[TUHS] Mac OS X is Unix

Steve Johnson scj at yaccman.com
Wed Jan 4 10:13:54 AEST 2017


Following on Doug's comment, when I wrote the portable C compiler my
vision was to separate out the machine independent parts of the
compiler (e.g, the lexer and parser) from the machine dependent parts
(those parts involving stack frames, instructions, etc.).  Then to
port the compiler, you could leave the machine independent code alone
(much of which was rather hairy, involving symbol tables,
optimizations, etc.) and simply describe the instructions and calling
sequence in the machine dependent files.  The preprocessor was
actually pretty important in carrying this out, because there were a
fair number of machine characteristics ( such as byte order and word
size) that were handled identically across different architectures but
the actual values were different.  These were handled by defining
some preprocessor macros.

Several years later, I had moved to development and was responsible
for shipping a variety of different compilers, most resting on the PCC
base.  At one point, I was appalled to see that somebody had put some
code into one of the machine-independent files that was bracketed with
"# ifdef VAX".  There followed a most difficult conversation with the
perpetrator who kept insisting that, after all, the code WAS the same
on all machines...

----- Original Message -----
From: "Doug McIlroy" <doug at cs.dartmouth.edu>
To:<tuhs at minnie.tuhs.org>
Cc:
Sent:Tue, 03 Jan 2017 15:19:08 -0500
Subject:Re: [TUHS] Mac OS X is Unix

 > 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".

 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.

 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!
 Doug

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