minimal loss of information when deallocating inodes?

[Robert Breckinridge Beatie]

In article <981 at vsi.COM>, friedl at vsi.COM (Stephen J. Friedl) writes:
> 
> It seems to me that marking the mode field to zero is sufficient
> for this, as any other kind of file has at least one high-order
> bit set (even regular files).  This is from <sys/stat.h> from
> System V:
> 
> 	#define	S_IFMT	0170000		/* type of file		*/
> 	#define		S_IFDIR	0040000	/* directory		*/
> 	#define		S_IFCHR	0020000	/* character special	*/
> 	#define		S_IFBLK	0060000	/* block special	*/
> 	#define		S_IFREG	0100000	/* regular		*/
> 	#define		S_IFIFO	0010000	/* fifo			*/
> 
> Comments?

Not so much a comment as a question.  I was looking into this question
the other day and was not able to find a completely satisfactory answer.

According to the SVID MKNOD(BA_OS)

	The mode of the new file is initialized from the argument
	_mode_.  Where the value of _mode_ is interpreted as  follows:

		0170000 file type; one of the following:
			0010000 FIFO-special
			0020000 character-special
			0040000 directory
			0060000 block-special
			0100000 or 000000 ordinary-file
			           ^^^^^^
(Unfortunately, I wasn't able to find anything more useful.)

Now it doesn't say anything about the mode parameter being placed
directly into the di_mode field of the disk inode, but it doesn't
say that it won't either.  Well, I assumed that calling mknod with
the mode field set to 000xxxx creates a file with mode equal to
010xxxx and that inodes whose di_mode field are zero are in fact
unallocated since that seems to be the case on most systems I've
checked.  But is there any guarantee that on a "normal" system V
file system this assumption won't blow up in my face.

A pointer to the appropriate section in the SVID would be most
reassuring.  Assurances from a few wizards would be somewhat less
reassuring, but would definitely be appreciated.
-- 
Breck Beatie	    				(408)748-8649
{uunet,ames!coherent}!aimt!breck  OR  breck at aimt.uu.net
"Sloppy as hell Little Father.  You've embarassed me no end."