[Secure Coding] master: C: Add example for unsigned overflow check (e97e4dc)

Repository : http://git.fedorahosted.org/git/?p=secure-coding.git

On branch  : master

> ---------------------------------------------------------------
commit e97e4dc0e06ef037b2042a251c17e1f4a66ccc66
Author: Florian Weimer <fweimer(a)redhat.com&gt;
Date:   Fri Jun 6 16:49:27 2014 +0200

    C: Add example for unsigned overflow check


> ---------------------------------------------------------------
 defensive-coding/en-US/C-Language.xml              |   11 +++++++++++
 ...etic-mult.xml => C-Arithmetic-add_unsigned.xml} |    9 ++++++---
 defensive-coding/src/C-Arithmetic-add.c            |   14 ++++++++++++++
 3 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/defensive-coding/en-US/C-Language.xml
b/defensive-coding/en-US/C-Language.xml
index b1eeec0..8f6f74d 100644
--- a/defensive-coding/en-US/C-Language.xml
+++ b/defensive-coding/en-US/C-Language.xml
@@ -103,8 +103,19 @@
 	<para>
 	  Perform the calculation in the corresponding unsigned type
 	  and use bit fiddling to detect the overflow.
+	  <xref linkend="ex-Defensive_Coding-C-Arithmetic-add_unsigned"/>
+	  shows how to perform an overflow check for unsigned integer
+	  addition.  For three or more terms, all the intermediate
+	  additions have to be checked in this way.
 	</para>
       </listitem>
+    </itemizedlist>
+    <example id="ex-Defensive_Coding-C-Arithmetic-add_unsigned">
+      <title>Overflow checking for unsigned addition</title>
+      <xi:include href="snippets/C-Arithmetic-add_unsigned.xml"
+		  xmlns:xi="http://www.w3.org/2001/XInclude" />
+    </example>
+    <itemizedlist>
       <listitem>
 	<para>
 	  Compute bounds for acceptable input values which are known
diff --git a/defensive-coding/en-US/snippets/C-Arithmetic-mult.xml
b/defensive-coding/en-US/snippets/C-Arithmetic-add_unsigned.xml
similarity index 68%
copy from defensive-coding/en-US/snippets/C-Arithmetic-mult.xml
copy to defensive-coding/en-US/snippets/C-Arithmetic-add_unsigned.xml
index ecb27a0..4ea1747 100644
--- a/defensive-coding/en-US/snippets/C-Arithmetic-mult.xml
+++ b/defensive-coding/en-US/snippets/C-Arithmetic-add_unsigned.xml
@@ -3,12 +3,15 @@
 ]>
 <!-- Automatically generated file.  Do not edit. -->
 <programlisting language="C">
+void report_overflow(void);
+
 unsigned
-mul(unsigned a, unsigned b)
+add_unsigned(unsigned a, unsigned b)
 {
-  if (b &#38;&#38; a &#62; ((unsigned)-1) / b) {
+  unsigned sum = a + b;
+  if (sum &#60; a) { // or sum &#60; b
     report_overflow();
   }
-  return a * b;
+  return sum;
 }
 </programlisting>
diff --git a/defensive-coding/src/C-Arithmetic-add.c
b/defensive-coding/src/C-Arithmetic-add.c
index 3e70286..95b403e 100644
--- a/defensive-coding/src/C-Arithmetic-add.c
+++ b/defensive-coding/src/C-Arithmetic-add.c
@@ -15,3 +15,17 @@ add(int a, int b)
   return result;
 }
 //-
+
+//+ C Arithmetic-add_unsigned
+void report_overflow(void);
+
+unsigned
+add_unsigned(unsigned a, unsigned b)
+{
+  unsigned sum = a + b;
+  if (sum < a) { // or sum < b
+    report_overflow();
+  }
+  return sum;
+}
+//-