avoiding file corruption

[Bryan Olson]

Paddy wrote:
> I've never done this in anger so feel free to mock (a little :-).
> 
> I'd have a fixed field at the beginning of the field that can hold the
> hostname process number, and access time of a writing process, togeher
> with a sentinal value that means "no process has access to the file".
> 
> A program would:
>  1. wait a random time.
>  2. open for update the file
>  3. read the locking data
>  4. If it is already being used by another process then goto 1.
>  5. write the process's locking data and time into the lock field.
>  6 Modify the files other fields.
>  7 write the sentinal value to the locking field.
>  8. Close and flush the file to disk.

That doesn't really work; you have still have a race condition.

Locking the file is the good solution, but operating systems
vary in how it works. Other reasonable solutions are to re-name
the file, work with the renamed version, then change it back
after closing; and to use "lock files", which Wikipedia explains
near the bottom of the "File locking" article.


-- 
--Bryan