stephenf@elecvax.SUN (Stephen Frede) (05/29/84)
Newsgroups: net.bugs,net.lang.c Subject: Bug (?) in C preprocessor Note the following behaviour of the C preprocessor (V7 and System V): #define FRED jim #define QUOTE(x) "x" QUOTE(FRED) ______________________ The output of the C preprocessor when fed this fragment is "FRED" Obviously, it is applying the QUOTE() macro on FRED, before re-scanning and since "FRED" is then quoted, no substitution takes place. If the order of evaluation were the other way, ie to evaluate what is inside the parentheses first, before applying the macro QUOTE to it, the result would be "jim" (which is actually what I wanted when I discovered this). I suggest that this order of evaluation is more sensible, and conforms more closely to what C itself (and almost all other programming languages) does (evaluating expressions from within the most deeply nested level of parentheses first). I will probably be changing the preprocessor here to do this, but would like to see what the net thinks first. Has anyone else come across this problem? Aside: If this change is made, then it allows constructs like #define QUOTE(x) "x" #include QUOTE(NAME.h) where NAME is defined to the preprocessor on the command line at compile time. Whether this is a terrible thing to do or not is another story. I wanted it so I could modify maketerms.c in the nroff system so that I could make terminal driving tables one at a time (a single "make" at the top of our source directory would then install a new terminal driving table in /usr/lib/terms without having to create a whole set. Also, new driving tables could then be added without modifying maketerms.c. Stephen Frede Dept. Computer Science University of NSW P.O. Box 1 Kensington 2033 AUSTRALIA ...!decvax!mulga!stephenf:elec70b
boyd@basser.UUCP (05/29/84)
No. The reference manual states:
"Each occurrance of an identifier mentioned in the
formal parameter list of the definition is replaced
by the corresponding token string from the call."
"...the replacement string is rescanned for more
defined identifiers."
"Text inside a string or character constant is not
subject to replacement."
Hence:
#define quote(x) "x"
#define gooeys sooterkin
quote(gooeys) expands to "gooeys" and not "sooterkin".
I have written a C preprocessor that adheres to the most
recent standard. The standard has these "additions":
#elif expression
and
defined(name) in #if expressions
I wouldn't advise a change to the preproccessor to do this alternate
expansion because by definition it would NOT be the C preproccessor.
---
>From the VB can of Boyd Roberts. ...!decvax!mulga!boyd:basserstephenf@elecvax.UUCP (05/29/84)
> From: boyd@basser.SUN (Boyd Roberts) > References: <204@elecvax.SUN> > Organization: Dept of C.S., University of Sydney > > No. The reference manual states: > > "Each occurrance of an identifier mentioned in the > formal parameter list of the definition is replaced > by the corresponding token string from the call." It also state that subsequent occurrences of identifiers used in ordinary "#define"s are replaced by the appropriate token-string wherever they occur. What the manual doesn't specify is any order for replacement. > "...the replacement string is rescanned for more > defined identifiers." Exactly, but which ones come first? > "Text inside a string or character constant is not > subject to replacement." The identifier is not inside a string or character constant. It happens to be inside another macro, which may introduce quotes at that point, when replacement occurs. But not until replacement occurrs. > Hence: > > #define quote(x) "x" > #define gooeys sooterkin > > quote(gooeys) expands to "gooeys" and not "sooterkin". Your "hence" does not follow at all. quote(gooeys) may first be expanded to quote(sooterkin) and thence to "sooterkin" in complete accordance with the reference manual. It's not defined. > I wouldn't advise a change to the preproccessor to do this alternate > expansion because by definition it would NOT be the C preproccessor. By definition where? Not the C reference manual. - Stephen Frede ...!decvax!mulga!stephenf:elecvax
ed@mtxinu.UUCP (05/29/84)
If you change the behaviour of the preprocessor, remember that C requires that the order of evaluation of expressions be undefined. Therefore, depending on any evaluation order will yield unportable code. -- Ed Gould ucbvax!mtxinu!ed
guy@rlgvax.UUCP (Guy Harris) (05/30/84)
> I have written a C preprocessor that adheres to the most > recent standard. The standard has these "additions": > #elif expression > and > defined(name) in #if expressions The former is very nice - I wish it were available in the Reiser "cpp" that comes with just about every UNIX under the sun. The latter is actually in the most recent standard, if the S5 man page CPP(1) is considered the most recent standard (it also documents __FILE__ and __LINE__). The parentheses are optional (shades of the "return(x)" vs. "return x" debate!) according to CPP(1). Guy Harris {seismo,ihnp4,allegra}!rlgvax!guy