Re: vasnprintf: Silence -Wswitch-unreachable warning.

[Bruno Haible via Gnulib discussion list]

Hi Collin,

> On NetBSD I can see the following warning when compiling
> lib/c-vasnprintf.c:
> 
>     depbase=`echo c-vasnprintf.o | sed 's|[^/]*$|.deps/&|;s|\.o$||'`; gcc 
> -DHAVE_CONFIG_H -I. -I..  -DGNULIB_STRICT_CHECKING=1    -g -O2 -MT 
> c-vasnprintf.o -MD -MP -MF $depbase.Tpo -c -o c-vasnprintf.o c-vasnprintf.c 
> && mv -f $depbase.Tpo $depbase.Po
>     In file included from c-vasnprintf.c:46:
>     vasnprintf.c: In function 'c_vasnprintf':
>     vasnprintf.c:6662:35: warning: statement will never be executed 
> [-Wswitch-unreachable]
>      6662 |                     pad_ourselves = 1;
>           |                     ~~~~~~~~~~~~~~^~~
> 
> The cause is PAD_OURSELVES being initialized in a switch statement with
> no preceding case statement. In the C23 standard § 6.8.5.3, Example 7,
> it explains that initilizations and function calls with no preceding
> case statement cannot be reached.
> 
> It happens on a few other platforms IIRC, based on skimming the CI a
> while ago.

Yes, I saw this warning too, on some platforms. But warnings on non-GNU
platforms don't bother me enough.

> Any objections to the attached patch?

I prefer to conditionalize the case statements together with the code that
follows.

I also experimented with code like this:

                switch (dp->conversion)
                  {
# if !DCHAR_IS_TCHAR || ENABLE_UNISTDIO
                  /* If we need conversion from TCHAR_T[] to DCHAR_T[], we need
                     to perform the padding after this conversion.  Functions
                     with unistdio extensions perform the padding based on
                     character count rather than element count.  */
                  case 'c': case 's':
                    pad_ourselves = 1;
                    break;
# endif
# if NEED_PRINTF_FLAG_ZERO
                  case 'f': case 'F': case 'e': case 'E': case 'g': case 'G':
                  case 'a': case 'A':
                    pad_ourselves = 1;
                    break;
# endif
                  default:
                    pad_ourselves = prec_ourselves | group_ourselves;
                    break;
                  }

The advantage would be one less #if statement. gcc optimizes this code nicely.
But clang doesn't: <https://github.com/llvm/llvm-project/issues/147239>
So, I see nothing better than to add another #if statement.


2025-07-07  Bruno Haible  <br...@clisp.org>

        vasnprintf: Fix -Wswitch-unreachable warning.
        Reported by Collin Funk in
        <https://lists.gnu.org/archive/html/bug-gnulib/2025-07/msg00026.html>.
        * lib/vasnprintf.c (VASNPRINTF): Put a #if around a potentially
        unreachable switch case.

diff --git a/lib/vasnprintf.c b/lib/vasnprintf.c
index f46e8701bd..3e421b87e2 100644
--- a/lib/vasnprintf.c
+++ b/lib/vasnprintf.c
@@ -6648,19 +6648,21 @@ VASNPRINTF (DCHAR_T *resultbuf, size_t *lengthp,
 #if !((WIDE_CHAR_VERSION && MUSL_LIBC) || NEED_PRINTF_FLAG_LEFTADJUST) && 
(!DCHAR_IS_TCHAR || ENABLE_UNISTDIO || NEED_PRINTF_FLAG_ZERO || 
NEED_PRINTF_FLAG_ALT_PRECISION_ZERO || NEED_PRINTF_UNBOUNDED_PRECISION || 
NEED_PRINTF_FLAG_GROUPING || NEED_PRINTF_FLAG_GROUPING_INT)
                 switch (dp->conversion)
                   {
-# if !DCHAR_IS_TCHAR || ENABLE_UNISTDIO
+# if !DCHAR_IS_TCHAR || ENABLE_UNISTDIO || NEED_PRINTF_FLAG_ZERO
+#  if !DCHAR_IS_TCHAR || ENABLE_UNISTDIO
                   /* If we need conversion from TCHAR_T[] to DCHAR_T[], we need
                      to perform the padding after this conversion.  Functions
                      with unistdio extensions perform the padding based on
                      character count rather than element count.  */
                   case 'c': case 's':
-# endif
-# if NEED_PRINTF_FLAG_ZERO
+#  endif
+#  if NEED_PRINTF_FLAG_ZERO
                   case 'f': case 'F': case 'e': case 'E': case 'g': case 'G':
                   case 'a': case 'A':
-# endif
+#  endif
                     pad_ourselves = 1;
                     break;
+# endif
                   default:
                     pad_ourselves = prec_ourselves | group_ourselves;
                     break;