Saturday, April 24, 2010

NTFS junctions are a poor substitute for symbolic links

Recently, I have been working in a Windoze-XP environment using multiple sandboxes checked out from CVS. Each sandbox needs a large collection of third party libraries (headers, lib and .dll files). Each third party directory takes up around 2GB. This space very quickly mounts up. In Unix we would use symbolic links to make all the third party directories point to a common place. But what to do in Windoze? Some people would say that NTFS junctions is the answer. Well, they can help and I am using them for this purpose but there are some nasties and I am documenting them here. I once saw a signature on USENET which goes: "Those who do not understand Unix are condemned to reinvent it, poorly." This is certainly true of Windoze. It doesn't understand symbolic links so it is no suprise to learn that NTFS junctions are a very poor imitation. Here are several points to bear in mind:
  • They are a cross between hard links and symbolic links. Like the hard link, they can onloy refer to files on the same partition, but like symbolic links the path of the link is stored which means it can become invalid. This is because Windoze filesystems do not support links! Also, only directories can be linked.
  • Windoze-XP has insufficient support for NTFS junctions despite them first appearing in Windoze-2000. There are no commands to manipulate them. You will have to download and install the SysInternals commands and use the junction command.
  • More explicit support for NTFS junctions was added to Vista and is also in Windows 7. But even by these versions of Windoze, support is incomplete. When you use Windows Explorer to navigate to a directory that contains NTFS junctions, if you delete the directory it deletes the files that the junctions refer to!!! See for the gory details.
The only way to delete a junction safely is to the use the SysInternals junction command. There is no safe way to delete a directory that may contain junctions. Currently, I do not know what the solution is to these problems. The cygwin rm command does not help, it follows the junction references just like Windoze Explorer does. I have a feeling a special delete command may have to be written. I will probably have to write it, in Python, and it will probably use the SysInternals junction command to delete any NTFS junctions it finds.