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From: karzes@mfci.UUCP (Tom Karzes)
Newsgroups: comp.lang.c
Subject: cpp behavior
Message-ID: <1077@m3.mfci.UUCP>
Date: 12 Oct 89 17:48:47 GMT
Sender: karzes@mfci.UUCP
Reply-To: karzes@mfci.UUCP (Tom Karzes)
Organization: Multiflow Computer Inc., Branford Ct. 06405
Lines: 85

I have a couple of questions about correct cpp behavior.  I have tried
the following test on several different machines with pcc-derived compilers
(and presumably standard cpp's as well), and all give the same results:

% cat test1
#define M1()+++
M1()
#define M2+++
M2
#define M3()===
M3()
#define M4===
M4
% /lib/cpp test1
# 1 "test1"

+++

++

===

=
% 


The results for M1 and M3 were what I expected, but I also would have
expected the results for M2 to be identical to the results for M1, and
the results for M4 to be identical to the results for M3.  Note that
there appears to be something special about =, since cpp eats two of
them.  For other characters it only eats one.  But why should it eat
any?  Please try this on your own machine before responding (if for
no other reason than to see if it does the same thing).


Here's a second test, which again gives the same results on several
different machines:

% cat test2
#define M1 123
#if M1 == 123
xxx
#endif

#define M2() 456
#if M2() == 456
yyy
#endif

#define M3(x) x
#if M3(789) == 789
zzz
#endif
% /lib/cpp test2
# 1 "test2"


xxx



test2: 9: syntax error
 456 == 456
yyy
#endif


test2: 14: syntax error
 789 == 789
zzz
#endif
test2: 15: missing endif
% 


The M1 case did what I expected, i.e., it expanded M1, then evaluated
123 == 123, which resulted in a value of 1, so xxx was passed through.
However, for both the M2 and M3 cases it appears that the macro was
expanded, but rather than consuming the results, it instead placed
them into the output, and forgot that it was processing an #if directive.
This seems like a clear bug to me.  Even if there is some rule about only
using unparameterized macros in #if expressions (which seems stupid),
I would have expected a sane error message.  Please try this on your own
machine before responding (if for no other reason than to see if it does
the same thing).