[TUHS] Deleted lib1 and lib2 in v6, recoverable?

Norman Wilson norman at oclsc.org
Mon Dec 31 05:07:09 AEST 2018

Warner Losh:

  But wasn't it tsort that did the heavy lifting to get things in order?

  ar c foo.a `tsort *.o`

  Ranlib just made it fast by adding an index..


There's a little more than that to ranlib.

Without ranlib, ld made a single pass through each library,
loading the modules that resolved unresolved symbols.  If
a module itself had unresolved symbols (as many do) and
some of those symbols were defined in a module ld had
already passed, you were out of luck, unless you explicitly
told ld to run a second pass, e.g. cc x.o y.o -la -la.
Hence the importance of explicit ordering when building
the library archive, and the usefulness of tsort.

Ranlib makes a list of all the .o file in this archive and
the global symbols defined or used by each module, and
places the list first in the archive.  If ld is (as it
was) changed to recognize the index, it can then make a
list of all the object files needed from this archive, even
if needed only by some file it will load from the same
archive, then collect all required modules in a single pass.

Ld could all along have just made two passes through the
library, one to assemble the same list ranlib did in advance,
a second to load the files.  (Or perhaps a first pass to
load what it knows it needs and assemble the list, and a
second only if necessary.)  Presumably it didn't both to
make ld simpler and because disk I/O was much slower back
then (especially on a heavily-loaded time-sharing system,
something far less common today).  I suspect it would work
fine just to do it that way today.

Nowadays ranlib is no longer a separate program: ar
recognizes object files and maintains an index if any are
present.  I never especially liked that; ar is in
principle a general tool so why should it have a special
case for one type of file?  But in practice I don't know
anyone who uses ar for anything except libraries any more
(everyone uses tar for the general case, since it does a
better job).  Were I to wave flags over the matter I'd
rather push to ditch ar entirely save for compatibility
with the past, move to using tar rather than ar for object
libraries, and let ld do two passes when necessary rather
than requiring that libraries be specially prepared.  As
I say, I think modern systems are fast enough that that
would work fine.

Norman Wilson
Toronto ON

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