Here are some hints to debug with GNU Mach.

Kernel Debugger (KDB)

Mach has a built-in kernel debugger. Manual.

First, make sure to enable it. Either by using a pre-packaged gnumach-image-something-dbg, or by passing --enable-kdb to the ./configure invocation.

Then, reproduce the issue again. If something like a kernel trap happens, you will end up in the GNU Mach debugger. Otherwise, type control-alt-d to make Mach enter it by hand.

If you are running in kvm or qemu, it is convenient to use the curses frontend to be able to copy/paste.

To get the register values, type

show registers

To get a backtrace, type


, which will print both function return addresses and function parameters, such as


Run the addr2line tool on the return addresses:

$ addr2line -i -f -e /boot/gnumach 0x107cf1 0x1071bc 0x106831

This will print the source code lines of the backtrace.

To get the userland backtrace of the thread, you can use


To examine the backtrace of some given thread, use

show all thread/u

to get the whole listing of all tasks and threads. You can then use trace/t or trace/tu to trace a specific thread. gcc however uses the frame pointer as a normal register unless -fno-omit-frame-pointer is given. It can thus be useful to rebuild glibc with that option, by adding extra_cflags = -fno-omit-frame-pointer in glibc/debian/sysdeps/

To examine a variable, use nm /boot/gnumach to get the address of the variable (e.g. 0x123400), and use

x 0x123400

to read it. One can also write to it by using

w 0x123400

Another interesting feature is watching a variable, by using

watch 0x123400

and then type continue, to let Mach continue execution. The debugger will be entered again on any change in that variable. The watch is implemented in hardware, so it does not disturb or slow down execution at all. The same can be achieved programmatically, e.g. using

struct db_watchpoint watch = { .task = NULL, .loaddr= 0x40e, .hiaddr = 0x40e+2, .link = NULL};
db_set_hw_watchpoint(&watch, 0);


When you're running a system in QEMU you can directly use GDB on the running kernel.

debugging gnumach startup qemu gdb

Code Inside the Kernel

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__);
      case 1:
       printf ("%s: Invoking task_collect_scan.\n", __FUNCTION__);
       extern void task_collect_scan (void);
       task_collect_scan ();
       printf ("No idea what you want me to do.\n");

  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.

Writing to the Screen Buffer

If you're doing real low level debugging, you might want to put variations of the following snipped into the code, this code will write a # character at line [LINE], column [COLUMN] on the screen:

*((char *) 0xb8000 + 2 * ([LINE] * 80 + [COLUMN])) = '#';
halt_cpu ();

The call of halt_cpu will -- as the name suggests -- halt the system afterwards. This might be what you want or it might not, but it is needed at some place when running the kernel inside QEMU, as QEMU somehow decides not to update its display buffer anymore under certain conditions.

Halting the CPU and Examining Registers

IRC, freenode, #hurd, 2011-07-14

<braunr> one ugly trick i use when printf isn't available is to halt the
<braunr> then use info registers to know where the cpu is halted
<braunr> and you'll know if you reached that code or not
<braunr> (info registers is a qemu command)

Serial Console

IRC, freenode, #hurd, 2011-11-13

<youpi> use console=com0
<youpi> to activate the console on the first serial port

ud2 instruction

IRC, freenode, #hurd, 2013-10-31

<phcoder> GNU Mach 1.3.99
<phcoder> Running on xen-3.0-x86_64.
<phcoder> AT386 boot: physical memory from 0x0 to 0x40000000
<youpi> \o/
<phcoder> well when loaded through pvgrub2 i hangs without any message
<phcoder> any pointers on debugging?
<youpi> I usually put the ud2 instruction along the path to see where it