[[meta copyright="Copyright © 2007 Free Software Foundation, Inc."]]
[[meta license="Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no Invariant
Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
is included in the section entitled
[[GNU_Free_Documentation_License|/fdl.txt]]."]]

Mach has a built-in kernel debugger.
[Manual](http://www.gnu.org/software/hurd/gnumach-doc/Kernel-Debugger.html).


When you're [[running_a_system_in_QEMU|hurd/running/qemu]] you can directly
[use GDB on the running
kernel](http://fabrice.bellard.free.fr/qemu/qemu-doc.html#SEC36).


Alternatively you can use an approach like this one: add the following code
snippet to `device/ds_routines.c`'s `ds_device_open` function, right at the top
of the function, and modify the code as needed.

      void D (char *s)
      {
        switch (s[0] - '0')
          {
          case 0:
           printf ("Hello from %s!\n", __FUNCTION__);
           break;
          case 1:
           printf ("%s: Invoking task_collect_scan.\n", __FUNCTION__);
           extern void task_collect_scan (void);
           task_collect_scan ();
           break;
          default:
           printf ("No idea what you want me to do.\n");
           break;
          }
      }
    
      if (name && name[0] == 'D')
        D (name + 1);

Then boot your system and do something like this:

    # devprobe D0
    Hello from D!
    # devprobe D1
    D: Invoking task_collect_scan.
    # devprobe D2
    No idea what you want me to do.

This is especially useful if you need to manually trigger some stuff inside the
running kernel, as with the *D1* example.