Restricting D applications to a single instance

[Preetpal]

On Monday, 22 February 2021 at 08:00:06 UTC, Ali Çehreli wrote:
> I achieve it with 'flock' (man 2 flock) on Linux. My case is 
> different thoug: It allows me to have single writer and many 
> readers of program data, which is kept inside a cache 
> directory. All instances start with read permissionss to 
> exclude any potential writer. Once a writer grabs the lock for 
> writing, no other writer or reader can exist, etc. A nice side 
> effect that I realized was since there is only one writer, the 
> lock file itself stores progress information so that other 
> instances can say something like "Waiting for the writer to 
> finish: 75%" by reading from the lock file itself. :)
>
> Works like a charm except flock operations are not atomic; so 
> some care is needed. For example, the writer cannot assume 
> there were no other writers when it "drops" its access right to 
> a reader. It has to check some signature to make sure that 
> everything is expected.

Implementing progress reporting in that way is cool. :)

I'll looking into using flock to implement this functionality 
when I port this program to FreeBSD/Linux (I'll probably try 
implementing this functionality in another application first as I 
only infrequently use those operating systems as desktop 
environments).