Index: lib/Fuzzer/CMakeLists.txt
===================================================================
--- lib/Fuzzer/CMakeLists.txt
+++ lib/Fuzzer/CMakeLists.txt
@@ -10,15 +10,17 @@
   endif()
   add_library(LLVMFuzzerNoMainObjects OBJECT
     FuzzerCrossOver.cpp
-    FuzzerTraceState.cpp
     FuzzerDriver.cpp
     FuzzerExtFunctionsDlsym.cpp
     FuzzerExtFunctionsWeak.cpp
     FuzzerIO.cpp
+    FuzzerIOPosix.cpp
+    FuzzerIOWindows.cpp
     FuzzerLoop.cpp
     FuzzerMutate.cpp
     FuzzerSHA1.cpp
     FuzzerTracePC.cpp
+    FuzzerTraceState.cpp
     FuzzerUtil.cpp
     FuzzerUtilDarwin.cpp
     FuzzerUtilLinux.cpp
Index: lib/Fuzzer/FuzzerDriver.cpp
===================================================================
--- lib/Fuzzer/FuzzerDriver.cpp
+++ lib/Fuzzer/FuzzerDriver.cpp
@@ -13,6 +13,7 @@
 #include "FuzzerInterface.h"
 #include "FuzzerInternal.h"
 #include "FuzzerIO.h"
+#include "FuzzerIOPlatform.h"
 #include "FuzzerMutate.h"
 #include "FuzzerRandom.h"
 
Index: lib/Fuzzer/FuzzerIO.h
===================================================================
--- lib/Fuzzer/FuzzerIO.h
+++ lib/Fuzzer/FuzzerIO.h
@@ -41,18 +41,5 @@
 
 void Printf(const char *Fmt, ...);
 
-bool IsFile(const std::string &Path);
-
-void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
-                             std::vector<std::string> *V, bool TopDir);
-
-char GetSeparator();
-
-FILE* OpenFile(int Fd, const char *Mode);
-
-int CloseFile(int Fd);
-
-int DuplicateFile(int Fd);
-
 }  // namespace fuzzer
 #endif  // LLVM_FUZZER_IO_H
Index: lib/Fuzzer/FuzzerIO.cpp
===================================================================
--- lib/Fuzzer/FuzzerIO.cpp
+++ lib/Fuzzer/FuzzerIO.cpp
@@ -8,29 +8,25 @@
 //===----------------------------------------------------------------------===//
 // IO functions.
 //===----------------------------------------------------------------------===//
+#include "FuzzerIOPlatform.h"
 #include "FuzzerIO.h"
 #include "FuzzerExtFunctions.h"
 #include "FuzzerDefs.h"
-#include <iterator>
+#include <algorithm>
+#include <cstdarg>
+#include <cstdio>
 #include <fstream>
-#include <dirent.h>
+#include <iterator>
 #include <sys/types.h>
 #include <sys/stat.h>
+#if LIBFUZZER_POSIX
 #include <unistd.h>
-#include <cstdarg>
-#include <cstdio>
+#endif
 
 namespace fuzzer {
 
 static FILE *OutputFile = stderr;
 
-bool IsFile(const std::string &Path) {
-  struct stat St;
-  if (stat(Path.c_str(), &St))
-    return false;
-  return S_ISREG(St.st_mode);
-}
-
 long GetEpoch(const std::string &Path) {
   struct stat St;
   if (stat(Path.c_str(), &St))
@@ -38,29 +34,6 @@
   return St.st_mtime;
 }
 
-void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
-                             std::vector<std::string> *V, bool TopDir) {
-  auto E = GetEpoch(Dir);
-  if (Epoch)
-    if (E && *Epoch >= E) return;
-
-  DIR *D = opendir(Dir.c_str());
-  if (!D) {
-    Printf("No such directory: %s; exiting\n", Dir.c_str());
-    exit(1);
-  }
-  while (auto E = readdir(D)) {
-    std::string Path = DirPlusFile(Dir, E->d_name);
-    if (E->d_type == DT_REG || E->d_type == DT_LNK)
-      V->push_back(Path);
-    else if (E->d_type == DT_DIR && *E->d_name != '.')
-      ListFilesInDirRecursive(Path, Epoch, V, false);
-  }
-  closedir(D);
-  if (Epoch && TopDir)
-    *Epoch = E;
-}
-
 Unit FileToVector(const std::string &Path, size_t MaxSize, bool ExitOnError) {
   std::ifstream T(Path);
   if (ExitOnError && !T) {
@@ -121,23 +94,25 @@
 
 std::string DirPlusFile(const std::string &DirPath,
                         const std::string &FileName) {
-  return DirPath + "/" + FileName;
+  return DirPath + GetSeparator() + FileName;
 }
 
 void DupAndCloseStderr() {
-  int OutputFd = dup(2);
+  int OutputFd = DuplicateFile(2);
   if (OutputFd > 0) {
-    FILE *NewOutputFile = fdopen(OutputFd, "w");
+    FILE *NewOutputFile = OpenFile(OutputFd, "w");
     if (NewOutputFile) {
       OutputFile = NewOutputFile;
       if (EF->__sanitizer_set_report_fd)
         EF->__sanitizer_set_report_fd(reinterpret_cast<void *>(OutputFd));
-      close(2);
+      CloseFile(2);
     }
   }
 }
 
-void CloseStdout() { close(1); }
+void CloseStdout() {
+  CloseFile(1);
+}
 
 void Printf(const char *Fmt, ...) {
   va_list ap;
Index: lib/Fuzzer/FuzzerIOPlatform.h
===================================================================
--- /dev/null
+++ lib/Fuzzer/FuzzerIOPlatform.h
@@ -0,0 +1,34 @@
+//===- FuzzerIOPlatform.h - Internal header for IO plat. utils --*- C++ -* ===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Interface for IO platform specific functions.
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_FUZZER_IO_PLATFORM_H
+#define LLVM_FUZZER_IO_PLATFORM_H
+
+#include <string>
+#include <vector>
+
+namespace fuzzer {
+
+// Platform specific functions.
+bool IsFile(const std::string &Path);
+
+void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
+                             std::vector<std::string> *V, bool TopDir);
+
+char GetSeparator();
+
+FILE* OpenFile(int Fd, const char *Mode);
+
+int CloseFile(int Fd);
+
+int DuplicateFile(int Fd);
+
+}  // namespace fuzzer
+#endif  // LLVM_FUZZER_IO_PLATFORM_H
Index: lib/Fuzzer/FuzzerIOPosix.cpp
===================================================================
--- /dev/null
+++ lib/Fuzzer/FuzzerIOPosix.cpp
@@ -0,0 +1,74 @@
+//===- FuzzerIOPosix.cpp - IO utils for Posix. ----------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// IO functions implementation using Posix API.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerDefs.h"
+#if LIBFUZZER_POSIX
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include <cstdarg>
+#include <cstdio>
+#include <dirent.h>
+#include <fstream>
+#include <iterator>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+namespace fuzzer {
+
+bool IsFile(const std::string &Path) {
+  struct stat St;
+  if (stat(Path.c_str(), &St))
+    return false;
+  return S_ISREG(St.st_mode);
+}
+
+void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
+                             std::vector<std::string> *V, bool TopDir) {
+  auto E = GetEpoch(Dir);
+  if (Epoch)
+    if (E && *Epoch >= E) return;
+
+  DIR *D = opendir(Dir.c_str());
+  if (!D) {
+    Printf("No such directory: %s; exiting\n", Dir.c_str());
+    exit(1);
+  }
+  while (auto E = readdir(D)) {
+    std::string Path = DirPlusFile(Dir, E->d_name);
+    if (E->d_type == DT_REG || E->d_type == DT_LNK)
+      V->push_back(Path);
+    else if (E->d_type == DT_DIR && *E->d_name != '.')
+      ListFilesInDirRecursive(Path, Epoch, V, false);
+  }
+  closedir(D);
+  if (Epoch && TopDir)
+    *Epoch = E;
+}
+
+char GetSeparator() {
+  return '/';
+}
+
+FILE* OpenFile(int Fd, const char* Mode) {
+  return fdopen(Fd, Mode);
+}
+
+int CloseFile(int fd) {
+  return close(fd);
+}
+
+int DuplicateFile(int Fd) {
+  return dup(Fd);
+}
+
+}  // namespace fuzzer
+#endif // LIBFUZZER_POSIX
Index: lib/Fuzzer/FuzzerIOWindows.cpp
===================================================================
--- /dev/null
+++ lib/Fuzzer/FuzzerIOWindows.cpp
@@ -0,0 +1,142 @@
+//===- FuzzerIOWindows.cpp - IO utils for Windows. ------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// IO functions implementation for Windows.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerDefs.h"
+#if LIBFUZZER_WINDOWS
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+#include <cstdarg>
+#include <cstdio>
+#include <fstream>
+#include <io.h>
+#include <iterator>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <windows.h>
+
+namespace fuzzer {
+
+bool IsFile(const std::string &Path) {
+  DWORD Att = GetFileAttributesA(Path.c_str());
+
+  if (Att == INVALID_FILE_ATTRIBUTES) {
+    Printf("GetFileAttributesA() failed for \"%s\" (Error code: %lu).\n",
+        Path.c_str(), GetLastError());
+    return false;
+  }
+
+  if (Att & FILE_ATTRIBUTE_NORMAL)
+    return true;
+
+  if (Att & FILE_ATTRIBUTE_DIRECTORY)
+    return false;
+
+  HANDLE FileHandle(
+      CreateFileA(Path.c_str(), 0, FILE_SHARE_READ, NULL, OPEN_EXISTING,
+                  FILE_FLAG_BACKUP_SEMANTICS, 0));
+
+  if (FileHandle == INVALID_HANDLE_VALUE) {
+    Printf("CreateFileA() failed for \"%s\" (Error code: %lu).\n", Path.c_str(),
+        GetLastError());
+    return false;
+  }
+
+  DWORD FileType = GetFileType(FileHandle);
+
+  if (FileType == FILE_TYPE_UNKNOWN) {
+    Printf("GetFileType() failed for \"%s\" (Error code: %lu).\n", Path.c_str(),
+        GetLastError());
+    CloseHandle(FileHandle);
+    return false;
+  }
+
+  if (FileType != FILE_TYPE_DISK) {
+    CloseHandle(FileHandle);
+    return false;
+  }
+
+  CloseHandle(FileHandle);
+  return true;
+}
+
+void ListFilesInDirRecursive(const std::string &Dir, long *Epoch,
+                             std::vector<std::string> *V, bool TopDir) {
+  auto E = GetEpoch(Dir);
+  if (Epoch)
+    if (E && *Epoch >= E) return;
+
+  std::string Path(Dir);
+  assert(!Path.empty());
+  if (Path.back() != '\\')
+      Path.push_back('\\');
+  Path.push_back('*');
+
+  // Get the first directory entry.
+  WIN32_FIND_DATAA FirstFind;
+  HANDLE FindHandle(FindFirstFileA(Path.c_str(), &FirstFind));
+  if (FindHandle == INVALID_HANDLE_VALUE)
+  {
+    Printf("No file found in: %s.\n", Dir.c_str());
+    return;
+  }
+
+  do {
+    size_t FilenameLen = strlen(FirstFind.cFileName);
+    if ((FilenameLen == 1 && FirstFind.cFileName[0] == '.') ||
+        (FilenameLen == 2 && FirstFind.cFileName[0] == '.' &&
+                             FirstFind.cFileName[1] == '.'))
+      continue;
+
+    std::string FileName = DirPlusFile(Dir, FirstFind.cFileName);
+
+    DWORD Att = GetFileAttributesA(FileName.c_str());
+
+    if (Att == INVALID_FILE_ATTRIBUTES) {
+      Printf("GetFileAttributesA() failed for \"%s\" (Error code: %lu).\n",
+          FileName.c_str(), GetLastError());
+      FindClose(FindHandle);
+      return;
+    }
+
+    if (Att & FILE_ATTRIBUTE_DIRECTORY)
+      ListFilesInDirRecursive(FileName, Epoch, V, false);
+    else if (IsFile(FileName))
+      V->push_back(FileName);
+  } while (FindNextFileA(FindHandle, &FirstFind));
+
+  DWORD LastError = GetLastError();
+  if (LastError != ERROR_NO_MORE_FILES)
+    Printf("FindNextFileA failed (Error code: %lu).\n", LastError);
+
+  FindClose(FindHandle);
+
+  if (Epoch && TopDir)
+    *Epoch = E;
+}
+
+char GetSeparator() {
+  return '\\';
+}
+
+FILE* OpenFile(int Fd, const char* Mode) {
+  return _fdopen(Fd, Mode);
+}
+
+int CloseFile(int Fd) {
+  return _close(Fd);
+}
+
+int DuplicateFile(int Fd) {
+  return _dup(Fd);
+}
+
+}  // namespace fuzzer
+#endif // LIBFUZZER_WINDOWS