uunet!mcsun!inria!chorus!francois
From: francois@chorus.fr (Francois Armand)
Newsgroups: comp.unix.questions
Subject: Sharing Open File Current Offset? (Looking for example of applications)
Keywords: Files
Message-ID: <6011@chorus.fr>
Date: 20 Aug 90 13:04:07 GMT
Sender: news@chorus.fr
Lines: 50



	I am looking for some _real_ _uses_ of current offset sharing
by real applications running on any flavor of UNIX systems.

To be more precise, suppose you have a program that opens(2) a file "foo"
for read/write operations, and then forks(2).The parent and 
the child processes now share the same open file description, and in
particular the current offset within the file and the flags (O_SYNC, 
O_APPEND, O_NDELAY...). 

It seems to me that if both the parent and the child want to make any
sensible use of that shared access to the file, they need to synchronize
themselves by some mechanism (signals, shared memory, file locks...).

In addition, within the Unix file system sources which I have read, I have 
never seen any "synchronization" mechanism to provide a deterministic way
of computing the current offset when, for example, two read(2) operations
are issued in parallel by two processes sharing the same open file. Thus, I
really wonder how one could deal without any synchronization between user's
programs.

I have never seen any program using this property! Thus, my question
is: do Unix systems really need to make the current offset be shared
between parent and child processes. 

I would like to know if somebody has ever seen or written an application
using that property of the UNIX fork(2)... I am not interested in having 
example of applications sharing open files such as terminals, pipes or
sockets (for which current offset is meaningless).

Could you mail me a description of the examples you are aware of, I
will summarize to the net.

I would be happy to get detailed information about the use of the mechanism,
ie: why is the application written that way, how processes synchronize 
themselves, what flags are set for the shared open files (for example,
setting the O_APPEND flag, changes the behaviour of the current offset...),
do the programs use lseek(2)... and any kind of information that may be
relevant.

Thanks in advance

Francois ARMAND

Francois ARMAND
Chorus Systemes				Tel: +33 (1) 30 57 00 22
6 Avenue Gustave Eiffel			Fax: +33 (1) 30 57 00 66
78182 St-Quentin-en-Yvelines Cedex	Email: francois@chorus.fr (Internet)
FRANCE