lock access to serial port

[kkadrese]

hello group,

how to get ttyS0 serial port for exclusive access? I have a python
script that uses this device with AT commands. I need that two
instances can call simultaneosuly this python script but only one of
them gets the device. I tried fcntl.flock, it was just ignored, put
writtable file LCK..ttyS0 in /var/lock, tried ioctl (but I may not
know what are appropriate arguments), googled half a day for various
phrases, error messages etc....without success.

please help,

Andra

 

[taco]

hello group,

how to get ttyS0 serial port for exclusive access? I have a python
script that uses this device with AT commands. I need that two
instances can call simultaneosuly this python script but only one of
them gets the device. I tried fcntl.flock, it was just ignored, put
writtable file LCK..ttyS0 in /var/lock, tried ioctl (but I may not
know what are appropriate arguments), googled half a day for various
phrases, error messages etc....without success.

please help,

Andra

not sure if I understand you well, but how about a server application which
accepts 2 clients which can send messages which are output on the serial
device? The serial device access protected with a mutex while receiving
messages done in 2 threads or something.
taco

 

[kkadrese]

not the goal setup but the way I tried it out - a webpage to send an
sms to mobile phone. This is an php what calls a python script. A
python script opens serial device, checks for gsm network, sends a
message - on all the way I try to make sure that appropriate answers
are received from the gsm device so that I know that all is going well
(and if not - cancel it with error message).

Testing it with sending an sms from two open webpages, I normally got
that one message was sent and for the other I received "try once more,
happened that and that". But I got also an inadmissible situation -
the received sms contained also unwanted text. As far as I can realize
how to deal with it is not allowing writing to the device for two
processes simultaneously.


Andra

 

[Grant Edwards]

how to get ttyS0 serial port for exclusive access? I have a python
script that uses this device with AT commands. I need that two
instances can call simultaneosuly this python script but only one of
them gets the device. I tried fcntl.flock, it was just ignored, put
writtable file LCK..ttyS0 in /var/lock,

Using a lock file is the traditional method of providing
mutually exclusive access to a serial port.

tried ioctl (but I may not know what are appropriate
arguments), googled half a day for various phrases, error
messages etc....without success.

It's unclear what "without success" means. Lockfiles have been
used for decades, and they work fine as long as all of the
applications follow the rules.

 

[kkadrese]

hmm, now it seems I get to the locking
try:
ser = serial.Serial(dev, 19200, rtscts=1, timeout=1)
if ser.isOpen():
try:
fcntl.lockf(ser,fcntl.LOCK_EX)
but get into some weirdness - trying to lock does not time out! The
script stops at this line and waits ?for ever? (it waits for 15
minutes at least, that I have tried).

Andra

 

[Andra]

I tried ready-made commands for file locking, and turned to that LCK
file just in case some permissions are wrong and that's the reason the
commands fail.