[TUHS] Mac OS X is Unix

Dan Cross crossd at gmail.com
Wed Jan 4 13:50:18 AEST 2017

On Tue, Jan 3, 2017 at 3:19 PM, Doug McIlroy <doug at cs.dartmouth.edu> wrote:

> > keeping the code I work on portable between Linux and the Mac requires
> > more than a bit of ‘ifdef’ hell.
> Curmudgeonly comment: I bristle at the coupling of "ifdef" and "portable".
> Ifdefs that adjust code for different systems are prima facie
> evidence of NON-portability. I'll buy "configurable" as a descriptor
> for such ifdef'ed code, but not "portable".

Here, here.

And, while I am venting about ifdef:
> As a matter of sytle, ifdefs are global constructs. Yet they often
> have local effects like an if statement. Why do we almost always write
> #ifdef LINUX
>         linux code
> #else
>         default unix code
> #endif
> instead of the much cleaner
>         if(LINUX)
>                 linux code
>         else
>                 default unix code
> In early days the latter would have cluttered precious memory
> with unfreachable code, but now we have optimizing compilers
> that will excise the useless branch just as effectively as cpp.

Interesting, but I'm curious how this would work in the context of C (or a
C-like variant)? The code must parse and type-check in accordance with the
existing standard, no? So if the `if(LINUX)` branch referred to, say,
Linux-specific structure members, then how would the compiler recognize
that avoid spitting out a diagnostic/erroring out? The existing C language
seems defined to expressly disallow this sort of thing.

What might be interesting would be an additional lexical token that would
tag a particular expression as, "compile-time evaluated" with the
consequence that the compiler would ignore the following statement or block
if it appeared in a conditional (or something along those lines). It would
be an error if such a conditional did not evaluate to a compile-time
constant. So one might say something along the lines of, `if #(LINUX) { ...
}` (or whatever syntax was reasonable and allowable with respect to the
existing language) and this would be ignored by the compiler on systems for
which LINUX evaluated to logical false (0 or whatever the kids call it
these days). I imagine this would open other cans of worms, though.

Much as the trait of overeating has been ascribed to our
> hunter ancestors' need to eat fully when nature provided,
> so overuse of ifdef echos coding practices tuned to
> the capabilities of bygone computing systems.
> "Ifdef hell" is a fitting image for what has to be one of
> Unix's least felicitous contributions to computing. Down
> with ifdef!

I've learned two hard lessons about #ifdef.

First, one should strive to avoid them by programming to an abstract
interface, with "system" specific implementations of that interface that
are selected by the build system (make, mk, whatever). I've further found
that one rarely needs complicated shell scripts for things like that.

Second, if one cannot avoid them, one should #ifdef around a particular
feature, not a system. For example, instead of `#if defined(__linux__) ||
defined(__FreeBSD__) || defined(__OpenBSD__) || ... // Use mmap()` all over
the place, one should just, `#ifdef PROJECT_USE_MMAP` and `#define
PROJECT_USE_MMAP` for one's project somewhere (possibly based on system
there or whatever). Then adding a new system is relatively easier: one
defines a set of symbols for the new system, instead of trying to find
every place where __linux__ is tested and adding to that.

Ideally, one would avoid #if, though.

        - Dan C.
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