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#### 10.6.4 Cleaning Up from Nonlocal Exits

The `unwind-protect` construct is essential whenever you temporarily put a data structure in an inconsistent state; it permits you to make the data consistent again in the event of an error or throw. (Another more specific cleanup construct that is used only for changes in buffer contents is the atomic change group; Atomic Changes.)

Special Form: unwind-protect body-form cleanup-forms…

  `unwind-protect` executes body-form with a guarantee that the cleanup-forms will be evaluated if control leaves body-form, no matter how that happens. body-form may complete normally, or execute a `throw` out of the `unwind-protect`, or cause an error; in all cases, the cleanup-forms will be evaluated.

  If body-form finishes normally, `unwind-protect` returns the value of body-form, after it evaluates the cleanup-forms. If body-form does not finish, `unwind-protect` does not return any value in the normal sense.

  Only body-form is protected by the `unwind-protect`. If any of the cleanup-forms themselves exits nonlocally (via a `throw` or an error), `unwind-protect` is not guaranteed to evaluate the rest of them. If the failure of one of the cleanup-forms has the potential to cause trouble, then protect it with another `unwind-protect` around that form.

  The number of currently active `unwind-protect` forms counts, together with the number of local variable bindings, against the limit `max-specpdl-size` (see Local Variables).

For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:

    (let ((buffer (get-buffer-create " *temp*")))
      (with-current-buffer buffer
        (unwind-protect
            body-form
          (kill-buffer buffer))))

You might think that we could just as well write `(kill-buffer (current-buffer))` and dispense with the variable `buffer`. However, the way shown above is safer, if body-form happens to get an error after switching to a different buffer! (Alternatively, you could write a `save-current-buffer` around body-form, to ensure that the temporary buffer becomes current again in time to kill it.)

Emacs includes a standard macro called `with-temp-buffer` which expands into more or less the code shown above (see Current Buffer). Several of the macros defined in this manual use `unwind-protect` in this way.

Here is an actual example derived from an FTP package. It creates a process (see Processes) to try to establish a connection to a remote machine. As the function `ftp-login` is highly susceptible to numerous problems that the writer of the function cannot anticipate, it is protected with a form that guarantees deletion of the process in the event of failure. Otherwise, Emacs might fill up with useless subprocesses.

    (let ((win nil))
      (unwind-protect
          (progn
            (setq process (ftp-setup-buffer host file))
            (if (setq win (ftp-login process host user password))
                (message "Logged in")
              (error "Ftp login failed")))
        (or win (and process (delete-process process)))))

This example has a small bug: if the user types `C-g` to quit, and the quit happens immediately after the function `ftp-setup-buffer` returns but before the variable `process` is set, the process will not be killed. There is no easy way to fix this bug, but at least it is very unlikely.

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