Debugging

Debugging code running on the JNode platform is no easy task. The platform currently has none of the nice debugging support that you normally find on a mature Java platform; no breakpoints, no looking at object fields, stack frames, etc.

Instead, we typically have to resort to sending messages to the system Logger, adding traceprint statements and calling 'dumpStack' and 'printStackTrace'. Here are some other pointers:

  • If you are debugging a JNode command, you may be able to use ShellEmu or TestHarness to run and debug JNode specific code in your development environment. (This may not work if your command makes use of JNode specific services.)
  • A lot of classes have no JNode platform dependencies, and can be debugged using JUnit tests running in your development sandbox.
  • If you are debugging low-level JNode code, you can use "Unsafe.debug(...)" calls and the (so called) Kernel debugger to get trace information without causing object allocation. This is particularly important when debugging the JNode memory management, etc where any object allocation could trigger a kernel panic.
  • Beware of the effects of adding debug code on JNode system performance, and on timing related bugs; e.g. race conditions.

There is also a simple debugger that can be used in textmode to display threads and their stacktraces. Press Alt-SysRq to enter the debugger and another Alt-SysRq to exit the debugger. Inside the debugger, press 'h' for usage information.

Note: the Alt-SysRq debugger isn't working at the moment: see this issue.

Kernel debugger

A very simple kernel debugger has been added to the JNode nano-kernel. This debugger is able to send all data outputted to the console (using Unsafe.debug) to another computer via a null-modem cable connected to COM1.

From the other computer you can give simple commands to the debugger, such as dump the processor thread queues and print the current thread.

The kernel debugger can be enabled by adding " kdb" to the grub kernel command line, or by activating it in JNode using a newly added command: "kdb".

Ewout

‹ Is this a reasonable use of plugin extensions? explain a bit about GNU classpath ›

Using "remoteout" to record console / logger output

The remoteout command allows you to send a copy of console output and logger output to a remote TCP or UDP receiver. This allows you to capture console output for bug reports, and in the cases where JNode is crashing.

Before you run the command, you need to set up a receiver application on the remote host to accept and record the output. More details (including a brief note on the JNode RemoteReceiver application) may be found in the remoteout command page. Please read the Bugs section as well!