Re: tcpdump equivalent for the serial port

[Jan-Benedict Glaw <jbglaw@lug-owl.de>]

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On Fri, 2003-09-12 16:23:32 +0200, Nicolae Mihalache <mache@abcpages.com>
wrote in message <3F61D6E4.2050604@abcpages.com>:
> Jan-Benedict Glaw wrote:
> >That way, you don't monitor the serial port itself, but the 
> >application
> >using it. For me, that has always been enough:-)
> >
> This seems a good ideea. Probably I will not use exactly strace but 
> develop something using the same library it uses.

There isn't really a library involved. strace is basically a hugh
wrapper around the ptrace() system call.

> I will still keep the cable break-out solution as an option but 
> definitely I wanted to be able to monitor the data without interfering 
> with the electic signaling on the cable.

Then you most probably want to go the preload lib way. That is, you
build a small lib containing open(), close(), read() and write() which
you call the original program with:

$ LD_PRELOAD=/path/to/your/lib.so ./serial_app

The lib needs to call the libc routines, of course. You can do this by
dlopen()ing libc (RTLD_NEXT) and get the original function (pointer)
from it (by calling dlsym()).

This way, you get access to all parameters (you're able to get the
open()ed file name, compare it to /dev/ttyS.*) and save the resultin fd
from the libc open() call.

On upcoming read()s and write()s, you simply always compare the supplied
fd with the initial fd() from the open() call to a serial device. It's
really that easy:)

MfG, JBG

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