Plan 9 had the distinct advantage of a constant system interface at the
source level. X11 did not, but it also made essentially no attempt to
abstract it away, so the lines starting #ifdef often outnumbered the actual
code. I couldn't make hide nor hair of it, and had no way to reliably test
any change.

C with #ifdefs is not portable, it is a collection of 2^n overlaid
programs, where n is the number of distinct #if[n]def tags. It's too bad
the problems of that approach were not appreciated by the C standard
committee, who mandated the #ifndef guard approach that I'm sure could
count as a provable billion dollar mistake, probably much more. The cost of
building #ifdef'ed code, especially with C++, which decided to be more
fine-grained about it, is unfathomable.

Google alone might well count for many millions of dollars in wasted
compilation equipment. I remember giving a Plan 9 demo to someone soon
after I got to Google. None of the features of the system were of interest.
The thing that astounded my audience was the ability to build the kernel on
a P90 in 20 seconds or so, and the window system in under 3. At that time,
a build of a Google server would require hours on a large distcc cluster.

I still shudder to think of it. It's worse now, of course, far worse, but
Google has far larger clusters to handle it and some improvement in
tooling. However, the #ifdefs persist.


Tom Cargill warned Bjarne about this around 1984, but the plea fell on deaf
ears.

-rob


On Mon, Feb 28, 2022 at 12:07 PM Douglas McIlroy <
douglas.mcilroy@dartmouth.edu> wrote:

> > The X11 tree was a heavily ifdef-ed.  And it needed to be, I don't have
> > an answer as to how you would reuse all that code on different hardware
> > in a better way.
>
> Plan 9 did it with #include. The name of the included file was the same for
> every architecture. Only the search path for include files changed. Done
> with
> care, this eliminates the typical upfront #ifdefs.that define constants
> and set
> flags.
>
> Other preprocessor conditionals can usually be replaced by a regular if,
> letting
> the compiler optimize away the unwanted alternative. This makes
> conditionals
> obey the scope rules of C.
>
> Doug
>