Is this correct action for the c compiler/preprocessor ??

[der Mouse]

> Consider the folowing short program.

> #define BAD_SEGMENT     29
> #define ERROR(number)   printf("Error number %d\n",number);
> main() {
>        ERROR(BAD_SEGMENT);
> }

> what should this program do ????

     This  is open  to interpretation.  All current preprocessors I know
of substitute  "number"  inside the double  quotes as  well.  Of course,
this is not serious; merely rewrite ERROR as

#define ERROR(n) printf("Error number %d\n",n);

     PS.  There really should be no  semicolon after the definition;  as
it stands the result of expanding

 ERROR(BAD_SEGMENT);

is

  printf("Error BAD_SEGMENT %d\n",BAD_SEGMENT);;

which makes a difference  if it  is the then clause of  an  if statement
without  braces (yes, Virginia,  there are people who write that sort of
statement).)

     This seems to be a  common  stumbling  block.   There  seems to  be
confusion  between   *expanding  macros*  inside  a  quoted  string  and
*replacing  macro  formals*  inside  a  quoted  string.    The  compiler
designers originally  chose (perhaps not conciously)  to replace formals
inside strings, which is in fact useful for debugging:

#define LOGINTEGER(i) printf("i = %d\n",i)

which not  only prints out the value desired but  prints  the expression
producing it, as in

 LOGINTEGER(table[index]);

which expands into

  printf("table[index] = %d\n",table[index]);

a nice feature.  Of course, the statement

 printf("LOGINTEGER(foo)\n");

will not be touched.  Macros are not expanded  inside double quotes; but
inside double quotes in the replacement string, formals  get changed  to
actuals.  This actually has more frequent use in a definition like

#define CTRLCHAR(c) ('c'&0x1f)

(used on  ASCII machines).   Here, it is necessary that c be substituted
inside the (single) quotes,  so  for uniformity if nothing else the same
should happen with double quotes.

> Is this correct ???

     Apparently it is de-facto correct.  Anyone with  a copy of the ANSI
C standard want to answer this?
-- 
					der Mouse

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