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Wed Jul 21 12:43:15 CEST 2010

gdb as an application console

I'm doing a short 1-2 month project on an Atmel ARM7 AT91SAMX512 using
eCos.  I thought about bringing up a Scheme or PF clone on the
machine.  My work is mostly of low-level nature, fairly tied to the
device (Flash storage management), so an interactive on-target
debugging approach is useful.

However, this would take too long to get going.  Additionally, the
board I'm working on didn't have a console input, just output for
logging.  I decided to use just gdb with Segger's J-Link gdbserver on
an old windows box connected to the target using JTAG.

I did need to tweak some eCos compilation flags to make sure that
functions that are not referenced in the code still end up on the
target, preventing them to be tossed out by the linker.  This required
removal of: (man gcc)

       -ffunction-sections
       -fdata-sections

           Place each function or data item into its own section in
           the output file if the target supports arbitrary sections.

It turns out that test-driven development (use a separate eCos test
application to scaffold the code you're writing) combined with the
possibility to interactively execute code on the machine is more than
enough to get the necessary interactivity.

The approach I used:

   - Define some "registers", i.e. instances of the objects under test

   - Define operations on them that have short names, and don't take
     excessive amount of arguments (i.e. 0 or 1).

This gives you a small working set to use interactively using the gdb
"p" command, which allows you to execute individual functions on the
target as long as it has an execution stack setup.

> p x_dump(123)

The x_dump() function in my case would take an integer argument, fetch
a block from Flash into one of the registers, and display it in
human-readable formatting.

This makes me wonder: how powerful is the gdb scripting language?  The
on-target functions can be strung together in sequences like this:

define dump2
  p dump(0)
  p dump(1)
end

Doesn't seem to be very powerful.  There seems to be a Python
interface though[1].

[1] http://sourceware.org/gdb/current/onlinedocs/gdb/Python.html#Python



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