string: Fix "mismatched allocation function" warnings regarding strndup

[Bruno Haible]

When compiling GNU poke 2.1 with GCC 11 (on Linux/x86_64), I see these
warnings:

../../libpoke/libpoke.c:221:7: warning: 'rpl_free' called on pointer returned 
from a mismatched allocation function [-Wmismatched-dealloc]
../../libpoke/pkl-ast.c:700:7: warning: 'rpl_free' called on pointer returned 
from a mismatched allocation function [-Wmismatched-dealloc]

The cause is that the program invokes strndup() and then frees the result
using free(), which at this point in the program macroexpands to rpl_free().
This situation should not cause a warning.

This patch fixes it, by using the same idiom for 'strndup' as for 'strdup'.


2022-02-09  Bruno Haible  <bruno@clisp.org>

        string: Fix "mismatched allocation function" warnings regarding strndup.
        * lib/string.in.h (strndup): Mark with _GL_ATTRIBUTE_MALLOC and
        _GL_ATTRIBUTE_DEALLOC_FREE. For GCC >= 11: Declare also when the
        platform already declares the function or when the module 'strndup' is
        not in use.

diff --git a/lib/string.in.h b/lib/string.in.h
index 03e6a17a36..c9432948c1 100644
--- a/lib/string.in.h
+++ b/lib/string.in.h
@@ -563,22 +563,35 @@ _GL_WARN_ON_USE (strncat, "strncat is unportable - "
 #   undef strndup
 #   define strndup rpl_strndup
 #  endif
-_GL_FUNCDECL_RPL (strndup, char *, (char const *__s, size_t __n)
-                                   _GL_ARG_NONNULL ((1)));
+_GL_FUNCDECL_RPL (strndup, char *,
+                  (char const *__s, size_t __n)
+                  _GL_ARG_NONNULL ((1))
+                  _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE);
 _GL_CXXALIAS_RPL (strndup, char *, (char const *__s, size_t __n));
 # else
-#  if ! @HAVE_DECL_STRNDUP@
-_GL_FUNCDECL_SYS (strndup, char *, (char const *__s, size_t __n)
-                                   _GL_ARG_NONNULL ((1)));
+#  if !@HAVE_DECL_STRNDUP@ || __GNUC__ >= 11
+_GL_FUNCDECL_SYS (strndup, char *,
+                  (char const *__s, size_t __n)
+                  _GL_ARG_NONNULL ((1))
+                  _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE);
 #  endif
 _GL_CXXALIAS_SYS (strndup, char *, (char const *__s, size_t __n));
 # endif
 _GL_CXXALIASWARN (strndup);
-#elif defined GNULIB_POSIXCHECK
-# undef strndup
-# if HAVE_RAW_DECL_STRNDUP
+#else
+# if __GNUC__ >= 11
+/* For -Wmismatched-dealloc: Associate strndup with free or rpl_free.  */
+_GL_FUNCDECL_SYS (strndup, char *,
+                  (char const *__s, size_t __n)
+                  _GL_ARG_NONNULL ((1))
+                  _GL_ATTRIBUTE_MALLOC _GL_ATTRIBUTE_DEALLOC_FREE);
+# endif
+# if defined GNULIB_POSIXCHECK
+#  undef strndup
+#  if HAVE_RAW_DECL_STRNDUP
 _GL_WARN_ON_USE (strndup, "strndup is unportable - "
                  "use gnulib module strndup for portability");
+#  endif
 # endif
 #endif