Utility to create NTFS junctions points.

tools/junction/junction.cpp

+// 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[])

[cmp, 2012/03/14 17:43:38]

no space after main

[szager, 2012/03/14 19:04:04]

Done.

+{
+  char buf[MAX_PATH*sizeof(WCHAR) + sizeof(REPARSE_DATA_BUFFER)]={'\0'};
+  char* dest_path=NULL;

[cmp, 2012/03/14 17:43:38]

spaces around = here and below

[szager, 2012/03/14 19:04:04]

Done.

+  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: 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) {

[cmp, 2012/03/14 17:45:11]

This code seems to assume that it's running on a NTFS partition.  Is that a
requirement?  Is it possible to check and exit early if that requirement isn't
met?

[szager, 2012/03/14 19:04:04]

Added this check below.

+    fprintf(stderr, "%s: No such animal.\n", src_path);

[cmp, 2012/03/14 17:43:38]

:)  Any objection to changing this to "Invalid file attributes."?

[szager, 2012/03/14 19:04:04]

Done.

+    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:

[cmp, 2012/03/14 17:43:38]

Is there a way to check for the next 2 cases before attempting to create the
directory?

[szager, 2012/03/14 19:04:04]

There's no ordering implied in a case statement, other than checking the value
of the integer on which you're switching.  I can reorder the statements if you'd
like, but it will be a no-op change.

[cmp, 2012/03/14 19:10:10]

I mean what if you moved the tests for these conditions (directory already
exists, some part of the intermediate path doesn't exist) above into the tests
before line 95.  Here it seems that you're mixing the action with the tests.  I
will be fine with that, though, just checking if there are tests you could use
above to look for these conditions instead.

+          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;

[cmp, 2012/03/14 17:43:38]

In the other error conditions you return -1, but you return 1 here.  Is that
significant?  This error seems similar enough that it would be consistent to
return -1 here, too, I suppose.

[szager, 2012/03/14 19:04:04]

I usually use -1 to indicate that the program exited before the 'main' action
was even attempted, but 1 to indicate that the 'main' action was attempted and
failed.  That's just personal preference, so if there's a more appropriate
heuristic, that's fine with me.

[cmp, 2012/03/14 19:10:10]

Ok, I'm sold. :)

+  }
+
+  return 0;
+}