commit e79ad2dbcb14d1e66f6edead4ff87b62e575a8e7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Sep 16 14:07:54 2015 +0100

    Implement filesystem::canonical() without realpath
    
    	PR libstdc++/67173
    	* acinclude.m4 (GLIBCXX_CHECK_FILESYSTEM_DEPS): Check _XOPEN_VERSION
    	and PATH_MAX for _GLIBCXX_USE_REALPATH.
    	* config.h.in: Regenerate.
    	* configure: Regenerate.
    	* src/filesystem/dir.cc: Define _XOPEN_VERSION.
    	* src/filesystem/ops.cc: Likewise.
    	(canonical) [!_GLIBCXX_USE_REALPATH]: Add alternative implementation.
    	* testsuite/experimental/filesystem/operations/canonical.cc: New.

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 64c9b7e..364a7d2 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -3926,7 +3926,7 @@ dnl
   AC_LANG_SAVE
   AC_LANG_CPLUSPLUS
   ac_save_CXXFLAGS="$CXXFLAGS"
-  CXXFLAGS="$CXXFLAGS -fno-exceptions"
+  CXXFLAGS="$CXXFLAGS -fno-exceptions -D_XOPEN_SOURCE=700"
 dnl
   AC_MSG_CHECKING([for struct dirent.d_type])
   AC_CACHE_VAL(glibcxx_cv_dirent_d_type, [dnl
@@ -3947,13 +3947,24 @@ dnl
   AC_MSG_CHECKING([for realpath])
   AC_CACHE_VAL(glibcxx_cv_realpath, [dnl
     GCC_TRY_COMPILE_OR_LINK(
-      [#include <stdlib.h>],
-      [char *tmp = realpath((const char*)NULL, (char*)NULL);],
+      [
+       #include <stdlib.h>
+       #include <unistd.h>
+      ],
+      [
+       #if _XOPEN_VERSION < 500
+       #error
+       #elif _XOPEN_VERSION >= 700 || defined(PATH_MAX)
+       char *tmp = realpath((const char*)NULL, (char*)NULL);
+       #else
+       #error
+       #endif
+      ],
       [glibcxx_cv_realpath=yes],
       [glibcxx_cv_realpath=no])
   ])
   if test $glibcxx_cv_realpath = yes; then
-    AC_DEFINE(_GLIBCXX_USE_REALPATH, 1, [Define if realpath is available in <stdlib.h>.])
+    AC_DEFINE(_GLIBCXX_USE_REALPATH, 1, [Define if usable realpath is available in <stdlib.h>.])
   fi
   AC_MSG_RESULT($glibcxx_cv_realpath)
 dnl
diff --git a/libstdc++-v3/src/filesystem/dir.cc b/libstdc++-v3/src/filesystem/dir.cc
index 016a78d..cfa44b1 100644
--- a/libstdc++-v3/src/filesystem/dir.cc
+++ b/libstdc++-v3/src/filesystem/dir.cc
@@ -22,6 +22,8 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
+#define _XOPEN_SOURCE 700
+
 #include <experimental/filesystem>
 #include <utility>
 #include <stack>
diff --git a/libstdc++-v3/src/filesystem/ops.cc b/libstdc++-v3/src/filesystem/ops.cc
index cefb927..45daf34 100644
--- a/libstdc++-v3/src/filesystem/ops.cc
+++ b/libstdc++-v3/src/filesystem/ops.cc
@@ -22,6 +22,8 @@
 // see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 // <http://www.gnu.org/licenses/>.
 
+#define _XOPEN_SOURCE 700
+
 #include <experimental/filesystem>
 #include <functional>
 #include <stack>
@@ -96,23 +98,98 @@ namespace
 fs::path
 fs::canonical(const path& p, const path& base, error_code& ec)
 {
-  path can;
+  const path pa = absolute(p, base);
+  path result;
 #ifdef _GLIBCXX_USE_REALPATH
-  char* buffer = nullptr;
-#if defined(__SunOS_5_10) && defined(PATH_MAX)
-  buffer = (char*)::malloc(PATH_MAX);
-#endif
-  if (char_ptr rp = char_ptr{::realpath(absolute(p, base).c_str(), buffer)})
+  char_ptr buf{ nullptr };
+# if _XOPEN_VERSION < 700
+  // Not safe to call realpath(path, NULL)
+  buf.reset( (char*)::malloc(PATH_MAX) );
+# endif
+  if (char* rp = ::realpath(pa.c_str(), buf.get()))
     {
-      can.assign(rp.get());
+      if (buf == nullptr)
+	buf.reset(rp);
+      result.assign(rp);
       ec.clear();
+      return result;
+    }
+  if (errno != ENAMETOOLONG)
+    {
+      ec.assign(errno, std::generic_category());
+      return result;
     }
-  else
-    ec.assign(errno, std::generic_category());
-#else
-  ec = std::make_error_code(std::errc::not_supported);
 #endif
-  return can;
+
+  auto fail = [&ec, &result](int e) mutable {
+      if (!ec.value())
+	ec.assign(e, std::generic_category());
+      result.clear();
+  };
+
+  if (!exists(pa, ec))
+    {
+      fail(ENOENT);
+      return result;
+    }
+  // else we can assume no unresolvable symlink loops
+
+  result = pa.root_path();
+
+  deque<path> cmpts;
+  for (auto& f : pa.relative_path())
+    cmpts.push_back(f);
+
+  while (!cmpts.empty())
+    {
+      path f = std::move(cmpts.front());
+      cmpts.pop_front();
+
+      if (f.compare(".") == 0)
+	{
+	  if (!is_directory(result, ec))
+	    {
+	      fail(ENOTDIR);
+	      break;
+	    }
+	}
+      else if (f.compare("..") == 0)
+	{
+	  auto parent = result.parent_path();
+	  if (parent.empty())
+	    result = pa.root_path();
+	  else
+	    result.swap(parent);
+	}
+      else
+	{
+	  result /= f;
+
+	  if (is_symlink(result, ec))
+	    {
+	      path link = read_symlink(result, ec);
+	      if (!ec.value())
+		{
+		  if (link.is_absolute())
+		    {
+		      result = link.root_path();
+		      link = link.relative_path();
+		    }
+		  else
+		    result.remove_filename();
+
+		  cmpts.insert(cmpts.begin(), link.begin(), link.end());
+		}
+	    }
+
+	  if (ec.value() || !exists(result, ec))
+	    {
+	      fail(ENOENT);
+	      break;
+	    }
+	}
+    }
+  return result;
 }
 
 fs::path
diff --git a/libstdc++-v3/testsuite/experimental/filesystem/operations/canonical.cc b/libstdc++-v3/testsuite/experimental/filesystem/operations/canonical.cc
new file mode 100644
index 0000000..0bcb85b
--- /dev/null
+++ b/libstdc++-v3/testsuite/experimental/filesystem/operations/canonical.cc
@@ -0,0 +1,74 @@
+// Copyright (C) 2015 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-options "-std=gnu++11 -lstdc++fs" }
+// { dg-require-filesystem-ts "" }
+
+#include <experimental/filesystem>
+#include <testsuite_hooks.h>
+#include <testsuite_fs.h>
+
+namespace fs = std::experimental::filesystem;
+
+void
+test01()
+{
+  std::error_code ec;
+  fs::path p = "";
+  canonical( p, ec );
+  VERIFY( !ec );
+
+  canonical( __gnu_test::nonexistent_path(), ec );
+  VERIFY( !ec );
+
+  p = "/";
+  p = canonical( p, ec );
+  VERIFY( p == "/" );
+  VERIFY( ec );
+
+  p = "/.";
+  p = canonical( p, ec );
+  VERIFY( p == "/" );
+  VERIFY( ec );
+
+  p = "/..";
+  p = canonical( p, ec );
+  VERIFY( p == "/" );
+  VERIFY( ec );
+
+  p = "/../.././.";
+  p = canonical( p, ec );
+  VERIFY( p == "/" );
+  VERIFY( ec );
+
+  p = "/dev/stdin";
+  if (exists(p))
+    {
+      auto p2 = canonical(p);
+      if (is_symlink(p))
+        VERIFY( p != p2 );
+      else
+        VERIFY( p == p2 );
+      VERIFY( canonical(p2) == p2 );
+    }
+}
+
+int
+main()
+{
+  test01();
+}