Hello, 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")))
The (time) procedure is just modified ,time from the REPL, full code at the end
of the email.
I started with the usual C way, alarm, and tested it with a sleep:
(display "Sleep:\n")
(alarm 1)
(time (λ () (sleep 3)))
That does seems to work fine:
Sleep:
Alarm!
;; Result: 1
;; 1.000361s real time, 0.000119s run time. 0.000000s spent in GC.
However when I moved to the sockets, I quickly hit a wall:
(display "read-char:\n")
(let* ((ai (car (getaddrinfo "www.gnu.org" "http")))
(s (socket (addrinfo:fam ai)
(addrinfo:socktype ai)
(addrinfo:protocol ai))))
(connect s (addrinfo:addr ai))
(alarm 1)
(time (λ () (read-char s))))
That does hang for a long time:
read-char:
Alarm!
;; Result: #<eof>
;; 51.742715s real time, 0.000375s run time. 0.000000s spent in GC.
The alarm was obviously triggered, but the read-char was not interrupted. I
would have expected to receive (maybe as an exception) EINTR.
When I try non-blocking code:
(display "read-char (non-block):\n")
(let* ((ai (car (getaddrinfo "www.gnu.org" "http")))
(s (socket (addrinfo:fam ai)
(addrinfo:socktype ai)
(addrinfo:protocol ai))))
(connect s (addrinfo:addr ai))
(let ((flags (fcntl s F_GETFL)))
(fcntl s F_SETFL (logior O_NONBLOCK flags)))
(alarm 1)
(time (λ () (read-char s))))
It still does not work:
read-char (non-block):
Alarm!
;; Result: #<eof>
;; 51.581392s real time, 0.000371s run time. 0.000000s spent in GC.
I would have expected to receive EWOULDBLOCK. But let's not get distracted by
the non-blocking variant.
So, what would be a good way to do this?
Thank you for any advice on this.
Have a nice day,
Tomas Volf
Here is full script I used for playing with this:
;;; I took this code from ,time implementation.
(use-modules (ice-9 format))
(define (time thunk)
(let* ((gc-start (gc-run-time))
(real-start (get-internal-real-time))
(run-start (get-internal-run-time))
(result (thunk))
(run-end (get-internal-run-time))
(real-end (get-internal-real-time))
(gc-end (gc-run-time)))
(define (diff start end)
(/ (- end start) 1.0 internal-time-units-per-second))
(format #t ";; Result: ~s\n" result)
(format #t ";; ~,6Fs real time, ~,6Fs run time. ~,6Fs spent in GC.\n"
(diff real-start real-end)
(diff run-start run-end)
(diff gc-start gc-end))
result))
(sigaction SIGALRM (lambda _ (display "Alarm!\n")))
(display "Sleep:\n")
(alarm 1)
(time (λ () (sleep 3)))
(display "read-char:\n")
(let* ((ai (car (getaddrinfo "www.gnu.org" "http")))
(s (socket (addrinfo:fam ai)
(addrinfo:socktype ai)
(addrinfo:protocol ai))))
(connect s (addrinfo:addr ai))
(alarm 1)
(time (λ () (read-char s))))
(display "read-char (non-block):\n")
(let* ((ai (car (getaddrinfo "www.gnu.org" "http")))
(s (socket (addrinfo:fam ai)
(addrinfo:socktype ai)
(addrinfo:protocol ai))))
(connect s (addrinfo:addr ai))
(let ((flags (fcntl s F_GETFL)))
(fcntl s F_SETFL (logior O_NONBLOCK flags)))
(alarm 1)
(time (λ () (read-char s))))
--
There are only two hard things in Computer Science:
cache invalidation, naming things and off-by-one errors.
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