[argyllcms] Re: ubuntu-10.10-desktop-amd64: dispcal -D5 -? hangs
- From: Gerhard Fuernkranz <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Sat, 23 Apr 2011 14:19:05 +0200
Am 21.04.2011 06:06, schrieb Graeme Gill:
Gerhard Fuernkranz wrote:
If open() blocks on a serial port, then a typical reason is that modem control is enabled for this
tty (i.e. the CLOCAL flag not set, see "man termios"), but the attached device does not
assert the DSR signal. I.e. open() waits until the device reports "I am ready".
Invoke "stty -a -F /dev/ttyS2" in order to check the settings of the tty. Look for "clocal"
(-> no modem control) versus "-clocal" (-> modem control) in the output.
Hi Gerhard,
is there anything I could do from a progromatic point of
view to avoid such a hang ?
Hello Graeme,
you can use pass the O_NONBLOCK flag to open() in order to avoid that it blocks.
Using O_NONBLOCK is also a prerequisite for turning turn on the CLOCAL flag,
because tcsetattr() requires an open file descriptor. Take care however, that
subsequent read() and write() calls will be non-blocking too, when the tty was
opened with O_NONBLOCK (e.g. read() would then immediately return -1, with
errno==EAGAIN, if no buffered received data are available for immediate
retrieval, and not wait for data).
Presumably I don't want to alter the CLOCAL flag for a serial port that
isn't connected to an instrument. Perhaps there's a better way of
getting a list of potential instrument serial ports that doesn't
involve opening them ?
Unfortunately I don't know any method either for enumerating existing serial ports in
Linux. But even if such a function was available, one would still not know which kind
of device is connected to the port w/o trying to talk to the device. In order not to
conflict with other programs which are currently accessing serial ports (e.g. getty,
minicom, etc.), you could try to lock a serial port before accessing it (keeping it
locked as long you keep it open, and don't touch ports which can't be locked). IMO
most program accessing serial ports use UCCP-style lock files (residing in the
directory /var/lock/ on Linux, e.g. /var/lock/LCK..tty<XX>). See
http://docs.freebsd.org/info/uucp/uucp.info.UUCP_Lock_Files.html. For instance, when
I connect minicom to tty8, then I see the following file
# cat /var/lock/LCK..tty8
15605 minicom root
which prevents other applications honoring UUCP lock files from accessing the
port simultaneously.
Regards,
Gerhard
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