Embedded Linux Primer: A Practical Real-World Approach

15.5. Additional Remote Debug Options

Sometimes you might want to use a serial port for remote debugging. For other tasks, you might find it useful to attach the debugger to a process that is already running. These simple but useful operations are detailed here.^[6]

^[6] Refer back to Listing 14-5 in Chapter 13

15.5.1. Debugging via Serial Port

Debugging via serial port is quite straightforward. Of course, you must have a serial port available on your target that is not being used by another process, such as a serial console. The same limitation applies to your host. A serial port must be available. If both of these conditions can be met, simply replace the IP:Port specification passed to gdbserver with a serial port specification. Use the same technique when connecting to your target from your host-based GDB.

On your target:

root@coyote:/workspace # gdbserver /dev/ttyS0 ./tdemo Process ./tdemo created; pid = 698 Remote debugging using /dev/ttyS0

From your host:

$ xscale_be-gdb -q tdemo (gdb) target remote /dev/ttyS1 Remote debugging using /dev/ttyS1 0x40000790 in ?? ()

15.5.2. Attaching to a Running Process

It is often advantageous to connect to a process to examine its state while it is running instead of killing the process and starting it again. With gdbserver, it is trivial:

root@coyote:/workspace # ps ax | grep tdemo 1030 pts/0 Sl+ 0:00 ./tdemo root@coyote:/workspace # gdbserver localhost:2001 --attach 1030 Attached; pid = 1030 Listening on port 2001

When you are finished examining the process under debug, you can issue the gdb detach command. This detaches the gdbserver from the application on the target and terminates the debug session. The application continues where it left off. This is a very useful technique for examining a running program. Be aware, though, that when you attach to the process, it halts, waiting for instructions from you. It will not resume execution until instructed to do so, using either the continue command or the detach command. Also note that you can use the detach command at almost any time to end the debug session and leave the application running on the target.