Re: [9fans] Include guards and multiple includes

[geoff@plan9.bell-labs.com]

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.