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From: jeff@samna.UUCP (jeff)
Newsgroups: comp.unix.i386,comp.unix.xenix,comp.unix.wizards
Subject: Re: floating point exception status not inherited by exec
Message-ID: <240@samna.UUCP>
Date: 26 Mar 90 21:39:45 GMT
References: <795@s7.Morgan.COM>
Reply-To: jeff@samna.UUCP ()
Followup-To: comp.unix.i386
Organization: Draughtsman's Contractors
Lines: 28

In article <795@s7.Morgan.COM> amull@Morgan.COM (Andrew P. Mullhaupt) writes:
>I need to preserve the state of the floating point exception mask
>across an exec(). Experiments show that the exception mask and the
>sticky bit status seems to be preserved across fork() (i.e. is
>inherited by a child) but when exec is invoked, the exceptions may
>change. This has ummm - 'unpleasant' consequences. Note that it
>is not sufficient to work at the level of SIGFPE, but it is actually
>required to specify the floating point exception mask and sticky bit
>status to different values than the (otherwise sensible) defaults.

Hmm, I'm fairly sure that the "sticky bit" ordinarily refers to the bit in
the file permissions which causes an executable to hang around in
the swap space even when it's not in use.  I don't think anyone would
want this inherited across an exec (Imagine putting the sticky bit on
your shell and having it inherited across exec's - pretty soon you'd
run out of swap as every single program in /bin, /usr/bin, /usr/ucb, etc.
gets sucked into swap).  I think you're referring to something other
than the sticky bit.

As for the exception mask, I think it's only fair to a process being
exec'd that it know that the floating point chip is in some reasonably
well-defined state when it begins execution - I'd suspect that's why
it's being re-initialized.  If you really need the exception mask to
be set a certain way, why not pass the state to the new process as
an argument and let the new process set the exception mask to the
desired value.

Jeff