First off, it has often been said and i will say it again: "Unix" is not an operating system, but a trademark. A system is "Unix", because its manufacturer bought the license to bear this trademark. "Linux" (to be precise: its manufacturers) did not purchase anything (mostly because there is no money to do so involved in Linux) and this is the only reason "Linux" is not a "Unix", but only "unixlike". This is about the same difference as "sparkling wine" versus "champagne": there is none. "Champagne" is the sparkling wine produced in the french area called Champagne, but that's it. You could create a sparkling wine which is absolutely identical to champagne - you would still not be allowed to call it "champagne" it you don't do so in the Champagne (and probably meet some other legalities too), but that would be about it.

The only real standard of any value is the POSIX standard. Linux is in most regards, AFAIK, compatible against this OS blueprint and so is HP-Ux (to which extent they actually conform i don't know - it is not the point of the argument).

Having said this: the real difference you perceive in your example above is probably the difference in the respective compiler suits. Try "gcc" in both environments (i suppose the HP-Ux compiler is not available in a Linux version) and probably the differences will vanish.

I hope this helps.

bakunin