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Re: Fwd: Can GDB interact with serial ports on remote targets?


On 02/20/2015 01:45 PM, Brendan J wrote:
Hi Luis, thanks for your response.

Hmm, yes I now realise I was remotely aware of semihosting.
Unfortunately my project is running on bare-metal (no OS, no u-boot or
UEFI or anything) so I don't think semihosting is quite appropriate.
As I understand it, typical use of semihosting goes like this:

You're debugging an application on a remote target, and that target
has an OS, and that OS has support for semihosting. The application
issues a "read" system call, and the OS handles it by issuing a
semihosting debug call (as opposed to actually reading the file or
UART or whatever).

That is to say, the system under debug needs to be modified to support
semihosting, which I'd really like to avoid doing in my case. Please
correct me if my understanding of semihosting is wrong.

Yes, unfortunately you need a minimal support in your debugging stub (in your case the probe?) to allow semihosting calls.

Otherwise, are there any other options?

At this moment i don't recall any other elegant options, just the usual brute force ones by means of breakpoints and manually changing memory from within the debugger.

I suppose GDB could be instructed to send/receive program input/output through a separate channel, but it would need to know when to do so.


On Fri, Feb 20, 2015 at 3:14 PM, Luis Machado <> wrote:
Hi Brendan,

On 02/20/2015 12:27 PM, Brendan J wrote:

I am using GDB to debug a remote target: I start GDB then type `target
remote foo:1234`. I do I/O with the target via a serial port. So I
have to have two terminals open: one with picocom (connected to
something like /dev/ttyUSB0) and one with GDB (connected to OpenOCD
via a socket). To be clear: the debug connection is *not over the
serial port*, it's over a totally separate JTAG interface.

As you know, when you debug a "normal" (i.e. not "remote") inferior in
GDB, its stdin and stdout are multiplexed into GDB's TTY so that you
can interact with it while it's running [1].

Is it possible to achieve that for a remote target - that is: can GDB
connect to the serial port itself so I can do I/O with the target from
within the GDB session?

If not, is this something that might be feasible? Maybe GDB could
multiplex its I/O so that while the inferior is running it passes
characters to/from an external tool like picocom? (As you can see I'm
fairly ignorant about this whole issue at the moment).


PS: I'm using arm-none-eabi-gdb, in case that happens to be relevant.


It sounds like what you want is semihosting support, an I/O operation that
is initiated on the remote end, passing through the host and then back again
to the remote end.

For example, the remote program needs to read a character. In this case the
remote debug agent issues a vFile request with the right system call, GDB
receives it, the user enters the character via GDB and then GDB sends the
reply back to the target. The remote program then reads the character and
continues executing.

You can check the documentation here:

Check this one as well:

This feature is only available on all-stop mode.


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