fix article

homepage/knowledge/c78.html

@@ -48,38 +48,56 @@ This page documents those incompatibilities and relative oddities.
 I have never done (and probably never will do) extensive programming in pre-ANSI C.
 These incompatibilities were discovered out of Appendix C in <I>The C Programming Language, 2nd ed.</I> but are described further.
 </P>
-<!--<H2 ID="octal89"><CODE>8</CODE> and <CODE>9</CODE> as valid octal digits</H2>
+<H2 ID="octal89"><CODE>8</CODE> and <CODE>9</CODE> as valid octal digits</H2>
+<UL>
+<LI><A HREF="https://minnie.tuhs.org/cgi-bin/utree.pl?file=V6/usr/source/c">UNIX V6 C compiler source directory</A></LI>
+</UL>
 <PRE>
 main()
 {
-	int i;
+	printf("%d\n%d\n%d\n%d\n%d\n%d\n%d\n%d\n%d\n%d\n",
-	for(i = 0; i &lt; 10; ++i)
+		00, 01, 02, 03, 04, 05, 06, 07, 08, 09
-		printf("%c\t%o\n", i + '0', i);
+	);
 }
 </PRE>
 <P>
-In pre-ANSI C, <CODE>8</CODE> and <CODE>9</CODE> were valid octal digits corresponding to <CODE>10</CODE> and <CODE>11</CODE>.
+On my machine, the GNU C compiler emits the following errors (this omits warnings) for the preceding piece of code:
+</P>
+<PRE>
+snippet.c:4:49: error: invalid digit "8" in octal constant
+    4 |                 00, 01, 02, 03, 04, 05, 06, 07, 08, 09
+      |                                                 ^~
+snippet.c:4:53: error: invalid digit "9" in octal constant
+    4 |                 00, 01, 02, 03, 04, 05, 06, 07, 08, 09
+      |                                                     ^~
+</PRE>
+<P>
+This is because in C (both pre-standardization and post-ANSI) integer constants with leading zeroes are parsed as octal (base 8) numbers.
+In pre-ANSI C, <CODE>8</CODE> and <CODE>9</CODE> were valid octal digits corresponding to <CODE>010</CODE>(b8) and <CODE>11</CODE>(0b8).
 This is documented in <I>The C Programming Language</I>, Appendix A, subsection 2.4.1, and evidenced by the preceding code block.
 The following is output of the compiled program in UNIX V6 on an emulated PDP-11:
 </P>
 <PRE>
-0       0
+0
-1       1
+1
-2       2
+2
-3       3
+3
-4       4
+4
-5       5
+5
-6       6
+6
-7       7
+7
-8       10
+8
-9       11
+9
 </PRE>
 <P>
 This behavior exists within <CODE>/usr/source/c/c0t.s</CODE>, which is a PDP-11 assembler file that parses integer constants (as far as I can tell).
 This file provides <CODE>getnum</CODE>, a function used in <CODE>/usr/source/c/c00.c</CODE>.
-My theory is that this behavior is a side-effect of a very efficient but imperfect method of parsing integer constants from a file stream,
+My theory is that this behavior is a side-effect of a very efficient but imperfect method of parsing integer constants from a file stream.
-and that this implementation was "good enough" and informally the behavior of a C compiler supplied invalid octal constants was just undefined.
+</P>
+<P>
+This wouldn't be a significant or even noticeable error; most programmers wouldn't use <CODE>8</CODE> or <CODE>9</CODE> as octal digits anyway.
+If not for its documentation within <I>The C Programming Language</I> it would probably be an obscure bug.
+This is also mentioned in <I>The C Programming Language, 2nd ed.</I> in Appendix C.
 </P>
--->
 </BODY>
 </HTML>