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[wwwdocs] Add porting_to.html for GCC 5 (again)
- From: Marek Polacek <polacek at redhat dot com>
- To: GCC Patches <gcc-patches at gcc dot gnu dot org>
- Date: Wed, 14 Jan 2015 23:07:53 +0100
- Subject: [wwwdocs] Add porting_to.html for GCC 5 (again)
- Authentication-results: sourceware.org; auth=none
A few months ago I posted the "porting to" document for GCC 5.
But I never got around to commit it, so here it is again, this
time with feewing. I fixed description of GNU89 "extern inline",
added some more snippets, and mentioned -Wc9?-c??-compat warnings.
I plan to commit this tomorrow.
--- porting_to.html.mp 2014-10-22 17:25:42.122367884 +0200
+++ porting_to.html 2015-01-14 22:38:24.939867012 +0100
@@ -0,0 +1,255 @@
+<html>
+
+<head>
+<title>Porting to GCC 5</title>
+</head>
+
+<body>
+<h1>Porting to GCC 5</h1>
+
+<p>
+The GCC 5 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 in order to support new standards, or relaxed
+in standards-conforming ways to facilitate compilation or run-time
+performance. Some of these changes are not visible to the naked eye
+and will not cause problems when updating from older versions.
+</p>
+
+<p>
+However, some of these changes are visible, and can cause grief to
+users porting to GCC 5. This document is an effort to identify major
+issues and provide clear solutions in a quick and easily searched
+manner. Additions and suggestions for improvement are welcome.
+</p>
+
+<h2>C language issues</h2>
+
+<h3>Default standard is now GNU11</h3>
+
+<p>GCC defaults to <code>-std=gnu11</code> instead of <code>-std=gnu89</code>.
+This brings several changes that the users should be aware of. The following
+paragraphs describe some of these changes and suggest how to deal with them.
+
+Some users might prefer to stay with gnu89, in which case we suggest to use
+the <code>-std=gnu89</code> command-line option, perhaps by putting it in
+<code>override CFLAGS</code> or similarly in the Makefile.</p>
+
+<p>To ease the migration process, GCC offers two new warning options,
+<code>-Wc90-c99-compat</code> and <code>-Wc99-c11-compat</code>. The
+former warns about features not present in ISO C90, but present in ISO C99
+and the latter warns about features not present in ISO C99, but present in
+ISO C11. See the GCC manual for more info.</p>
+
+<h4>Different semantics for inline functions</h4>
+<p>While <code>-std=gnu89</code> employs the GNU89 inline semantics,
+<code>-std=gnu11</code> uses the C99 inline semantics. The C99 inline semantics
+requires that if a function with external linkage is declared with
+<code>inline</code> function specifier, it also has to be defined in the same
+translation unit. Consequently, GCC now warns if it sees a TU such as the
+following:</p>
+
+<pre><code>
+ inline int foo (void);
+</code></pre>
+
+<p>This example now gives the following diagnostic:</p>
+
+<pre>
+<b>f.c:1:12:</b> <b style='color:magenta'>warning:</b> inline function <b>'foo'</b> declared but never defined
+ inline int foo (void);
+ <b style='color:lime'>^</b>
+</pre>
+
+<p>Furthermore, there is a difference between <code>extern inline</code> and
+<code>inline</code>:
+<ul>
+ <li>C99 <code>inline</code>: no externally visible function is generated;
+ if the function is referenced in this TU, external definition has to
+ exist in another TU; same as GNU89 <code>extern inline</code> with no
+ redefinition;</li>
+ <li>C99 <code>extern inline</code>: externally visible function is generated;
+ same as GNU89 <code>inline;</code></li>
+ <li>GNU89 <code>inline</code>: same as C99 <code>extern inline</code>;</li>
+ <li>GNU89 <code>extern inline</code>: no externally visible function is generated;
+ no equivalent in C99, because redefinition is not permitted.</li>
+</ul></p>
+
+(Fortunately <code>static inline</code> is the same in both C99 and GNU89.)
+<p>In other words, ISO C99 requires that exactly one C source file has the
+callable copy of the inline function. Consider the following program:</p>
+
+<pre><code>
+ inline int
+ foo (void)
+ {
+ return 42;
+ }
+
+ int
+ main (void)
+ {
+ return foo ();
+ }
+</code></pre>
+
+<p>The program above will not link with the C99 inline semantics, because there
+is not an out-of-line function <code>foo</code> generated. To fix this, either
+mark the function <code>foo</code> as <code>extern</code>, or add the following
+declaration:</p>
+
+<pre><code>
+ extern inline int foo (void);
+</code></pre>
+
+<p>This ensures that an externally visible function be emitted.
+To enforce the GNU89 inline semantics, you can either use the
+<code>-fgnu89-inline</code> command-line option, or mark a function with the
+<code>gnu_inline</code> attribute. For example:</p>
+
+<pre><code>
+ __attribute__ ((gnu_inline)) inline int
+ foo (void)
+ {
+ return 42;
+ }
+</code></pre>
+
+<p>A program which used GNU89 <code>extern inline</code> may fail in the new
+standard due to multiple definition errors:</p>
+
+<pre><code>
+ extern inline int
+ foo (void)
+ {
+ return 42;
+ }
+
+ int
+ foo (void)
+ {
+ return 23;
+ }
+
+ int
+ main (void)
+ {
+ return foo ();
+ }
+</code></pre>
+
+<h4>Some warnings are enabled by default</h4>
+
+<p>The C99 mode enables some warnings by default. For instance, GCC warns
+about missing declarations of functions:</p>
+
+<pre><code>
+ int
+ foo (void)
+ {
+ return bar ();
+ }
+</code></pre>
+
+<p>This example now gives the following diagnostic:</p>
+
+<pre>
+<b>w.c:4:10:</b> <b style='color:magenta'>warning:</b> implicit declaration of function <b>'bar'</b> [-Wimplicit-function-declaration]
+ return bar ();
+ <b style='color:lime'>^</b>
+</pre>
+
+<p>To suppress this warning add the proper declaration:</p>
+
+<pre><code>
+ int bar (void);
+</code></pre>
+
+<p>or use <code>-Wno-implicit-function-declaration</code>.</p>
+
+<p>Another warning that is now turned on by default is the warning about
+implicit int, as in the following snippet:</p>
+
+<pre><code>
+ foo (u)
+ {
+ return u;
+ }
+</code></pre>
+
+<p>This example now gives the following diagnostic:</p>
+
+<pre>
+<b>q.c:1:1:</b> <b style='color:magenta'>warning:</b> return type defaults to <b>'int'</b> [-Wimplicit-int]
+ foo (u)
+ <b style='color:lime'>^</b>
+<b>q.c:</b> In function <b>'foo'</b>:
+<b>q.c:1:1:</b> <b style='color:magenta'>warning:</b> type of <b>'u'</b> defaults to <b>'int'</b> [-Wimplicit-int]
+</pre>
+
+<p>To suppress this warning just add the proper types:</p>
+
+<pre><code>
+ int
+ foo (int u)
+ {
+ return u;
+ }
+</code></pre>
+
+<p>or use <code>-Wno-implicit</code> or <code>-Wno-implicit-int</code>.</p>
+
+<p>Another warning that is now turned on by default is the warning about
+returning no value in function returning non-void:</p>
+
+<pre><code>
+ int
+ foo (void)
+ {
+ return;
+ }
+</code></pre>
+
+<p>This example now gives the following diagnostic:</p>
+
+<pre>
+<b>q.c:4:3:</b> <b style='color:magenta'>warning:</b> <b>'return'</b> with no value, in function returning non-void
+ return;
+ <b style='color:lime'>^</b>
+</pre>
+
+<p>The fix is either to specify a proper return value, or to declare the return
+value of <code>foo</code> as <code>void</code>.
+
+<h4>Initializing statics with compound literals</h4>
+
+<p>Previously, initializing objects with static storage duration with compound
+literals was only allowed in the GNU89 mode. This restriction has been lifted
+and currently it is possible to do this even in C99/C11 mode. The following
+snippet is an example of such initialization:</p>
+
+<pre><code>
+ struct T { int i; };
+ struct S { struct T t; };
+ static struct S s = (struct S) { .t = { 42 } };
+</code></pre>
+
+<p>We used to reject such code in C99/C11 mode:</p>
+
+<pre>
+<b>q.c:3:29:</b> <b style='color:red'>error:</b> initializer element is not constant
+ static struct S s = (struct S) { .t = { 42 } };
+ <b style='color:lime'>^</b>
+</pre>
+
+<p>Note that using <code>-Wpedantic</code> will cause a warning be emitted:</p>
+
+<pre>
+<b>q.c:3:29:</b> <b style='color:magenta'>warning:</b> initializer element is not constant [-Wpedantic]
+ static struct S s = (struct S) { .t = { 42 } };
+ <b style='color:lime'>^</b>
+</pre>
+
+</body>
+</html>
Marek