Change target for symbolic link in windows

Question

How can I change target of already existing symbolic link. Any solution would work: console, powershell,etc.

Important think is to make this change atomically so solution to delete link and then create a new one do not work for me.

Ashbaugh

If you wrap the deletion and recreation in a transaction I believe it will be atomic. – Hoogh

What kind of transaction? File system operation like move file on the same disk I think are atomic. Two file operation are not. Correct me please if I'm wrong. – Ashbaugh

See my answer. Support for transactional NTFS was added in Windows Vista. – Hoogh

The answer from @Gyan below absolutely works. I also ran tests and it works for me. – Anyways

Answer 1

This seems to be possible with the ZwFsControlFile function using the FSCTL_SET_REPARSE_POINT control code. At least that's what I gleaned from how Far Manager does it via Process Monitor.

Casavant

As of Mar 2017 Far Manager uses DeviceIoControl function to change/edit either symlink or junction target(s). – Osteomalacia

Answer 2

You could use transactional NTFS. See the following functions:

• CreateTransaction
• DeleteFileTransacted
• CreateSymbolicLinkTransacted
• CommitTransaction

The downside is that MS are deprecating support for transactions. In particular transactions are not available in the new file system being introduced in Windows Server 2012.

Hoogh

Thank you, Harry. Never though that it is possible to have transaction. Unfortunately deprecation is a bad sign so I'm not sure I'll be able to use it. – Ashbaugh

Answer 3

You can try creating a new symbolic link, and then renaming the new link to overwrite the old.

There are some possibilities mentioned here:

Is an atomic file rename (with overwrite) possible on Windows?

Mcglynn

Answer 4

A slight modification of LSemi's method works for me in Windows 7 CMD console

mklink TempLink NewTarget

copy /l /y TempLink OldLink

del TempLink

I've a process that reads OldLink multiple times a second and with this method I'm able to continuously update OldLink to new targets without causing a read error. Strictly speaking, this isn't probably atomic but the time taken to effect the symlink copy must be so small, that it doesn't interfere.

Bioastronautics

What does /l do? It's not listed for Windows Server copy. – Gerianne

Windows 7 copy /? shows this /L If the source is a symbolic link, copy the link to the target instead of the actual file the source link points to. – Bioastronautics

This answer is brilliant. Why was it downvoted? If anything, @Gyan, you can add more details, like output of dir to show exactly how the file system changes after each step. I tested your steps: In a separate Cygwin window, I run a simple infinite loop: while true; do cat symlink.txt; done. Then I used your trick to update symlink.txt to point to different file. Never once did my infinite read file loop fail. Works perfectly. I wonder how copy executes this operation! It is annoying that MKLINK does not have a /F (force) option to do the same. UNIX has it. – Anyways