This is a revised version. I reworded the paragraph dealing with __STDC_VERSION__, made some clarifications wrt %a, and added some text wrt cpp -P issue.
Ok? Index: porting_to.html =================================================================== RCS file: /cvs/gcc/wwwdocs/htdocs/gcc-5/porting_to.html,v retrieving revision 1.3 diff -u -r1.3 porting_to.html --- porting_to.html 10 Feb 2015 11:12:20 -0000 1.3 +++ porting_to.html 18 Feb 2015 12:01:50 -0000 @@ -24,6 +24,17 @@ manner. Additions and suggestions for improvement are welcome. </p> +<h2>Preprocessor issues</h2> + +<p>The preprocessor started to emit line markers to properly distinguish +whether a macro token comes from a system header, or from a normal header +(see <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60723";>PR60723</a>). +These new markers can cause intriguing problems, if the packages aren't ready +to handle them. To stop the preprocessor from generating the <code>#line</code> +directives, use the <code>-P</code> option, documented +<a href="https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html#Preprocessor-Options";>here</a>. +</p> + <h2>C language issues</h2> <h3>Default standard is now GNU11</h3> @@ -251,6 +262,105 @@ <b style='color:lime'>^</b> </pre> +<h4><code>__STDC_VERSION__</code> macro</h4> + +<p>As the default mode changed to C11, the <code>__STDC_VERSION__</code> +standard macro, introduced in C95, is now defined by default, and has +the value <code>201112L</code>. + +Typically, this macro is used as in the following:</p> + +<pre><code> + #if !defined __STDC_VERSION__ || __STDC_VERSION__ < 199901L + /* ... */ + #else + # include <stdint.h> + #endif +</code></pre> + +<p>You can check the macro using <code>gcc -dM -E -std=gnu11 - < /dev/null | grep STDC_VER</code>.</p> + +<h4>Different meaning of the <code>%a *scanf</code> conversion specification</h4> + +<p>In C89, the GNU C library supports dynamic allocation via the <code>%as</code>, +<code>%aS</code>, and <code>%a[...]</code> conversion specification; see +<a href="https://www.gnu.org/software/libc/manual/html_node/Dynamic-String-Input.html#Dynamic-String-Input";> +this</a> for more info. +But in C99, the <code>a</code> conversion specifier is a synonym for <code>f</code> +(float), so the compiler expects an argument of type <code>float *</code>. Different +meaning of <code>%a</code> is a change in semantics, and in combination with the +<code>-Wformat</code> warning option the compiler may emit additional warnings:</p> + +<pre><code> + #include <stdio.h> + + int + main (void) + { + char *s; + scanf ("%as", &s); + } +</code></pre> + +<pre> +<b>q.c:7:10:</b> <b style='color:magenta'>warning:</b> format <b>'%a'</b> expects argument of type <b>'float *'</b>, but argument 2 has type <b>'char **'</b> [-Wformat=] + scanf ("%as", &s); + <b style='color:lime'>^</b> +</pre> + +<p>To use the dynamic allocation conversion specifier in C99 and C11, specify +<code>m</code> as a length modifier, specified by POSIX.1-2008. That is, use +<code>%ms</code> or <code>%m[...]</code>.</p> + +<h3>New warnings</h3> + +<p>Several new warnings have been added to the C front end. One of the new +warnings is that GCC now warns about non-standard predefined identifiers with +the <code>-Wpedantic</code> option. For instance:</p> + +<pre><code> + void + foo (void) + { + const char *s = __FUNCTION__; + } +</code></pre> + +<pre> +<b>q.c:4:19:</b> <b style='color:magenta'>warning:</b> ISO C does not support <b>'__FUNCTION__'</b> predefined identifier [-Wpedantic] + const char *s = __FUNCTION__; + <b style='color:lime'>^</b> +</pre> + +<p>The fix is either to use the standard predefined identifier <code>__func__</code> +(since C99), or to use the <code>__extension__</code> keyword:<p/> + +<pre><code> + const char *s = __extension__ __FUNCTION__; +</code></pre> + +<h2>C++ language issues</h2> + +<h3>Converting <code>std::nullptr_t</code> to <code>bool</code></h3> + +<p>Converting <code>std::nullptr_t</code> to <code>bool</code> in the C++11 +mode now requires direct-initialization. This has been changed in +<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1423";>DR 1423</a>. +As a consequence, the following is invalid:</p> + +<pre><code> + bool b = nullptr; +</code></pre> + +<p>but the following is valid:</p> + +<pre><code> + bool b(nullptr); +</code></pre> + +It is recommended to use <code>true</code>, resp. <code>false</code> keywords +in such cases. + <h3>Links</h3> <p> Marek
