Re: 1 file, multiple threads

From: Jeff Shannon (jeff_at_ccvcorp.com)
Date: 11/27/04 

Date: Fri, 26 Nov 2004 15:43:06 -0800

Jason wrote:

>>If I have multiple threads reading from the same file, would that be a
>>problem?
>>
>>
>
>As long as you open each file with 'r' or 'rb' access only, it is not
>a problem. I believe you can even write to that file from one (but
>only one) thread while reading the file in multiple other threads.
>
>

You *can*, but due to buffering issues, it's likely that the reader
threads will not see changes made by the writer thread properly, and may
have issues with separate disk-reads which nominally stop/start at the
same location not actually matching because the underlying disk file has
changed. (Note that disk reads, which go into a buffer, do not
necessarily correlate in any predictable way to calls to the read*()
family of functions.)

In order to ensure consistent access to a mutable (i.e. not read-only)
file from multiple threads, it would be necessary to ensure that only
one thread was accessing the file at a given instant (i.e. use some form
of locking/synchronizing mechanism), and to be careful to flush all
buffers both before and after any file access. If the file can change
at all, then the only time that a given thread can make *any*
assumptions about the state of the file is during a single section in
which that thread has exclusive access to the file.

Much simpler to designate a single file-handler 'server', and have each
thread access the file only through the intermediary of this server
(which could be implemented as a separate thread itself). The server
then manages all of the file buffers, both in and out, and can ensure
that each access happens in a consistent way.

Jeff Shannon
Technician/Programmer
Credit International

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