From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <91b5ebacd2423c345a89544c3d45ed34@plan9.bell-labs.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] Include guards and multiple includes
Date: Mon, 23 Oct 2006 16:23:03 -0400
From: geoff@plan9.bell-labs.com
In-Reply-To: <2ce618c636a8a74af13258cdeaa93de0@plan9.jp>
MIME-Version: 1.0
Content-Type: text/plain; charset="US-ASCII"
Content-Transfer-Encoding: 7bit
Topicbox-Message-UUID: d3598e4c-ead1-11e9-9d60-3106f5b1d025

I'd think the set of cpp keywords needed would be: ifdef, ifndef,
else, endif, define, undef, and include.  `line' is typically
generated by other preprocessors (e.g., yacc) not by people.  `pragma'
is compiler-dependent, and we have uses for it on Plan 9, but most
uses are in system-supplied headers files, not ones you'd write;
`#pragma incomplete' is an exception.

The main use we advocated in the paper for conditional compilation was
providing default values, thus:

#ifndef COPYSIZE
#define COPYSIZE 8192
#endif

and Henry Spencer suggested that this idiom might have been profitably
bundled up as `#override symbol value' (`#ifndefdef' in the paper).

The paper dates from an era of considerable diversity among Unix
systems and the beginnings of migration to ANSI/ISO C and POSIX, so it
was a time of transition.  These days I'm not using conditional
compilation or even cpp much.  Plan 9 has shown us better alternatives
to conditional compilation, and looking at some of my recent code, I
see that I #include headers and #define an occasional macro for some
expression and not much else.  enums have replaced #define for integer
constants, and that was always my main use for #define.