Subj : Committing file changes
To : Coridon Henshaw
From : David Noon
Date : Mon Aug 14 2000 01:23 pm
Hi Coridon,
Replying to a message of Coridon Henshaw to All:
CH> What's the proper way to 'checkpoint' an open file so as to ensure
CH> that the file's control structures are consistant on disk? I would
CH> have thought that calling fflush() after every file write would be
CH> sufficient, but a recent trap proved that calling fflush() after file
CH> writes was no protection against CHKDSK truncating the file well
CH> before the last write. I suppose I could close and reopen the file
CH> after every update, but I was hoping to find a more elegant solution.
How low do you want to go?
Firstly, you should not be doing buffered I/O if your updates must be committed
immediately, so you should not use fopen() and fwrite() without a setbuf() call
to suppress buffer allocation. Better yet, you should consider using open() and
write() instead, and use the UNIX-like unbuffered I/O routines. If you want to
be a real fundamentalist, you should use DosOpen() and DosWrite() without
risking the CRTL tampering with your data flow.
Moreover, if your data resources are critically important then you should be
handling any traps that occur in your program and cleaning up the critical data
resources in an orderly manner. This is far and away the most professional
approach to the situation. About the only things you can't handle are kernel
level traps and power outages.
In your situation, I would have used the second facility before considering any
intermediate commits.
Another consideration is choice of language. The more robust languages have
RTL's that perform a formal close on all files, even when the application code
fails to handle an error. This is because their RTL's catch all unhandled
application errors, and they keep track of all files opened through the RTL.
Hence, either PL/I or COBOL would have been a far better choice of language
than C/C++, if you are not rolling your own trap handler. The ultimate moral
is: code your own trap handler, or use a language that provides one built-in.
Regards
Dave
<Team PL/I>
--- FleetStreet 1.25.1
* Origin: My other computer is an IBM S/390 (2:257/609.5)
