The following code :

	#define hash_hash # ## #
	#define mkstr(a) # a
	#define in_between(a) mkstr(a)
	#define join(c, d) in_between(c hash_hash d)

	char p[] = join(x, y);

should produce :

	char p[] = "x ## y";

instead of :

	char p[] = "xy";

I think stringify() is called too many. That's why I propose this fix.
However, even though it fixes this problem, I cannot tell you of any collateral damages ;)

Phil;

Reference: /n/sources/patch/applied/cpp-bad-stringify
Date: Fri Nov 30 10:45:46 CET 2007
Signed-off-by: xigh@free.fr

--- /sys/src/cmd/cpp/cpp.c	Fri Nov 30 10:40:28 2007
+++ /sys/src/cmd/cpp/cpp.c	Fri Nov 30 10:40:25 2007
@@ -69,7 +69,7 @@
 			trp->tp += 1;
 			control(trp);
 		} else if (!skipping && anymacros)
-			expandrow(trp, NULL);
+			expandrow(trp, NULL, 0);
 		if (skipping)
 			setempty(trp);
 		puttokens(trp);
@@ -213,7 +213,7 @@
 
 	case KLINE:
 		trp->tp = tp+1;
-		expandrow(trp, "<line>");
+		expandrow(trp, "<line>", 0);
 		tp = trp->bp+2;
 	kline:
 		if (tp+1>=trp->lp || tp->type!=NUMBER || tp+3<trp->lp
--- /sys/src/cmd/cpp/cpp.h	Fri Nov 30 10:40:34 2007
+++ /sys/src/cmd/cpp/cpp.h	Fri Nov 30 10:40:31 2007
@@ -109,11 +109,11 @@
 void	doadefine(Tokenrow *, int);
 void	doinclude(Tokenrow *);
 void	doif(Tokenrow *, enum kwtype);
-void	expand(Tokenrow *, Nlist *);
+void	expand(Tokenrow *, Nlist *, int);
 void	builtin(Tokenrow *, int);
 int	gatherargs(Tokenrow *, Tokenrow **, int, int *);
 void	substargs(Nlist *, Tokenrow *, Tokenrow **);
-void	expandrow(Tokenrow *, char *);
+void	expandrow(Tokenrow *, char *, int);
 void	maketokenrow(int, Tokenrow *);
 Tokenrow *copytokenrow(Tokenrow *, Tokenrow *);
 Token	*growtokenrow(Tokenrow *);
--- /sys/src/cmd/cpp/eval.c	Fri Nov 30 10:40:41 2007
+++ /sys/src/cmd/cpp/eval.c	Fri Nov 30 10:40:37 2007
@@ -116,7 +116,7 @@
 	}
 	ntok = trp->tp - trp->bp;
 	kwdefined->val = KDEFINED;	/* activate special meaning of defined */
-	expandrow(trp, "<if>");
+	expandrow(trp, "<if>", 0);
 	kwdefined->val = NAME;
 	vp = vals;
 	op = ops;
--- /sys/src/cmd/cpp/include.c	Fri Nov 30 10:40:47 2007
+++ /sys/src/cmd/cpp/include.c	Fri Nov 30 10:40:43 2007
@@ -18,7 +18,7 @@
 		goto syntax;
 	if (trp->tp->type!=STRING && trp->tp->type!=LT) {
 		len = trp->tp - trp->bp;
-		expandrow(trp, "<include>");
+		expandrow(trp, "<include>", 0);
 		trp->tp = trp->bp+len;
 	}
 	if (trp->tp->type==STRING) {
--- /sys/src/cmd/cpp/macro.c	Fri Nov 30 10:40:54 2007
+++ /sys/src/cmd/cpp/macro.c	Fri Nov 30 10:40:50 2007
@@ -138,7 +138,7 @@
  * Flag is NULL if more input can be gathered.
  */
 void
-expandrow(Tokenrow *trp, char *flag)
+expandrow(Tokenrow *trp, char *flag, int inmacro)
 {
 	Token *tp;
 	Nlist *np;
@@ -170,7 +170,7 @@
 		if (np->flag&ISMAC)
 			builtin(trp, np->val);
 		else {
-			expand(trp, np);
+			expand(trp, np, inmacro);
 		}
 		tp = trp->tp;
 	}
@@ -184,7 +184,7 @@
  * (ordinarily the beginning of the expansion)
  */
 void
-expand(Tokenrow *trp, Nlist *np)
+expand(Tokenrow *trp, Nlist *np, int inmacro)
 {
 	Tokenrow ntr;
 	int ntokc, narg, i;
@@ -214,7 +214,8 @@
 			dofree(atr[i]);
 		}
 	}
-	doconcat(&ntr);				/* execute ## operators */
+	if(!inmacro)
+		doconcat(&ntr);				/* execute ## operators */
 	hs = newhideset(trp->tp->hideset, np);
 	for (tp=ntr.bp; tp<ntr.lp; tp++) {	/* distribute hidesets */
 		if (tp->type==NAME) {
@@ -359,7 +360,7 @@
 				insertrow(rtr, 1, atr[argno]);
 			else {
 				copytokenrow(&tatr, atr[argno]);
-				expandrow(&tatr, "<macro>");
+				expandrow(&tatr, "<macro>", 1);
 				insertrow(rtr, 1, &tatr);
 				dofree(tatr.bp);
 			}
@@ -452,7 +453,7 @@
 			error(ERROR, "Stringified macro arg is too long");
 			break;
 		}
-		if (tp->wslen && (tp->flag&XPWS)==0)
+		if (tp->wslen) // && (tp->flag&XPWS)==0)
 			*sp++ = ' ';
 		for (i=0, cp=tp->t; i<tp->len; i++) {	
 			if (instring && (*cp=='"' || *cp=='\\'))