[SOLVED] Weird socat behavior for linked serial device file.

crabbit · 03-28-2018, 01:19 PM

I am writing a program to monitor serial devices (over tcp) and I use socat to link a serial device file to a tcp port as follows.

socat pty,link=/dev/ttyTCP1,raw,b19200,echo=0 tcp-listen:5001,forever,max-children=1,fork,reuseaddr

when I issue the socat command It creates /dev/ttyTCP1, my program runs, opens the file and processes data that is recieved as expected.

ps output

Code:

root      5005     1  0 18:07 pts/1    00:00:00 socat pty,link=/dev/ttyTCP1,raw,b19200,echo=0 tcp-listen:5001,forever,max-children=1,fork,reuseaddr
root      5022  5005  0 18:07 pts/1    00:00:00 socat pty,link=/dev/ttyTCP1,raw,b19200,echo=0 tcp-listen:5001,forever,max-children=1,fork,reuseaddr

If the serial device is disconnected from the network and I wait a couple of minutes the forked child process remains running and /dev/ttyTCP1 still exists.

when I plug the serial device back in I get a new PID for the child listener process.

Code:

root      5005     1  0 18:07 pts/1    00:00:00 socat pty,link=/dev/ttyTCP1,raw,b19200,echo=0 tcp-listen:5001,forever,max-children=1,fork,reuseaddr
root      5356  5005  0 18:12 pts/1    00:00:00 socat pty,link=/dev/ttyTCP1,raw,b19200,echo=0 tcp-listen:5001,forever,max-children=1,fork,reuseaddr

And the device file no long shows up using ls which is very weird since my program that has been running the whole time, which opens and keeps open the device file, can still read new data sent by the serial device through that file.

Is this a normal behavior or am I missing something?

crabbit · 03-28-2018, 02:03 PM

found my own answer...

If the file is deleted, the file handle remains open and can still be used (This is not what some people expect). The file will not really be deleted until the last handle is closed.

Further, socat does not actually create a "file", it creates a symlink to a /dev/pts