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Conference turris::decc_bugs

Title:	DEC C Problem Reporting Forum
Notice:	Report DEC C++ problems in TURRIS::C_PLUS_PLUS
Moderator:	CXXC::REPETETCHEON

Created:	Fri Nov 13 1992
Last Modified:	Fri Jun 06 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	1299
Total number of notes:	6249

1267.0. "POSSIBLE DEC C PREPROCESSOR BUG (OPENVMS)" by TAV02::MILCHIN () Tue Feb 25 1997 02:37

	HI!

	It seems that DEC C preprocessor (OPENVMS VAX and ALPHA) has a problem
with macro expansion. If the macro definition contains another macro
on its right side, the result of the right-side macro expansion string
is padded by a single-blank.
	Look at the following example:

#define MMMDEF() M
#define MMM(Z) MMMDEF()Z

int main() {
 int i;
 extern int MMM(_a)(int);
 i = MMM(_a)(5);
 return 1;
}

	$ CC/PREPROC/DEFINE=M=ABC A

# 1 "TSC:[MICHAEL]A.C;8"



int main() {
 int i;
 extern int ABC _a(int);
 i = ABC _a(5);
 return 1;
}

	Indeed, the compilation will produce errors of the type %CC-E-NOSEMI,...
	The problem disappears when compiling with /STANDARD=VAXC, but this
work-arround does not applicable to the customer.
(DEC C V5.5 for OPENVMS VAX and ALPHA).

		MICHAEL

T.R	Title	User	Personal Name	Date	Lines
1267.1	##	MUCTEC::BECKER	Hartmut B., VMS & Languages, Munich	`Tue Feb 25 1997 04:35`	23
	The ANSI C preprocessor has to tokenize the input stream into C language tokens. That is it places white spaces between the recognized tokens. Hence a concatenation with you construct isn't possible. It works with pre-ANSI preprocessors as vaxc. This was mentioned before in this conference and the DECC notes conference. There are also a lot of examples how to do token concatenation with macros in ANSI C. This is done with the ## operator which is evaluated in macro replacements. I suggest #if __STDC__ #define _MMMDEF(a,b) a##b #define MMMDEF(M,Z) _MMMDEF(M,Z) #define MMM(Z) MMMDEF(M,Z) #else #define MMMDEF() M #define MMM(Z) MMMDEF()Z #endif The additional macro is necessary to have M expanded before it is passed to the real concatenation, the ##. Hope this helps, Hartmut
1267.2		TAV02::MILCHIN		`Tue Feb 25 1997 09:22`	5
	RE -.1: Thank you for your quick and clear reply. I passed this information to the customer and he was satisfied. MICHAEL
1267.3	Avoid _MMMDEF, squirrelly __STDC__	CXXC::REPETE	Rich Peterson 381-1802 ZKO2-3/N30	`Wed Feb 26 1997 13:59`	24
	RE .1: > #if __STDC__ > #define _MMMDEF(a,b) a##b > #define MMMDEF(M,Z) _MMMDEF(M,Z) Hartmut's answer was great, but I'd add two little cautions to it: 1. The helper macro _MMMDEF should be named something else. An identifier beginning with an underscore followed by an uppercase letter (or another underscore) is reserved to the C language implementation and should never by declared or defined as a macro in a user's source code. 2. The test "#if __STDC__" is equivalent to "#if __STDC__ != 0". This will be TRUE for a strictly-conforming compiler mode (in which it is required that __STDC__ == 1), and also for a huge number of situations which are undefined by the standard (where the value of the macro is nonzero but not 1). In particular, it will be FALSE if __STDC__ is defined with a value of 0 - but defining __STDC__ to 0 just happens to be a relatively common practice among UNIX compilers (including Digital UNIX and System V compilers) to indicate a relaxed-ANSI mode, which normally means that K&R-style token pasting will not be done. So a better test in this case would be "#if defined (__STDC__)".