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	Thu, 9 Jul 1998 07:30:53 +1000
Date: Thu, 9 Jul 1998 07:30:53 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199807082130.HAA14372@godzilla.zeta.org.au>
To: freebsd-bugs@FreeBSD.ORG, r.carey@dcs.napier.ac.uk, rotel@indigo.ie
Subject: mount, floating point
Sender: owner-freebsd-bugs@FreeBSD.ORG
Precedence: bulk
X-Loop: FreeBSD.org

>> 1) "mount_msdos /dev/fd0a /floppy/"

Don't do that.  Use `mount -t msdos ...'.  mount(8) calls realpath()
to canonicalize the path, while mount_msdos(8) passes the path unchanged
to mount(2).  The path is stored by the kernel and if it is non-canonical
then it may confuse things later.

>> 2) "umount /floppy/": not currently mounted
>> 3) "umount /floppy": not currently mounted

It does confuse things later :-).  I think (2) should work but (3)
shouldn't.  umount already does one stat() too many, at least for
umount -f (the stat() may hang on nfs mount points before giving
unmount() a chance).  realpath() would be more likely to hang.

>> typedef double REAL;
>> # define  DIG DBL_DIG
>
>The bug is in your understanding of meaning of the DBL_DIG macro;
>see http://www.dinkumware.com/htm_cl/float.html.  Change the above
>to:
>
>#define DIG DBL_MANT_DIG
>
>and try again.

DBL_MANT_DIG is irrelevant (it's for base 2).  Try (DBL_DIG + 2).
Assuming that printf gets this right (I think it does) then the exact
value is d = 2.999...5x... where `5x' occurs DBL_DIG = 15 digits to the
right of the decimal point.  l = (unsigned)d is 2 because the assignment
rounes towards 0, while printf in %.15f format gives 3.000... because
printf rounds to neares and the value is a bit closer to precisely 3
than it is to 2.999...999 (DBL_DIG 9's).

Bruce

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