PATH_MAX is not exactly portable. Hack around it for now.

A reference to an invalid macro argument (\ not followed by a digit
between 1 and 9) will cause an access outside of the bounds of the
currentmacroargs array in sym_FindMacroArg().

Macro arg references are processed in two places:

In CopyMacroArg(): called when scanning tokens between "", {} and
arguments of a macro call. The only problem here is that it accepts \0
as valid and so calls sym_FindMacroArg with a invalid value.

In PutMacroArg(): called by the lexer automata when it encounters a
token matching \\[0-9]? (in other cases than above). So not only it
accepts \0 but also \ alone.
  Memo: In setuplex(), a rule is defined with a regex composed of up to
    three ranges of chars and takes the form:
      [FirstRange]
      or [FirstRange][SecondRange]?
      or [FirstRange]([SecondRange][Range]*)?
    On scanning, when several rules match, the first longuest one is
    choosen.

Regression test:
1)
SECTION "HOME", HOME
	db "\0"

2)
SECTION "HOME", HOME
	db \A

3)
SECTION "HOME", HOME
	db \

Makefile

@@ -79,7 +79,7 @@ rgbfix: ${rgbfix_obj}
 .c.o:
 	$Q${CC} ${REALCFLAGS} -c -o $@ $<
 
-src/asm/gameboy/locallex.c src/asm/globlex.c src/asm/lexer.c: src/asm/asmy.h
+src/asm/gameboy/locallex.o src/asm/globlex.o src/asm/lexer.o: src/asm/asmy.h
 src/asm/asmy.h: src/asm/asmy.c
 
 src/asm/asmy.y: ${yacc_pre}

src/asm/fstack.c

@@ -19,6 +19,10 @@
 #include "extern/err.h"
 #include "extern/strl.h"
 
+#ifndef PATH_MAX
+#define PATH_MAX 256
+#endif
+
 /*
  * RGBAsm - FSTACK.C (FileStack routines)
  *

src/asm/globlex.c

@@ -208,20 +208,30 @@ PutMacroArg(char *src, ULONG size)
 	char *s;
 
 	yyskipbytes(size);
+	if ((size == 2 && src[1] >= '1' && src[1] <= '9')) {
 		if ((s = sym_FindMacroArg(src[1] - '0')) != NULL) {
 			yyunputstr(s);
 		} else {
 			yyerror("Macro argument not defined");
 		}
+	} else {
+		yyerror("Invalid macro argument");
+	}
 	return (0);
 }
 
 ULONG 
 PutUniqueArg(char *src, ULONG size)
 {
+	char *s;
+
 	src = src;
 	yyskipbytes(size);
-	yyunputstr(sym_FindMacroArg(-1));
+	if ((s = sym_FindMacroArg(-1)) != NULL) {
+		yyunputstr(s);
+	} else {
+		yyerror("Macro unique label string not defined");
+	}
 	return (0);
 }
 
@@ -387,7 +397,7 @@ setuplex(void)
 
 	    id = lex_FloatAlloc(&tMacroArgToken);
 	lex_FloatAddFirstRange(id, '\\', '\\');
-	lex_FloatAddSecondRange(id, '0', '9');
+	lex_FloatAddSecondRange(id, '1', '9');
 	id = lex_FloatAlloc(&tMacroUniqueToken);
 	lex_FloatAddFirstRange(id, '\\', '\\');
 	lex_FloatAddSecondRange(id, '@', '@');

src/asm/lexer.c

@@ -437,7 +437,6 @@ CopyMacroArg(char *dest, size_t maxLength, char c)
 	int argNum;
 	
 	switch (c) {
-	case '0':
 	case '1':
 	case '2':
 	case '3':

src/asm/symbol.c

@@ -5,6 +5,7 @@
  *
  */
 
+#include <assert.h>
 #include <stdio.h>
 #include <string.h>
 #include <time.h>
@@ -431,6 +432,8 @@ sym_FindMacroArg(SLONG i)
 	if (i == -1)
 		i = MAXMACROARGS + 1;
 
+	assert(i-1 >= 0 &&
+	    i-1 < sizeof currentmacroargs / sizeof *currentmacroargs);
 	return (currentmacroargs[i - 1]);
 }