Hi,
>On 1/6/20 7:10 AM, Jonathan Wakely wrote:
>> GCC now defaults to <code>-fno-common</code>. As a result, global
>> variable accesses are more efficient on various targets. In C, global
>> variables with multiple tentative definitions will result in linker
>> errors.
>
> This is better. I'd also s/will/now/, since we're talking about the
> present behavior of GCC 10, not some future behavior.
Thanks for the suggestions, I've reworded it as:
GCC now defaults to <code>-fno-common</code>. As a result, global
variable accesses are more efficient on various targets. In C, global
variables with multiple tentative definitions now result in linker errors.
With <code>-fcommon</code> such definitions are silently merged during
linking.
Also changed the anchor name (I think it was a copy/paste from another entry).
Here the updated version:
diff --git a/htdocs/gcc-10/changes.html b/htdocs/gcc-10/changes.html
index
d6108269e977df2af29bd5c9149cc2136654ce05..45af7fa333cfff2155ff0346fe36855aa6ff940a
100644
--- a/htdocs/gcc-10/changes.html
+++ b/htdocs/gcc-10/changes.html
@@ -140,6 +140,13 @@ a work-in-progress.</p>
<li>In C2X mode, <code>-fno-fp-int-builtin-inexact</code> is
enabled by default.</li>
</ul></li>
+
+ <li>GCC now defaults to <code>-fno-common</code>. As a result, global
+ variable accesses are more efficient on various targets. In C, global
+ variables with multiple tentative definitions now result in linker
errors.
+ With <code>-fcommon</code> such definitions are silently merged during
+ linking.
+ </li>
</ul>
<h3 id="cxx">C++</h3>
diff --git a/htdocs/gcc-10/porting_to.html b/htdocs/gcc-10/porting_to.html
index
3256e8a35d00ce1352c169a1c6df6d8f120889ee..7d45a962d014fecece9bd52a13ca1799153672fe
100644
--- a/htdocs/gcc-10/porting_to.html
+++ b/htdocs/gcc-10/porting_to.html
@@ -29,9 +29,25 @@ and provide solutions. Let us know if you have suggestions
for improvements!
<h2 id="cpp">Preprocessor issues</h2>
-->
-<!--
<h2 id="c">C language issues</h2>
--->
+
+<h3 id="common">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="fortran">Fortran language issues</h2>
