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Re: [PATCH] PR85678: Change default to -fno-common

Hi Martin,

> I've noticed quite significant package failures caused by the revision.

How significant? Is it mostly the common mistake of forgetting extern?

> Would you please consider documenting this change in porting_to.html
> (and in changes.html) for GCC 10 release?

Sure, I already had a patch for changes.html - I've added an initial porting_to
as well:

[wwwdocs] Document -fcommon default change

Add an entry for the default change. Passes the W3 validator.

--
diff --git a/htdocs/gcc-10/changes.html b/htdocs/gcc-10/changes.html
index f0f0d312171a54afede176f06ce76f9c8abaebc4..980e4e591781d04aa888ba5988981006bd30dd1f 100644
--- a/htdocs/gcc-10/changes.html
+++ b/htdocs/gcc-10/changes.html
@@ -47,6 +47,13 @@ a work-in-progress.</p>
 
 <!-- .................................................................. -->
 <h2 id="general">General Improvements</h2>
+<p>The following GCC command line options have been introduced or improved.</p>
+<ul>
+  <li>GCC now defaults to <code>-fno-common</code>.  In C, global variables with
+      multiple tentative definitions will result in linker errors.
+      Global variable accesses are also more efficient on various targets.
+  </li>
+</ul>
 
 <p>The following built-in functions have been introduced.</p>
 <ul>
diff --git a/htdocs/gcc-10/porting_to.html b/htdocs/gcc-10/porting_to.html
new file mode 100644
index 0000000000000000000000000000000000000000..2e652f6aa4bd3259a316af0c72ab7eb96bab53b7
--- /dev/null
+++ b/htdocs/gcc-10/porting_to.html
@@ -0,0 +1,65 @@
+<!DOCTYPE html>
+<html lang="en">
+
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+<title>Porting to GCC 10</title>
+<link rel="stylesheet" type="text/css" href="https://gcc.gnu.org/gcc.css"; />
+</head>
+
+<body>
+<h1>Porting to GCC 10</h1>
+
+<p>
+The GCC 10 release series differs from previous GCC releases in
+<a href="changes.html">a number of ways</a>. Some of these are a result
+of bug fixing, and some old behaviors have been intentionally changed
+to support new standards, or relaxed in standards-conforming ways to
+facilitate compilation or run-time performance.
+</p>
+
+<p>
+Some of these changes are user visible and can cause grief when
+porting to GCC 10. This document is an effort to identify common issues
+and provide solutions. Let us know if you have suggestions for improvements!
+</p>
+
+
+<!--
+<h2 id="cpp">Preprocessor issues</h2>
+-->
+
+<h2 id="c">C language issues</h2>
+
+<h3 id="complit">Default to <code>-fno-common</code></h3>
+
+<p>
+  A common mistake in C is omitting <code>extern</code> when declaring a global
+  variable in a header file.  If the header is included by several files it
+  results in multiple definitions of the same variable.  In previous GCC
+  versions this error is ignored.  GCC 10 defaults to <code>-fno-common</code>,
+  which means a linker error will now be reported.
+  To fix this, use <code>extern</code> in header files when declaring global
+  variables, and ensure each global is defined in exactly one C file.
+  As a workaround, legacy C code can be compiled with <code>-fcommon</code>.
+</p>
+  <pre><code>
+      int x;  // tentative definition - avoid in header files
+
+      extern int y;  // correct declaration in a header file
+  </code></pre>
+
+<!--
+<h2 id="cxx">C++ language issues</h2>
+-->
+
+<!--
+<h2 id="fortran">Fortran language issues</h2>
+-->
+ 
+<!--
+<h2 id="links">Links</h2>
+-->
+
+</body>
+</html>
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