lock access to serial port

[kkadrese at gmail.com]

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