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…
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.
body-form finishes normally,
unwind-protect returns the value of
body-form, after it evaluates the
body-form does not finish,
unwind-protect does not return any value in the normal sense.
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*")))
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
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))
(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.