From a13fbc7d6de0022ea15f3663c47af8911aa294cd Mon Sep 17 00:00:00 2001
From: Deven Blake <trinity@trinity.moe>
Date: Tue, 31 May 2022 21:24:59 -0400
Subject: [PATCH] fix article

---
 homepage/knowledge/c78.html | 54 ++++++++++++++++++++++++-------------
 1 file changed, 36 insertions(+), 18 deletions(-)

diff --git a/homepage/knowledge/c78.html b/homepage/knowledge/c78.html
index 2a704fd..dffc9f5 100644
--- a/homepage/knowledge/c78.html
+++ b/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;
-	for(i = 0; i &lt; 10; ++i)
-		printf("%c\t%o\n", i + '0', i);
+	printf("%d\n%d\n%d\n%d\n%d\n%d\n%d\n%d\n%d\n%d\n",
+		00, 01, 02, 03, 04, 05, 06, 07, 08, 09
+	);
 }
 </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
-1       1
-2       2
-3       3
-4       4
-5       5
-6       6
-7       7
-8       10
-9       11
+0
+1
+2
+3
+4
+5
+6
+7
+8
+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,
-and that this implementation was "good enough" and informally the behavior of a C compiler supplied invalid octal constants was just undefined.
+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.
+</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>
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