Re: Exclusive file access

["Randy Brukardt" <randy@rrsoftware.com>]

"Simon Wright" <simon@pushface.org> wrote in message 
news:lyh9nexjed.fsf@pushface.org...
> ahlan@marriott.org writes:
>
>> Bruce B. tells me that there are routines in the package Directories
>> that can be used to check if the file is already open. However that
>> wouldn't be a full solution because there would be nothing to stop any
>> other program from opening the file and messing with it. However it
>> would be a start. If the file was already opened (by some other
>> process) then I could avoid opening it. Unfortunately I couldn't find
>> any such routine in Ada.Directories. Am I going blind?
...
> I think you are rather out of luck. I can see how it might be done on OS
> X [open(2) with O_EXLOCK, then write(2)].

It's trivial to do on Windows, one just needs to open the file with the 
correct sharing flags (and I think that the correct flag is the default). 
Sharing is a built-in part of the file descriptor in Windows; there's no 
need for separate locking so long as you want to do full-file exclusion (as 
in this case).

Typically, those sharing flags are made available through a Form parameter. 
Janus/Ada uses 'R' and 'W' to indicate read and write sharing 
(respectively). (So a form of "RW" opens a file allowing read and write 
sharing, while opening a file with "" - the default - opens it for exclusive 
access.) But this will be different on different compilers, plus it's 
specific to Windows.

                                            Randy.