[TUHS] nm on Third Edition .o files?'

Steve Johnson scj at yaccman.com
Wed Dec 21 06:23:44 AEST 2016

I had a funny and somewhat embarrassing experience around the whole
issue of structure name spaces.  When types were spreading through C,
we had a lot of discussion about whether we should require all
programs to call structures with a structure pointer (which would
allow a separate name space for structure members of each
structure).   We all thought it was a good idea, but the problem was
there was a lot of code out there that didn't conform, including a lot
of OS code.  As we started to port Unix to the Interdata (a 32-bit
machine), the situation became critical.  So I took a deep breath and
implemented the following: the compiler would use the old style
structure rules until I saw a "new style" structure reference in the
code.  At that point I would start to enforce the new rules, which
involved translating the symbol table, and starting to complain if the
types didn't match.   This was a very ugly hack, and I had inserted
many firewalls and assertions to check that the data structures were
correct after this translation.

Now, a word about error messages.  The PDP-11 didn't have much
memory.  But at the same time it didn't have much of a debugger in
those days.  So the main aim was to identify what went wrong in a
small number of characters so you could put in print statements and
run it again.  This meant that the message produced had to be short
but also unique.  I was producing error checks at a great rate in
this code and running out of synonyms for "corrupted" in my messages.

I "published" the code, and everything seemed to be working for a
couple of days.  Then Ken, who was working on the Interdata file
system, tried to compile a structure declaration that contained a
sizeof of a structure that was recursively declared inside of the
sizeof, and my code gave up the ghost.  Ken came into my office with
a strange look on his face and asked "What is a gummy structure?"

----- Original Message -----
 "Clem Cole" <clemc at ccc.com>

"Larry McVoy" <lm at mcvoy.com>
"TUHS main list" <tuhs at minnie.tuhs.org>, "Noel Chiappa"
<jnc at mercury.lcs.mit.edu>
Tue, 20 Dec 2016 11:21:41 -0500
Re: [TUHS] nm on Third Edition .o files?'

On Tue, Dec 20, 2016 at 11:09 AM, Larry McVoy <lm at mcvoy.com [1]>

I get that it doesn't scale but man, so nice when reading
 code to know that s.st_size is a stat structure.



[1] mailto:lm at mcvoy.com

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