I just wanna do something like this

(defun my-fun (reg-path) 
  "reads the value from the given Windows registry path."
     ...??...
)

is there a built-in fn that does this?

or is there a command-line tool builtin to windows that I can run to retrieve a reg value?

The way I am imagining doing it, is to run a .js file in cscript.exe that does the work.

ANSWER

(defun my-reg-read (regpath)
  "read a path in the Windows registry. This probably works for string 
  values only. If the path does not exist,开发者_开发技巧 it returns nil. "
  (let ((reg.exe (concat (getenv "windir") "\\system32\\reg.exe"))
        tokens last-token)

    (setq reg-value (shell-command-to-string (concat reg.exe " query " regpath))
          tokens (split-string reg-value nil t)
          last-token (nth (1- (length tokens)) tokens))

    (and (not (string= last-token "value.")) last-token)))

==> Thank you to Oleg.

Use reg command line utility.

Emacs command

(shell-command "REG QUERY KeyName" &optional OUTPUT-BUFFER ERROR-BUFFER)

allows you to run a shell command. The output is sent to the OUTPUT-BUFFER.

Here's what I did:

(defun my-reg-read (regpath)
  "read a path in the Windows registry"
  (let ((temp-f (make-temp-file "regread_" nil ".js"))
        (js-code "var WSHShell, value, regpath = '';try{ if (WScript.Arguments.length > 0){regpath = WScript.Arguments(0); WSHShell = WScript.CreateObject('WScript.Shell'); value = WSHShell.RegRead(regpath); WScript.Echo(value); }}catch (e1){ WScript.Echo('error reading registry: ' + e1);}")
        reg-value)
  (with-temp-file temp-f (insert js-code))
  (setq reg-value (shell-command-to-string (concat temp-f " " regpath)))
  (delete-file temp-f)
  reg-value ))

The elisp function creates a temporary file, then writes a bit of javascript logic into it. the javascript reads the windows registry for a given path. The elisp fn then runs the temporary file, passing the registry path to read. It deletes the file, then returns the result of running it.