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From: mybg@doc.ic.ac.uk (M Y Ben Gershon)
Newsgroups: comp.sys.acorn
Subject: Re: C release 3 preprocessor bug ??
Message-ID: <1991May16.125325.15323@doc.ic.ac.uk>
Date: 16 May 91 12:53:25 GMT
Organization: Department of Computing, Imperial College, University of London, UK.
Lines: 31
Nntp-Posting-Host: swan.doc.ic.ac.uk


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
page 406 of the manual, which describes the use of a pragma to effectively
give user-defined non-serious errors?  I have just noticed it, and find it
very useful!

This behaviour seems to be highly non-standard.  I discovered it when
working on a port from Unix, and it took a long time until I realised that a
preprocessor macro was defined, but equal to zero - which the Acorn compiler
seems to treat differently to an undefined macro.  Does anyone from Acorn
wish to comment on this?

Michael Ben-Gershon

mybg@doc.ic.ac.uk