10.3. On-Line Kernel Debugging Using DDB

While kgdb as an off-line debugger provides a very high level of user interface, there are some things it cannot do. The most important ones being breakpointing and single-stepping kernel code.

If you need to do low-level debugging on your kernel, there is an on-line debugger available called DDB. It allows setting of breakpoints, single-stepping kernel functions, examining and changing kernel variables, etc. However, it cannot access kernel source files, and only has access to the global and static symbols, not to the full debug information like kgdb does.

To configure your kernel to include DDB, add the options

options KDB

options DDB

to your config file, and rebuild. (See The FreeBSD Handbook for details on configuring the FreeBSD kernel).

Once your DDB kernel is running, there are several ways to enter DDB. The first, and earliest way is to use the boot flag -d. The kernel will start up in debug mode and enter DDB prior to any device probing. Hence you can even debug the device probe/attach functions. To use this, exit the loader's boot menu and enter boot -d at the loader prompt.

The second scenario is to drop to the debugger once the system has booted. There are two simple ways to accomplish this. If you would like to break to the debugger from the command prompt, simply type the command:

# sysctl debug.kdb.enter=1

Alternatively, if you are at the system console, you may use a hot-key on the keyboard. The default break-to-debugger sequence is Ctrl+Alt+ESC. For syscons, this sequence can be remapped and some of the distributed maps out there do this, so check to make sure you know the right sequence to use. There is an option available for serial consoles that allows the use of a serial line BREAK on the console line to enter DDB (options BREAK_TO_DEBUGGER in the kernel config file). It is not the default since there are a lot of serial adapters around that gratuitously generate a BREAK condition, for example when pulling the cable.

The third way is that any panic condition will branch to DDB if the kernel is configured to use it. For this reason, it is not wise to configure a kernel with DDB for a machine running unattended.

To obtain the unattended functionality, add:


to the kernel configuration file and rebuild/reinstall.

The DDB commands roughly resemble some gdb commands. The first thing you probably need to do is to set a breakpoint:

break function-name address

Numbers are taken hexadecimal by default, but to make them distinct from symbol names; hexadecimal numbers starting with the letters a-f need to be preceded with 0x (this is optional for other numbers). Simple expressions are allowed, for example: function-name + 0x103.

To exit the debugger and continue execution, type:


To get a stack trace of the current thread, use:


To get a stack trace of an arbitrary thread, specify a process ID or thread ID as a second argument to trace.

If you want to remove a breakpoint, use

del address-expression

The first form will be accepted immediately after a breakpoint hit, and deletes the current breakpoint. The second form can remove any breakpoint, but you need to specify the exact address; this can be obtained from:

show b


show break

To single-step the kernel, try:


This will step into functions, but you can make DDB trace them until the matching return statement is reached by:



This is different from gdb's next statement; it is like gdb's finish. Pressing n more than once will cause a continue.

To examine data from memory, use (for example):

x/wx 0xf0133fe0,40
x/hd db_symtab_space
x/bc termbuf,10
x/s stringbuf

for word/halfword/byte access, and hexadecimal/decimal/character/ string display. The number after the comma is the object count. To display the next 0x10 items, simply use:

x ,10

Similarly, use

x/ia foofunc,10

to disassemble the first 0x10 instructions of foofunc, and display them along with their offset from the beginning of foofunc.

To modify memory, use the write command:

w/b termbuf 0xa 0xb 0
w/w 0xf0010030 0 0

The command modifier (b/h/w) specifies the size of the data to be written, the first following expression is the address to write to and the remainder is interpreted as data to write to successive memory locations.

If you need to know the current registers, use:

show reg

Alternatively, you can display a single register value by e.g.

p $eax

and modify it by:

set $eax new-value

Should you need to call some kernel functions from DDB, simply say:

call func(arg1, arg2, ...)

The return value will be printed.

For a ps(1) style summary of all running processes, use:


Now you have examined why your kernel failed, and you wish to reboot. Remember that, depending on the severity of previous malfunctioning, not all parts of the kernel might still be working as expected. Perform one of the following actions to shut down and reboot your system:


This will cause your kernel to dump core and reboot, so you can later analyze the core on a higher level with kgdb(1).

call boot(0)

Might be a good way to cleanly shut down the running system, sync() all disks, and finally, in some cases, reboot. As long as the disk and filesystem interfaces of the kernel are not damaged, this could be a good way for an almost clean shutdown.


This is the final way out of disaster and almost the same as hitting the Big Red Button.

If you need a short command summary, simply type:


It is highly recommended to have a printed copy of the ddb(4) manual page ready for a debugging session. Remember that it is hard to read the on-line manual while single-stepping the kernel.

All FreeBSD documents are available for download at https://download.freebsd.org/ftp/doc/

Questions that are not answered by the documentation may be sent to <freebsd-questions@FreeBSD.org>.
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