In message <199512171732.MAA06281@glacier.MIT.EDU>Greg Hudson writes
>I'd like to propose a new scheme for doing builds out of a read-only
>source tree, and make an argument for why I think it's as convenient
>as possible.  I'll talk about the userland build process first, and
>then about the kernel build process at the end.

What I do for user-land code is is:
	- make obj/ links to /usr/obj on the read-only filesystem
	  (before it becomes readonly)
	
	- create, or mount, /usr/obj as a read-write filesystem
	  wherever is convenient.

To build kernels, /usr/src/sys/arch/compile/ needs to have a read-write
filesystem mounted on it.

This model only allows one ``build tree'' (/usr/obj) per machine.

I haven't found that to be a limitation.  If it's a limitation for
others, then I think the "read-only" status is a side issue; the
scheme above would, for example, allow compiling from source on a
CD-ROM (provided of course the ./obj symlinks were on the CD-ROM).

If what Gary is really after is building multiple object trees, on
a single machine, from a single source tree, then "lndir" is one
ugly, but time-worn, solution.