libstdc++: check for openat

rtems6.0 has fdopendir, and fcntl.h defines AT_FDCWD and declares
openat, but there's no openat in libc.  Adjust dir-common.h to not
assume ::openat just because of AT_FDCWD.


for  libstdc++-v3/ChangeLog

	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Check for
	openat.
	* configure, config.h.in: Rebuilt.
	* src/filesystem/dir-common.h (openat): Use ::openat if
	_GLIBCXX_HAVE_OPENAT.
	* src/filesystem/dir.cc (dir_and_pathname): Use dirfd if
	_GLIBCXX_HAVE_OPENAT.

Author: Alexandre Oliva

libstdc++-v3/acinclude.m4

@@ -4772,6 +4772,18 @@ dnl
   if test $glibcxx_cv_dirfd = yes; then
     AC_DEFINE(HAVE_DIRFD, 1, [Define if dirfd is available in <dirent.h>.])
   fi
+dnl
+  AC_CACHE_CHECK([for openat],
+    glibcxx_cv_openat, [dnl
+    GCC_TRY_COMPILE_OR_LINK(
+      [#include <fcntl.h>],
+      [int fd = ::openat(AT_FDCWD, "", 0);],
+      [glibcxx_cv_openat=yes],
+      [glibcxx_cv_openat=no])
+  ])
+  if test $glibcxx_cv_openat = yes; then
+    AC_DEFINE(HAVE_OPENAT, 1, [Define if openat is available in <fcntl.h>.])
+  fi
 dnl
   AC_CACHE_CHECK([for unlinkat],
     glibcxx_cv_unlinkat, [dnl

libstdc++-v3/config.h.in

@@ -292,6 +292,9 @@
 /* Define if <math.h> defines obsolete isnan function. */
 #undef HAVE_OBSOLETE_ISNAN
 
+/* Define if openat is available in <fcntl.h>. */
+#undef HAVE_OPENAT
+
 /* Define if poll is available in <poll.h>. */
 #undef HAVE_POLL
 

libstdc++-v3/configure

@@ -77177,6 +77177,61 @@ $as_echo "$glibcxx_cv_dirfd" >&6; }
 
 $as_echo "#define HAVE_DIRFD 1" >>confdefs.h
 
+  fi
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for openat" >&5
+$as_echo_n "checking for openat... " >&6; }
+if ${glibcxx_cv_openat+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+      if test x$gcc_no_link = xyes; then
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <fcntl.h>
+int
+main ()
+{
+int fd = ::openat(AT_FDCWD, "", 0);
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  glibcxx_cv_openat=yes
+else
+  glibcxx_cv_openat=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+else
+  if test x$gcc_no_link = xyes; then
+  as_fn_error $? "Link tests are not allowed after GCC_NO_EXECUTABLES." "$LINENO" 5
+fi
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <fcntl.h>
+int
+main ()
+{
+int fd = ::openat(AT_FDCWD, "", 0);
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_link "$LINENO"; then :
+  glibcxx_cv_openat=yes
+else
+  glibcxx_cv_openat=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $glibcxx_cv_openat" >&5
+$as_echo "$glibcxx_cv_openat" >&6; }
+  if test $glibcxx_cv_openat = yes; then
+
+$as_echo "#define HAVE_OPENAT 1" >>confdefs.h
+
   fi
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking for unlinkat" >&5
 $as_echo_n "checking for unlinkat... " >&6; }

libstdc++-v3/src/filesystem/dir-common.h

@@ -199,7 +199,7 @@ struct _Dir_base
 #endif
 
 
-#ifdef AT_FDCWD
+#if _GLIBCXX_HAVE_OPENAT && defined AT_FDCWD
     fd = ::openat(fd, pathname, flags);
 #else
     // If we cannot use openat, there's no benefit to using posix::open unless

libstdc++-v3/src/filesystem/dir.cc

@@ -120,7 +120,7 @@ struct fs::_Dir : std::filesystem::_Dir_base
   dir_and_pathname() const noexcept
   {
     const fs::path& p = entry.path();
-#if _GLIBCXX_HAVE_DIRFD
+#if _GLIBCXX_HAVE_DIRFD && _GLIBCXX_HAVE_OPENAT
     return {::dirfd(this->dirp), std::prev(p.end())->c_str()};
 #endif
     return {this->fdcwd(), p.c_str()};