Graceful Shutdown on an Unsupported Earlier Mac OS Version

FROM : Oleg Krupnov
DATE : Tue Nov 18 10:37:08 2008

While it is not my intention to support previous versions of Mac OS,
I'd like my users be informed politely that the latest OS version is
required and the application shut down gracefully.

What is the recommended way to do this?

Basically it boils down to 2 questions:

1) How do I query the current OS version in Cocoa?

2) Will my code even have the opportunity to check the OS version, or
an error will occur on the stage of loading and reported by the OS
loader, before a single line of my code gets executed? I'm asking this
because this would be the case on Windows when the required statically
linked DLLs, are missing or have incorrect version.

I do not have any previous version of Mac OS at my disposal, so I
can't test what happens. I wish Mac OS X is smart enough to take care
of the problem without the need of any coding. I want to make sure.