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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.
unwind-protect executes the body with a guarantee that the
cleanup-forms will be evaluated if control leaves body, no
matter how that happens. The body 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 the body forms finish normally, unwind-protect returns
the value of the last body form, after it evaluates the
cleanup-forms. If the body forms do not finish,
unwind-protect does not return any value in the normal sense.
Only the body 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 section 11.3 Local Variables).
For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:
(save-excursion
(let ((buffer (get-buffer-create " *temp*")))
(set-buffer buffer)
(unwind-protect
body
(kill-buffer buffer))))
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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 happens to get an
error after switching to a different buffer! (Alternatively, you could
write another save-excursion around the body, 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 section 27.2 The 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 section 37. 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)))))
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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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