Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!uunet!mcsun!ukc!edcastle!gtoal
From: gtoal@castle.ed.ac.uk (G Toal)
Newsgroups: comp.sys.acorn
Subject: Re: C release 3 preprocessor bug ??
Message-ID: <10383@castle.ed.ac.uk>
Date: 18 May 91 20:45:10 GMT
References: <1991May16.125325.15323@doc.ic.ac.uk>
Organization: Edinburgh University
Lines: 36

In article <1991May16.125325.15323@doc.ic.ac.uk> mybg@doc.ic.ac.uk (M Y Ben Gershon) writes:
>
>Am I crazy, or is there a subtle bug in C release 3?  I am surprised that
>nobody has mentioned it so far, but the ANSI standard, as far as I can
>gather, states that if a preprocessor macro is undefined, it is deemed to
>have a value of 0 (in fact 0L, to be precise).  According to the latest K&R,
>(which is the ANSI version), and Harbison & Steele, a macro defined as being
>0 is identical to a macro which is undefined, as above.  However, Cv3 treats
>them differently:  If a macro is defined as:
>
>    #define VAX 0
>
>and we then do:
>
>    #ifdef VAX
>    #error VAX is defined!
>    #endif
>
>the error message will be displayed.  By the way, who noticed the section on

Which reminds me of a general point when porting programs - for progs
with lots of target machines, some authors tend to use #if rather than
#ifdef so that they cal say things like '#if VAX || (UNIX && BSD42)'
-- so they have a header file where *all* the possible symbols are
defined - most as 0, one as 1.  Now a modern ANSI C programmer
comes along to port it, and accidentally puts in a '#ifdef VAX' (or
more likely '#ifndef VAX') and is suprised when the wrong thing happens.
It is *very* hard not to accidentally use ifdef in these situations - the
supplied '#if' style being a bit non-obvious.

Nowadays I use '#if defined(VAX) || defined(BSD42)'
almost exclusively. Much safer.  But of course, old compilers will
treat expressions with 'defined()' in them as an error and assume
the expression is FALSE. ho hum.

Graham