Index: lib/Fuzzer/FuzzerIOWindows.cpp
===================================================================
--- lib/Fuzzer/FuzzerIOWindows.cpp
+++ lib/Fuzzer/FuzzerIOWindows.cpp
@@ -18,6 +18,7 @@
 #include <fstream>
 #include <io.h>
 #include <iterator>
+#include <regex>
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <windows.h>
@@ -141,10 +142,45 @@
 }
 
 std::string DirName(const std::string &FileName) {
-  std::string Res = FileName;
-  PathRemoveFileSpecA(&Res[0]);
-  Res.resize(std::strlen(Res.c_str()));
-  return Res;
+// Location, like: \\?\UNC\Server\Share\  \\?\C:\  \\Server\Share\  \  C:\  C:
+//   ((\\\\\?\\((UNC(\\[^\\]+){2})|(.:))\\)|(\\\\([^\\]+\\){2})|(\\)|(.:\\?))?
+// Directories, like: SomeDir\AnotherDir\
+//   (([^\\]+\\)*)
+// File, like: SomeFile.txt
+//   ([^\\]+)?
+  static std::regex Reg(
+  R"(((\\\\\?\\((UNC(\\[^\\]+){2})|(.:))\\)|(\\\\([^\\]+\\){2})|(\\)|(.:\\?))?)"
+  R"((([^\\]+\\)*))"
+  R"(([^\\]+)?)");
+
+  std::smatch MatchRes;
+  if (!regex_match(FileName, MatchRes, Reg)) {
+    Printf("DirName() failed for \"%s\", invalid path.\n", FileName.c_str());
+    exit(1);
+  }
+
+  std::string Location(MatchRes[1]), Dir(MatchRes[11]), File(MatchRes[13]);
+
+  if (!Dir.empty()) {
+    Dir.pop_back(); // Remove trailing '\'.
+    if (File.empty()) { // Path ended in '\'.
+      assert(!Dir.empty());
+      // Remove file name from Dir.
+      size_t LastSep = Dir.find_last_of('\\');
+      if (LastSep == std::string::npos)
+        LastSep = 0;
+      Dir.erase(LastSep);
+    }
+  }
+
+  if (Location.empty()) { // Relative path.
+    if (Dir.empty())
+      Location = ".";
+    else
+      Location = ".\\";
+  }
+
+  return Location.append(Dir);
 }
 
 }  // namespace fuzzer