On Mon, 2024-12-09 at 20:58 +0100, Jakub Jelinek wrote:
> On Mon, Dec 09, 2024 at 02:44:42PM -0500, David Malcolm wrote:
> > +C23 brings the following changes:
> > +
> > +<h4 id="c23-empty-fn-prototypes-become-void">Function prototypes
> > with empty params change from implicit <code>int</code> to
> > <code>void</code></h4>
> > +
> > +<p> In C23 <code>()</code> in a function declaration means the
> > same as <code>(void)</code>, whereas previously it implicitly
> > declared the function to take an <code>int</code> parameter.</p>
>
> This isn't true. void foo (); used to be an unprototyped function,
> one could pass any arguments to it (of course without invoking UB
> only if it actually matched the arguments passed to it).
> So, with
> void foo ();
> int bar ();
> one can
> foo (1, 2.0, 3L, bar (2));
> if the definitions were
> void foo (int a, double b, long c, int d) {
> //...
> }
> int bar (int e) {
> //...
> }
> or K&R-ish
> void foo (f, g, h, i)
> int f;
> double g;
> long h;
> int i;
> {
> /* ... */
> }
> void bar (j)
> int j;
> {
> /* ... */
> }
>
> Implicit int was about completely undeclared functions in K&R/C89,
> those were assumed to return int. But we already error on those by
> default in C99 and later modes, so no need to describe it again.
Here's an updated version of the patch
OK to push? (we could tweak it in followups)
I'm working on followups covering other issues that showed up in:
https://fedoraproject.org/wiki/F42-gcc-15-mass-prebuild
Thanks
Dave
htdocs/gcc-15/changes.html | 12 +++++++
htdocs/gcc-15/porting_to.html | 67 ++++++++++++++++++++++++++++++++++-
2 files changed, 78 insertions(+), 1 deletion(-)
diff --git a/htdocs/gcc-15/changes.html b/htdocs/gcc-15/changes.html
index 1c690c4a..601d7760 100644
--- a/htdocs/gcc-15/changes.html
+++ b/htdocs/gcc-15/changes.html
@@ -96,6 +96,18 @@ a work-in-progress.</p>
<code>musttail</code> statement attribute</a> was added to enforce
tail calls.</li>
</ul>
+<h3 id="c">C</h3>
+<ul>
+ <li>GCC 15 changes the default language version for C compilation from
+ <a
href="https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-std-1";>-std=gnu17</a>
+ to
+ <a
href="https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-std-1";>-std=gnu23</a>.
+ If your code relies on older versions of the C standard, you will need to
+ either add
+ <a
href="https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-std-1";>-std=</a>
+ to your build flags, or port your code; see <a
href="porting_to.html#c23">the porting notes</a>.
+</ul>
+
<h3 id="cxx">C++</h3>
<ul>
diff --git a/htdocs/gcc-15/porting_to.html b/htdocs/gcc-15/porting_to.html
index 702cf507..385fa141 100644
--- a/htdocs/gcc-15/porting_to.html
+++ b/htdocs/gcc-15/porting_to.html
@@ -27,7 +27,72 @@ and provide solutions. Let us know if you have suggestions
for improvements!
<p>Note: GCC 15 has not been released yet, so this document is
a work-in-progress.</p>
-<!-- <h2 id="c">C language issues</h2> -->
+<h2 id="c">C language issues</h2>
+
+<h3 id="c23">C23 by default</h3>
+<!-- change of default was in commit 55e3bd376b2214e200fa76d12b67ff259b06c212
-->
+
+GCC 15 changes the default language version for C compilation from
+<a
href="https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-std-1";>-std=gnu17</a>
+to
+<a
href="https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-std-1";>-std=gnu23</a>.
+
+If your code relies on older versions of the C standard, you will need to
+either add <a
href="https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-std-1";>-std=</a>
+to your build flags, or port your code to C23.
+
+C23 brings the following changes:
+
+<h4 id="c23-fn-decls-without-parameters">Function declarations without
parameters</h4>
+<p>
+ The meaning of function declarations of the form
+ <code>rettype identifier ();</code>
+ such as
+ <code>char *strstr ();</code>
+ that are not a definition changed in C23.
+</p>
+<p>
+ In C17 and earlier, such function declarators specified no information
+ about the number or types of the parameters of the function (C17 6.7.6.3),
+ requiring users to know the correct number of arguments, with each passed
+ argument going through default argument promotion.
+</p>
+<p>
+ In C23 such declarations mean <code>(void)</code> i.e. a function taking
+ no arguments, which can lead to build failures on code that relied on
+ the earlier meaning, such as in
+ <a href="https://godbolt.org/z/11hzWYEeK";>this example</a>:
+</p>
+<pre><code>
+#include <signal.h>
+
+void test()
+{
+ void (*handler)();
+ handler = signal(SIGQUIT, SIG_IGN);
+}
+
+<source>: In function 'test':
+<source>:6:11: error: assignment to 'void (*)(void)' from incompatible
pointer type '__sighandler_t' {aka 'void (*)(int)'}
[-Wincompatible-pointer-types]
+ 6 | handler = signal(SIGQUIT, SIG_IGN);
+ | ^
+In file included from <source>:1:
+/usr/include/signal.h:72:16: note: '__sighandler_t' declared here
+ 72 | typedef void (*__sighandler_t) (int);
+ | ^~~~~~~~~~~~~~
+</code></pre>
+<p>
+ Code relying on a non-zero number of parameters (such as a single
+ <code>int</code>) can be fixed for C23 by adding the correct parameters
+ to the function declarator, such as in the above via:
+</p>
+<pre><code>
+ void (*handler)(int);
+</code></pre>
+<p>
+ or you can use <a
href="https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-std-1";>-std=</a>
+ to select an earlier version of the C standard.
+</p>
<h2 id="cxx">C++ language issues</h2>
--
2.46.0
