On Wed, 2025-01-15 at 18:22 +0000, Joseph Myers wrote: > On Wed, 15 Jan 2025, David Malcolm wrote: > > > Here's an updated version of the patch > > > > OK to push? (we could tweak it in followups) > > This will need updating to work together with Jakub's patch that also > adds > the C section to changes.html. > > > +<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> > > It's not limited to "not a definition", it also introduces a > prototype > (for calls following the definition) for () in a definition as well, > where > there wasn't one before. > > > +<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#ind > > ex-std-1">-std=</a> > > + to select an earlier version of the C standard. > > +</p> > > Maybe also mention the case where the function already has a > prototype in > some header (standard or in the project) and you can just remove the > non-prototype declaration and make sure to include the header? This > is > for legacy code that does things like > > void *malloc(); > > with its own declarations of standard functions, you may have a > better > idea of how common that is. >
Thanks. I've tweaked for the above and taken the liberty of pushing the following (in the hope of not letting "perfect be the enemy of the good"). If I've still got the wording wrong, can someone with better knowledge of C standards fix it, please. --- htdocs/gcc-15/porting_to.html | 64 ++++++++++++++++++++++++++++++++++- 1 file changed, 63 insertions(+), 1 deletion(-) diff --git a/htdocs/gcc-15/porting_to.html b/htdocs/gcc-15/porting_to.html index 39598b93..c446e309 100644 --- a/htdocs/gcc-15/porting_to.html +++ b/htdocs/gcc-15/porting_to.html @@ -41,7 +41,69 @@ 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: --> +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> + 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 such as the above 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 via: +</p> +<pre><code> + void (*handler)(int); +</code></pre> +<p> + In other cases the code is simply missing a <code>#include</code> of + the correct header, such as with: +<pre><code> + void *malloc(); +</code></pre> +<p> + These can be fixed by including the correct header and removing the + non-prototype declaration. +</p> +<p> + Alternatively 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> <h4 id="c23-new-keywords">New keywords</h4> <p> -- 2.46.0
