Watching serial port activity.

Sergei Organov osv at
Thu Jun 1 12:53:32 CEST 2006

Grant Edwards <grante at> writes:

> On 2006-05-31, Sergei Organov <osv at> wrote:
>> It seems that sniff on a real tty device could be implemented using the
>> same technique strace uses to intercept and show syscalls, though I'm
>> not aware of any sniffer application that does it.
> Using strace you can indeed trace read/write calls on user-specified
> file descriptors.

I've actually meant to take strace in source code and modify it for
particular purpose. The fact that it almost can do it in unmodified form
is quite impressive though.

> Figuring out which file descriptors to trace is the tricky part.

Provided I have port name, say, /dev/ttyM0, and have running application:

osv at osv ~$ fuser /dev/ttyM0
/dev/ttyM0:           5134
osv at osv ~$ ls -l /proc/5134/fd | grep /dev/ttyM0
lrwx------  1 osv osv 64 2006-05-29 15:33 8 -> /dev/ttyM0
osv at osv ~$

so I need to track fd #8 of the process with pid 5134. Guess one can
attach to a running process as gdb does it.

Alternatively, if the process to be sniffed sporadically opens/closes
the port at run-time, it is required to track open()/close() syscalls as
well as read()/write() ones, I think. Overall, seems not that trivial
but doable.


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