[parisc-linux] HPUX binary compatibility

[Stan Sieler]

Jeff writes:

>   > The problem with translators is that they don't work in all cases.
>   > 
>   > The most important case is where you want to link some .o files
>   > together: some are compiled for Linux, some for HP-UX. 
>   > 
>   > *That's* why having different system call numbers is important.

> You really don't want to do that.  I strongly recommend against it.  In

(Presuming "that" is "have different system call numbers")

> 5 years of working on a system which had hpux compatibility we had zero
> need to do this kind of stuff and it just makes things a lot more complicated
> than they need to be.

You're "compatible" if you can run an HP-UX app "out of the box" (e.g.,
restore it and run).  If you have to run it through a translator of some kind,
you aren't compatible.   (If the translator is part of the OS, and invoked 
automatically & invisibly, that's ok...been there, used that for 15 years 
on MPE)

So...how do you achieve this level of compatibility?  Simple...the HP-UX
system call numbers *must* be honored.  What does that mean, then, if
Linux has an identically named (not numbered) system call whose
semantics differ?  Precisely one thing: the kernel must be able to
differentiate between them.  There are two basic methods of doing this:

  1) different system call numbers

or 

  2) different gateway page for the system calls (e.g., 0xc0000000 for
     HP-UX, and 0xc0000040 for Linux).  (Note: I don't recall the exact
     digits HP-UX uses...the values above are for illustrative purposes)
     (The kernel could check to see which address had been used for the
     call)

Without such a mechanism, you can't tell the system calls apart.

So, the only unanswered question is: is HP-UX compatibility desired?  
If the answer is "yes", the subsequent question (how) was answered above.

-- 
Stan Sieler                                          sieler@allegro.com
                                         http://www.allegro.com/sieler/