Hi Martin,
Thank you for the careful analysis.
> The root cause for this issue seems to be that clang has a builtin for the
> strndup function, even if the target platform actually doesn't have it.
>
> This manifests itself during configure like this:
>
> configure:31903: checking whether strndup is declared
> configure:31903: x86_64-w64-mingw32-gcc -c -g -O2 conftest.c >&5
> conftest.c:267:10: warning: implicitly declaring library function
> 'strndup' with type 'char *(const char *, unsigned long long)'
> [-Wimplicit-function-declaration]
> (void) strndup;
> ^
> conftest.c:267:10: note: include the header <string.h> or explicitly
> provide a declaration for 'strndup'
> 1 warning generated.
> configure:31903: $? = 0
> configure:31903: result: yes
>
> This results in variables set up like this:
>
> ac_cv_func_strndup=no
This is correct and expected: mingw does not have strndup.
> ac_cv_have_decl_strndup=yes
This is wrong: it should be ac_cv_have_decl_strndup=no.
> GNULIB_STRNDUP = 1
> HAVE_DECL_STRNDUP = 1
> REPLACE_STRNDUP = 0
>
> Which then leads to the errors quoted above.
There are two possible ways to fix this issue:
* Modify AC_CHECK_DECL to disable the clang built-in declarations
for strndup and many other functions.
* Ignore the result of AC_CHECK_DECL when the compiler is clang,
and use the AC_CHECK_FUNC result instead.
The second option is work in many places, in particular because in some
cases the AC_CHECK_FUNC needs to know which libraries to add to LIBS
for the test. I therefore prefer the first one.
With this patch, the configure output should be:
checking whether the compiler is clang... yes
checking for compiler option needed when checking for declarations...
-Werror=implicit-function-declaration
checking whether strndup is declared... no
2020-01-04 Bruno Haible <br...@clisp.org>
Fix AC_CHECK_DECL so that it deactivates clang's built-in declarations.
Reported by Martin Storsjö <mar...@martin.st> in
<https://lists.gnu.org/archive/html/bug-gnulib/2020-01/msg00016.html>.
* m4/00gnulib.m4 (gl_COMPILER_CLANG, gl_COMPILER_PREPARE_CHECK_DECL):
New macros.
(_AC_CHECK_DECL_BODY, AC_CHECK_DECL): Augment.
diff --git a/m4/00gnulib.m4 b/m4/00gnulib.m4
index 1a1a1d7..77503ea 100644
--- a/m4/00gnulib.m4
+++ b/m4/00gnulib.m4
@@ -1,13 +1,14 @@
-# 00gnulib.m4 serial 3
+# 00gnulib.m4 serial 4
dnl Copyright (C) 2009-2020 Free Software Foundation, Inc.
dnl This file is free software; the Free Software Foundation
dnl gives unlimited permission to copy and/or distribute it,
dnl with or without modifications, as long as this notice is preserved.
dnl This file must be named something that sorts before all other
-dnl gnulib-provided .m4 files. It is needed until such time as we can
-dnl assume Autoconf 2.64, with its improved AC_DEFUN_ONCE and
-dnl m4_divert semantics.
+dnl gnulib-provided .m4 files. The first part is needed until such time
+dnl as we can assume Autoconf 2.64, with its improved AC_DEFUN_ONCE and
+dnl m4_divert semantics. The second part is needed until the clang fix
+dnl has been included in Autoconf.
# Until autoconf 2.63, handling of the diversion stack required m4_init
# to be called first; but this does not happen with aclocal. Wrapping
@@ -39,6 +40,66 @@ m4_version_prereq([2.63.263], [],
[m4_indir([_gl_DEFUN_ONCE([$1])])])])]dnl
[AC][_DEFUN([_gl_DEFUN_ONCE([$1])], [$2])])])
+# The following definitions arrange to use a compiler option
+# -Werror=implicit-function-declaration in AC_CHECK_DECL, when the
+# compiler is clang. Without it, clang implicitly declares "known"
+# library functions in C mode, but not in C++ mode, which would cause
+# Gnulib to omit a declaration and thus later produce an error in C++
+# mode. As of clang 9.0, these "known" functions are identified through
+# LIBBUILTIN invocations in the LLVM source file
+# llvm/tools/clang/include/clang/Basic/Builtins.def.
+AC_DEFUN([gl_COMPILER_CLANG],
+[
+ AC_REQUIRE([AC_PROG_CC])
+ AC_CACHE_CHECK([whether the compiler is clang],
+ [gl_cv_compiler_clang],
+ [AC_EGREP_CPP([barfbarf],[
+#ifdef __clang__
+barfbarf
+#endif
+ ],
+ [gl_cv_compiler_clang=yes],
+ [gl_cv_compiler_clang=no])
+ ])
+])
+AC_DEFUN([gl_COMPILER_PREPARE_CHECK_DECL],
+[
+ AC_REQUIRE([AC_PROG_CC])
+ AC_REQUIRE([gl_COMPILER_CLANG])
+ AC_CACHE_CHECK([for compiler option needed when checking for declarations],
+ [gl_cv_compiler_check_decl_option],
+ [if test $gl_cv_compiler_clang = yes; then
+ dnl Test whether the compiler supports the option
+ dnl '-Werror=implicit-function-declaration'.
+ save_ac_compile="$ac_compile"
+ ac_compile="$ac_compile -Werror=implicit-function-declaration"
+ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[]],[[]])],
+
[gl_cv_compiler_check_decl_option='-Werror=implicit-function-declaration'],
+ [gl_cv_compiler_check_decl_option=none])
+ ac_compile="$save_ac_compile"
+ else
+ gl_cv_compiler_check_decl_option=none
+ fi
+ ])
+ if test "x$gl_cv_compiler_check_decl_option" != xnone; then
+ ac_compile_for_check_decl="$ac_compile $gl_cv_compiler_check_decl_option"
+ else
+ ac_compile_for_check_decl="$ac_compile"
+ fi
+])
+dnl Redefine _AC_CHECK_DECL_BODY so that it references
ac_compile_for_check_decl
+dnl instead of ac_compile.
+m4_define([_AC_CHECK_DECL_BODY],
+[ ac_save_ac_compile="$ac_compile"
+ ac_compile="$ac_compile_for_check_decl"]
+m4_defn([_AC_CHECK_DECL_BODY])[ ac_compile="$ac_save_ac_compile"
+])
+ ])
+dnl Redefine AC_CHECK_DECL so that it starts with
+dnl AC_REQUIRE([gl_COMPILER_PREPARE_CHECK_DECL]).
+m4_define([AC_CHECK_DECL],
+ [AC_REQUIRE([gl_COMPILER_PREPARE_CHECK_DECL])]m4_defn([AC_CHECK_DECL]))
+
# gl_00GNULIB
# -----------
# Witness macro that this file has been included. Needed to force
