problems with RFCOMM connection

[CliffsDover]

Hi, thanks for your example code. I adopted it and try to run it on my STM32F2 board connected with an USB Bluetooth dongle. Currently I can find the device using
hcitool scan
and obtain information using
hcitool info
But the was no response using
l2ping XX:XX:XX:XX:XX:XX

My problem is that I want to connect to the device using RFCOMM and print the string sent from the device. But the is no data received. I tried both using
minicom -D /dev/rfcomm0
or simply
cat /dev/rfcomm0

The info log of btstack shows the connection is completed, but it seems that there is no RFCOMM_EVENT_INCOMING_CONNECTION or RFCOMM_EVENT_OPEN_CHANNEL_COMPLETE event, thus I can't obtain a valid rfcomm_channel_id. Therefore rfcomm_send_internal() is never called. Are there any possible reasons to cause this situation? Thanks!