holes in files

[David Burren]

Where can I find mention of the implementation of "holes" in files under
the BSD ffs or other filesystems?
Do I guess and assume that the relevant block pointer(s) have some
sentinel value (eg. 0) to flag the fact that there is no data?
It seems the logical explanation, but I'm surprised I haven't found it
mentioned (or am I blind?).

I've read McKusick, et. al, "A Fast File System for UNIX" and Leffler, et. al.
"The Design and Implementation of the 4.3BSD UNIX Operating System", but
have found nary a mention of holes.

One of the excercises at the end of the filesystem chapter in Leffler et. al.
mentions them, but that's the only lead.

I'd appreciate any pointers towards a definite answer on this.

BTW, under some OSes I've seen processes with sparse address maps produce
core dumps with holes in them.  I once had a user ask me "how can I make my
file a core file?"  He was a student trying to get around quotas....
_____________________________________________________________________________
David Burren [Athos]                          Email: david at bacchus.esa.oz.au
Software Development Engineer                 Phone: +61 3 819 4554
Expert Solutions Australia, Hawthorn, VIC     Fax:   +61 3 819 5580