Utility to create an NTFS junction point. Previously reviewed at:

create-ntfs-junction.c

+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+//
+// This is a simple utility for creating NTFS junction points on XP and later
+// versions of Windows.
+
+#define _WIN32_WINNT 0x0500
+
+#include <stdio.h>
+#include <windows.h>
+
+// This struct definition is absent from the system header files,
+// but is described here:
+//   http://msdn.microsoft.com/en-us/library/ff552012.aspx
+typedef struct _REPARSE_DATA_BUFFER {
+  ULONG  ReparseTag;
+  USHORT  ReparseDataLength;
+  USHORT  Reserved;
+  union {
+    struct {
+      USHORT SubstituteNameOffset;
+      USHORT SubstituteNameLength;
+      USHORT PrintNameOffset;
+      USHORT PrintNameLength;
+      ULONG Flags;
+      WCHAR PathBuffer[1];
+    } SymbolicLinkReparseBuffer;
+    struct {
+      USHORT SubstituteNameOffset;
+      USHORT SubstituteNameLength;
+      USHORT PrintNameOffset;
+      USHORT PrintNameLength;
+      WCHAR PathBuffer[1];
+    } MountPointReparseBuffer;
+    struct {
+      UCHAR DataBuffer[1];
+    } GenericReparseBuffer;
+  };
+} REPARSE_DATA_BUFFER;
+
+int main (int argc, char *argv[])
+{
+  char buf[MAX_PATH*sizeof(WCHAR) + sizeof(REPARSE_DATA_BUFFER)]={'\0'};
+  char* dest_path=NULL;
+  char* src_path=NULL;
+  char* src_link=NULL;
+  char* src_vol=NULL;
+  char* dest_vol=NULL;
+  HANDLE dir=NULL;
+  REPARSE_DATA_BUFFER* reparse = (REPARSE_DATA_BUFFER*) buf;
+  int path_len=0, data_len=0;
+  DWORD ioctl_return = 0xdeadbeef;
+
+  // To allow this to be used as a drop-in replacement for the posix ln command,
+  // allow (and ignore) extra flags.
+  if (argc != 3 && (argc !=4 || *argv[1] != '-')) {
+    fputs("Usage: create-ntfs-junction <destination dir> <source dir>\n", stderr);
+    return -1;
+  }
+
+  src_path = argv[argc-2];
+  path_len = strlen(src_path);
+  if (src_path[path_len-1] == '\\')
+    src_path[path_len-1] = '\0';
+
+  dest_path = argv[argc-1];
+  path_len = strlen(dest_path);
+  if (dest_path[path_len-1] == '\\')
+    dest_path[path_len-1] = '\0';
+
+  if (GetFileAttributes(src_path) == INVALID_FILE_ATTRIBUTES) {
+    fprintf(stderr, "%s: No such animal.\n", src_path);
+    return -1;
+  }
+
+  if (!GetVolumePathName(src_path, buf, MAX_PATH)) {
+    fprintf(stderr, "Couldn't get volume name for '%s'.\n", src_path);
+    return -1;
+  }
+  src_vol = _strdup(buf);
+
+  if (!GetVolumePathName(dest_path, buf, MAX_PATH)) {
+    fprintf(stderr, "Couldn't get volume name for '%s'.\n", dest_path);
+    return -1;
+  }
+  dest_vol = _strdup(buf);
+
+  if (strcmp(src_vol, dest_vol)) {
+    fprintf(stderr, "Cannot create junction point across volume boundary.\n");
+    fprintf(stderr, "  (from volume '%s' to volume '%s')\n", src_vol, dest_vol);
+    return -1;
+  }
+
+  // End of input sanity checks; file system modifications may now occur.
+
+  if (GetFileAttributes(dest_path) == INVALID_FILE_ATTRIBUTES) {
+    if (!CreateDirectory(dest_path, NULL)) {
+      switch(GetLastError()) {
+        case ERROR_ALREADY_EXISTS:
+          fprintf(stderr, "Can't create directory %s because it already exists "
+                  "(this should never happen).\n", dest_path);
+          return -1;
+          break;
+        case ERROR_PATH_NOT_FOUND:
+          fprintf(stderr, "Can't create directory %s because some part of the "
+                  "intermediate path doesn't exist.", dest_path);
+          return -1;
+          break;
+        default:
+          fprintf(stderr, "Unknown error occurred while trying to create "
+                  "directory %s.", dest_path);
+          return -1;
+          break;
+      }
+    }
+  }
+
+  dir = CreateFile(dest_path,
+                   GENERIC_WRITE,
+                   0,
+                   NULL,
+                   OPEN_EXISTING,
+                   FILE_FLAG_OPEN_REPARSE_POINT | FILE_FLAG_BACKUP_SEMANTICS,
+                   NULL);
+
+  strcpy_s(buf, 5, "\\??\\");
+  GetFullPathName(src_path, MAX_PATH, buf+4, NULL);
+  src_link = _strdup(buf);
+
+  memset(buf, 0, sizeof(buf));
+  path_len = MultiByteToWideChar(CP_ACP,
+                                 0,
+                                 src_link,
+                                 -1,
+                                 reparse->MountPointReparseBuffer.PathBuffer,
+                                 MAX_PATH*sizeof(WCHAR));
+
+  reparse->ReparseTag = IO_REPARSE_TAG_MOUNT_POINT;
+  reparse->ReparseDataLength = (path_len+2)*sizeof(WCHAR) + 6;
+  reparse->MountPointReparseBuffer.SubstituteNameLength =
+      (path_len-1) * sizeof(WCHAR);
+  reparse->MountPointReparseBuffer.PrintNameOffset =
+      path_len * sizeof(WCHAR);
+  data_len = reparse->ReparseDataLength + 8;
+
+  if (!DeviceIoControl(dir,
+                       FSCTL_SET_REPARSE_POINT,
+                       &buf,
+                       data_len,
+                       NULL,
+                       0,
+                       &ioctl_return,
+                       NULL)) {
+    fprintf(stderr, "Junction point creation failed (ioctl_return=0x%x) (%d)\n",
+            ioctl_return, GetLastError());
+    return 1;
+  }
+
+  return 0;
+}