[fpc-pascal]File errors

[Peter Brooks]

-----Original Message-----
From: fpc-pascal-admin at lists.freepascal.org
[mailto:fpc-pascal-admin at lists.freepascal.org] On Behalf Of James Mills
Sent: 05 May 2003 12:12
To: fpc-pascal at lists.freepascal.org
Subject: Re: [fpc-pascal]File errors

On Mon, May 05, 2003 at 11:52:46AM +0200, Peter Vreman wrote:
> > On Mon, May 05, 2003 at 11:35:14AM +0200, Peter Vreman wrote:
> >> > Hi,
> >> >
> >> > How do you check for file errors such as "Disk Full" before
attempting
> >> > to write out a file to disk ?
> >> > You can easily check if a file exists with the fileExists function...
> >> > Can something similar be done for other file errors ?
> >>
> >> You can check with diskfree() how much space is left on a disk
> >
> > Well the program I'm developing is irc services, any particular way
> > you'd reccomend recoding it so it doesn't crash because of a 'Disk Full'
> > error ?
> 
> You can turn of io-checking and check the result of ioresult
> 
> {$I-}
>  writeln(flog,logmessage)
> {$I+}
> if ioresult<>0 then
>  writeln(stderr,'Error writing to disk');

Is this considered good programming practise ? Reason I ask is that I
used to use borland's turbo pascal years ago and am aware of ioresult
but am interested in how people before me have done it (eg: on linux
systems).

The program I've been developing obviously have to be 100% stable :)

You can't be 100% stable on a multi-user system unless you use a write where
you flush the buffer to the disc and then do the error checking - even after
you have written your buffer another program of higher priority might fill
the disk so your buffer can't get to it and your error will only come at
your next write. 

The safest writes are to space that you already have allocated - random
writes. That is what databases do. In the case of a log-file, make your file
circular and pre-allocate it a certain size, then you will overwrite old
messages, but you will never fill up the disk.

This is a bit more work for people who read the log-file later, but you can
write a small reader to do it for them. There are lots of ways of doing
this, but to be safe it is best to write your new record to the circular
file - making sure it writes through any system buffers (a write with wait)
- then write the record number to the start of the file. That way your
log-file reader can easily show the last n records which are usually the
ones of interest.

On an msdros system you can make it fairly friendly by giving your circular
file an extension that you map to your reader - then people using it don't
need to be aware of how its done.


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