(Please ignore the wrong To: field, e-mail program is being silly)
>I am trying to figure out how to abort a read from a socket after some time
>elapses. >I failed to figure out how to do so.
>
> All code below runs after handler is set:
>
> (sigaction SIGALRM (lambda _ (display "Alarm!\n")))
Assuming the read-char takes too long, the kernel sends a SIGALRM to Guile.
Hence, the C signal handler is run (which sets some fields somewhere indicating
that this handler should be run later in the sense of system-async-mark), and
the syscall behind read-char returns EINTR.
As this is a fake error (passing it on as a Scheme exception would result in
rather messy semantics, e.g. consider the case where things are interrupted
twice in a row, time such that the exception handler itself is interrupted with
a new exception), Guile decides to retry the syscall, but before that, it
checks if there is a system-async-mark to be done, so it does that thing –
i.e., it displays “Alarm!\n” and merrily continue on reading a character.
Now, going by the rest of the mail, it appears you don’t just want to print a
message, you want to _abort_. So you should modify your handlers to abort,
i.e., call an escape continuation.
(define escaper (make-parameter #false)
(sigaction SIGALRM (lambda _ (let ((e (escaper))) (if e (e)))))
[set up sockets, etc.]
;; returns char on success, #false on timeout.
(define (read-char-or-timeout)
(let/ec e
[set up an alarm]
(parameterize ((escaper (lambda () (e #false))))
(read-char e)))) ; XXX add port argument, maybe stop the alarm afterwards?
(This assumes the signal handler is run in the same thread as the thread using
read-char-or-timeout, I don’t know if this is the case.)
This code has a bug, however: it might be the case that the signal is received
right after read-char finishes but before the parameterize ends. In that case,
the character is lost and read-char-or-timeout reports a “timeout”, losing the
character, which is most likely undesired.
So, if you do things like this, I think you should modify the signal handler to
instead set a a-timeout-happened flag, and let read-char-or-timeout read that
flag.
Best regards,
Maxime Devos
