How can I monitor data on a serial port in Linux?

Question

I'm debugging communications with a serial device, and I need to see all the data flowing both directions.

It seems like this should be easy on Linux, where the serial port is represented by a file. Is there some way that I can do a sort of "bi-directional tee", where I tell my program to connect to a pipe that copies the data to a file and also shuffles it to/from the actual serial port device?

I think I might even know how to write such a beast, but it seems non-trivial, especially to get all of the ioctls passed through for port configuration, etc.

Has anyone already built such a thing? It seems too useful (for people debugging serial device drivers) not to exist already.

Answer 1

strace is very useful for this. You have a visualisation of all ioctl calls, with the corresponding structure decoded. The following options seems particularly useful in your case:

-e read=set

Perform a full hexadecimal and ASCII dump of all the data read from file descriptors listed in the specified set. For example, to see all input activity on file descriptors 3 and 5 use -e read=3,5. Note that this is independent from the normal tracing of the read(2) system call which is controlled by the option -e trace=read.

-e write=set

Perform a full hexadecimal and ASCII dump of all the data written to file descriptors listed in the specified set. For example, to see all output activity on file descriptors 3 and 5 use -e write=3,5. Note that this is independent from the normal tracing of the write(2) system call which is controlled by the option -e trace=write.