Building and Debugging Kernels

This chapter is not about building kernel extensions (KEXTs). There are a number of good KEXT tutorials on Apple’s developer documentation site ( This chapter is about adding new in-kernel modules (optional parts of the kernel), building kernels, and debugging kernel and kernel extension builds.

The discussion is divided into three sections. The first, “Adding New Files or Modules,” describes how to add new functionality into the kernel itself. You should only add files into the kernel when the use of a KEXT is not possible (for example, when adding certain low-level motherboard hardware support).

The second section, “Building Your First Kernel,” describes how to build a kernel, including how to build a kernel with debugger support, how to add new options, and how to obtain sources that are of similar vintage to those in a particular version of OS X or Darwin.

The third section, “When Things Go Wrong: Debugging the Kernel,” tells how to debug a kernel or kernel module using ddb and gdb. This is a must-read for anyone doing kernel development.

Adding New Files or Modules

In this context, the term module is used loosely to refer to a collection of related files in the kernel that are controlled by a single config option at compile time. It does not refer to loadable modules (KEXTs). This section describes how to add additional files that will be compiled into the kernel, including how to add a new config option for an additional module.

Modifying the Configuration Files

The details of adding a new file or module into the kernel differ according to what portion of the kernel contains the file. If you are adding a new file or module into the Mach portion of the kernel, you need to list it in various files in xnu/osfmk/conf. For the BSD portion of the kernel, you should list it in various files in xnu/bsd/conf. In either case, the procedure is basically the same, just in a different directory.

This section is divided into two subsections. The first describes adding the module itself and the second describes enabling the module.

Adding the Files or Modules

In the appropriate conf directory, you need to add your files or modules into various files. The files MASTER, MASTER.ppc, and MASTER.i386 contain the list of configuration options that should be built into the kernel for all architectures, PowerPC, and i386, respectively.

These are supplemented by files, files.ppc, and files.i386, which contain associations between compile options and the files that are related to them for their respective architectures.

The format for these two files is relatively straightforward. If you are adding a new module, you should first choose a name for that module. For example, if your module is called mach_foo, you should then add a new option line near the top of files that is whitespace (space or tab) delimited and looks like this:

OPTIONS/mach_foo    optional mach_foo

The first part defines the name of the module as it will be used in #if statements in the code. (See “Modifying the Source Code Files” for more information.) The second part is always the word optional. The third part tells the name of the option as used to turn it on or off in a MASTER file. Any line with mach_foo in the last field will be enabled only if there is an appropriate line in a MASTER file.

Then, later in the file, you add

osfmk/foo/foo_main.c            optional mach_foo
osfmk/foo/foo_bar.c             optional mach_foo

and so on, for each new file associated with that module. This also applies if you are adding a file to an existing module. If you are adding a file that is not associated with any module at all, you add a line that looks like the following to specify that this file should always be included:

osfmk/crud/mandatory_file.c     standard

If you are not adding any modules, then you’re done. Otherwise, you also need to enable your option in one of the MASTER files.

Enabling Module Options

To enable a module option (as described in the files files), you must add an entry for that option into one of the MASTER files. If your code is not a BSD pseudo-device, you should add something like the following:

options MACH_FOO

Otherwise, you should add something like this:

pseudo-device   mach_foo

In the case of a pseudo-device (for example, /dev/random), you can also add a number. When your code checks to see if it should be included, it can also check that number and allocate resources for more than one pseudo-device. The meaning of multiple pseudo-devices is device-dependent. An example of this is ppp, which allocates resources for two simultaneous PPP connections. Thus, in the MASTER.ppc file, it has the line:

pseudo-device   ppp 2

Modifying the Source Code Files

In the OS X kernel, all source code files are automatically compiled. It is the responsibility of the C file itself to determine whether its contents need to be included in the build or not.

In the example above, you created a module called mach_foo. Assume that you want this file to compile only on PowerPC-based computers. In that case, you should have included the option only in MASTER.ppc and not in MASTER.i386. However, by default, merely specifying the file foo_main.c in files causes it to be compiled, regardless of compile options specified.

To make the code compile only when the option mach_foo is included in the configuration, you should begin each C source file with the lines

#include <mach_foo.h>
#if (MACH_FOO > 0)

and end it with

#endif /* MACH_FOO */

If mach_foo is a pseudo-device and you need to check the number of mach_foo pseudo-devices included, you can do further tests of the value of MACH_FOO.

Note that the file <mach_foo.h> is not something you create. It is created by the makefiles themselves. You must run make exporthdrs before make all to generate these files.

Building Your First Kernel

Before you can build a kernel, you must first obtain source code. Source code for the OS X kernel can be found in the Darwin xnu project on To find out your current kernel version, use the command uname -a. If you run into trouble, search the archives of the darwin-kernel and darwin-development mailing lists for information. If that doesn’t help, ask for assistance on either list. The list archives and subscription information can be found at

Next, you will need to compile several support tools. Get the bootstrap_cmds, Libstreams, kext_tools, IOKitUser, and cctools packages from Extract the files from these .tar packages, then do the following:

sudo mkdir -p /usr/local/bin
sudo mkdir -p /usr/local/lib
cd bootstrap_cmds-version/relpath.tproj
sudo make install
cd ../../Libstreams-version
sudo make install
cd ../cctools-version
sudo cp /usr/include/ar.h \

In the cctools package, modify the Makefile, and change the COMMON_SUBDIRS line (including the continuation line after it) to read:

COMMON_SUBDIRS = libstuff libmacho misc

Finally, issue the following commands:

make RC_OS=macos
sudo cp misc/seg_hack.NEW /usr/local/bin/seg_hack
cd ld
make RC_OS=macos kld_build
sudo cp static_kld/libkld.a /usr/local/lib
sudo ranlib /usr/local/lib/libkld.a

Now you’re done with the cctools project. One final step remains: compiling kextsymboltool. To do this, extract the kext_tools tarball, then do the following:

sudo mkdir -p /System/Library/Frameworks/IOKit.framework/Versions/A/PrivateHeaders/kext
cd /System/Library/Frameworks/IOKit.framework/
sudo ln -s Versions/A/PrivateHeaders PrivateHeaders
sudo cp PATH_TO_IOKITUSER/IOKitUser-version/kext.subproj/*.h PrivateHeaders/kext
cd PATH_TO_KEXT_TOOLS/kext_tools-version
gcc kextsymboltool.c -o kextsymboltool
sudo cp kextsymboltool /usr/local/bin

Congratulations. You now have all the necessary tools, libraries, and header files to build a kernel.

The next step is to compile the kernel itself. First, change directories into the xnu directory. Next, you need to set a few environment variables appropriately. For your convenience, the kernel sources contain shell scripts to do this for you. If you are using sh, bash, zsh, or some other Bourne-compatible shell, issue the following command:

source SETUP/

If you are using csh, tcsh, or a similar shell, use the following command:

source SETUP/setup.csh

Then, you should be able to type

make exporthdrs
make all

and get a working kernel in BUILD/obj/RELEASE_PPC/mach_kernel (assuming you are building a RELEASE kernel for PowerPC, of course).

If things don’t work, the darwin-kernel mailing list a good place to get help.

Building an Alternate Kernel Configuration

When building a kernel, you may want to build a configuration other than the RELEASE configuration (the default shipping configuration). Additional configurations are RELEASE_TRACE, DEBUG, DEBUG_TRACE, and PROFILE. These configurations add various additional options (except PROFILE, which is reserved for future expansion, and currently maps onto RELEASE).

The most useful and interesting configurations are RELEASE and DEBUG. The release configuration should be the same as a stock Apple-released kernel, so this is interesting only if you are building source that differs from that which was used to build the kernel you are already running. Compiling a kernel without specifying a configuration results in the RELEASE configuration being built.

The DEBUG configuration enables ddb, the in-kernel serial debugger. The ddb debugger is helpful to debug panics that occur early in boot or within certain parts of the Ethernet driver. It is also useful for debugging low-level interrupt handler routines that cannot be debugged by using the more traditional gdb.

To compile an alternate kernel configuration, you should follow the same basic procedure as outlined previously, changing the final make statement slightly. For example, to build the DEBUG configuration, instead of typing

make all

you type


and wait.

To turn on additional compile options, you must modify one of the MASTER files. For information on modifying these files, see the section “Enabling Module Options.”

When Things Go Wrong: Debugging the Kernel

No matter how careful your programming habits, sometimes things don’t work right the first time. Kernel panics are simply a fact of life during development of kernel extensions or other in-kernel code.

There are a number of ways to track down problems in kernel code. In many cases, you can find the problem through careful use of printf or IOLog statements. Some people swear by this method, and indeed, given sufficient time and effort, any bug can be found and fixed without using a debugger.

Of course, the key words in that statement are “given sufficient time and effort.” For the rest of us, there are debuggers: gdb and ddb.

Setting Debug Flags in Open Firmware

With the exception of kernel panics or calls to PE_enter_debugger, it is not possible to do remote kernel debugging without setting debug flags in Open Firmware. These flags are relevant to both gdb and ddb debugging and are important enough to warrant their own section.

To set these flags, you can either use the nvram program (from the OS X command line) or access your computer’s Open Firmware. You can access Open Firmware this by holding down Command-Option-O-F at boot time. For most computers, the default is for Open Firmware to present a command–line prompt on your monitor and accept input from your keyboard. For some older computers you must use a serial line at 38400, 8N1. (Technically, such computers are not supported by OS X, but some are usable under Darwin, and thus they are mentioned here for completeness.)

From an Open Firmware prompt, you can set the flags with the setenv command. From the OS X command line, you would use the nvram command. Note that when modifying these flags you should always look at the old value for the appropriate Open Firmware variables and add the debug flags.

For example, if you want to set the debug flags to 0x4, you use one of the following commands. For computers with recent versions of Open Firmware, you would type

printenv boot-args
setenv boot-args original_contents debug=0x4

from Open Firmware or

nvram boot-args
nvram boot-args="original_contents debug=0x4"

from the command line (as root).

For older firmware versions, the interesting variable is boot-command. Thus, you might do something like

printenv boot-command
setenv boot-command 0 bootr debug=0x4

from Open Firmware or

nvram boot-command
nvram boot-command="0 bootr debug=0x4"

from the command line (as root).

Of course, the more important issue is what value to choose for the debug flags. Table 20-1 lists the debugging flags that are supported in OS X.

Table 20-1  Debugging flags

Symbolic name





Halt at boot-time and wait for debugger attach (gdb).



Send kernel debugging printf output to console.



Drop into debugger on NMI (Command–Power, Command-Option-Control-Shift-Escape, or interrupt switch).



Send kernel debugging kprintf output to serial port.



Make ddb (kdb) the default debugger (requires a custom kernel).



Output certain diagnostic info to the system log.



Allow debugger to ARP and route (allows debugging across routers and removes the need for a permanent ARP entry, but is a potential security hole)—not available in all kernels.



Support old versions of gdb on newer systems.



Disable graphical panic dialog.

The option DB_KDP_BP_DIS is not available on all systems, and should not be important if your target and host systems are running the same or similar versions of OS X with matching developer tools. The last option is only available in Mac OS 10.2 and later.

Avoiding Watchdog Timer Problems

Macintosh computers have various watchdog timers designed to protect the system from certain types of failures. There are two primary watchdog timers in common use: the power management watchdog timer (not present on all systems) and the system crash watchdog timer. Both watchdogs are part of the power management hardware.

The first of these, the power management watchdog timer, is designed to restore the system to a known safe state in the event of unexpected communication loss between the power management hardware and the CPU. This timer is only present in G4 and earlier desktops and laptops and in early G5 desktops. More specifically, it is present only in machines containing a PMU (Power Management Unit) chip.

Under normal circumstances, when communication with the PMU chip is lost, the PMU driver will attempt to get back in sync with the PMU chip. With the possible exception of a momentary loss of keyboard and mouse control, you probably won't notice that anything has happened (and you should never even experience such a stall unless you are writing a device driver that disables interrupts for an extended period of time).

The problem occurs when the disruption in communication is caused by entering the debugger while the PMU chip is in one of these "unsafe" states. If the chip is left in one of these "unsafe" states for too long, it will shut the computer down to prevent overheating or other problems.

This problem can be significantly reduced by operating the PMU chip in polled mode. This prevents the watchdog timer from activating. You should only use this option when debugging, however, as it diminishes performance and a crashed system could overheat.

To disable this watchdog timer, add the argument pmuflags=1 to the kernel's boot arguments. See “Setting Debug Flags in Open Firmware” for information about how to add a boot argument.

The second type of watchdog timer is the system crash watchdog timer. This is normally only enabled in OS X Server. If your target machine is running OS X Server, your system will automatically reboot within seconds after a crash to maximize server uptime. You can disable this automatic reboot on crash feature in the server administration tool.

Choosing a Debugger

There are two basic debugging environments supported by OS X: ddb and gdb. ddb is a built-in debugger that works over a serial line. By contrast, gdb is supported using a debugging shim built into the kernel, which allows a remote computer on the same physical network to attach after a panic (or sooner if you pass certain options to the kernel).

For problems involving network extensions or low-level operating system bringups, ddb is the only way to do debugging. For other bugs, gdb is generally easier to use. For completeness, this chapter describes how to use both ddb and gdb to do basic debugging. Since gdb itself is well documented and is commonly used for application programming, this chapter assumes at least a passing knowledge of the basics of using gdb and focuses on the areas where remote (kernel) gdb differs.

Using gdb for Kernel Debugging

gdb, short for the GNU Debugger, is a piece of software commonly used for debugging software on UNIX and Linux systems. This section assumes that you have used gdb before, and does not attempt to explain basic usage.

In standard OS X builds (and in your builds unless you compile with ddb support), gdb support is built into the system but is turned off except in the case of a kernel panic.

Of course, many software failures in the kernel do not result in a kernel panic but still cause aberrant behavior. For these reasons, you can pass additional flags to the kernel to allow you to attach to a remote computer early in boot or after a nonmaskable interrupt (NMI), or you can programmatically drop into the debugger in your code.

You can cause the test computer (the debug target) to drop into the debugger in the following ways:

  • debug on panic

  • debug on NMI

  • debug on boot

  • programmatically drop into the default debugger

    The function PE_enter_debugger can be called from anywhere in the kernel, although if gdb is your default debugger, a crash will result if the network hardware is not initialized or if gdb cannot be used in that particular context. This call is described in the header pexpert/pexpert.h.

After you have decided what method to use for dropping into the debugger on the target, you must configure your debug host (the computer that will actually be running gdb). Your debug host should be running a version of OS X that is comparable to the version running on your target host. However, it should not be running a customized kernel, since a debug host crash would be problematic, to say the least.

When using gdb, the best results can be obtained when the source code for the customized kernel is present on your debug host. This not only makes debugging easier by allowing you to see the lines of code when you stop execution, it also makes it easier to modify those lines of code. Thus, the ideal situation is for your debug host to also be your build computer. This is not required, but it makes things easier. If you are debugging a kernel extension, it generally suffices to have the source for the kernel extension itself on your debug host. However, if you need to see kernel-specific structures, having the kernel sources on your debug host may also be helpful.

Once you have built a kernel using your debug host, you must then copy it to your target computer and reboot the target computer. At this point, if you are doing panic-only debugging, you should trigger the panic. Otherwise, you should tell your target computer to drop into the debugger by issuing an NMI (or by merely booting, in the case of debug=0x1).

Next, unless your kernel supports ARP while debugging (and unless you enabled it with the appropriate debug flag), you need to add a permanent ARP entry for the target. It will be unable to answer ARP requests while waiting for the debugger. This ensures that your connection won’t suddenly disappear. The following example assumes that your target is with an IP number of

$ ping -c 1 target_host_name
ping results: ....
$ arp -an ( 00:a0:13:12:65:31
$ sudo arp -s 00:a0:13:12:65:31
$ arp -an ( at00:a0:13:12:65:31 permanent

Now, you can begin debugging by doing the following:

gdb /path/to/mach_kernel
source /path/to/xnu/osfmk/.gdbinit
p proc0
source /path/to/xnu/osfmk/.gdbinit
target remote-kdp

Note that the mach kernel passed as an argument to gdb should be the symbol–laden kernel file located in BUILD/obj/DEBUG_PPC/mach_kernel.sys (for debug kernel builds, RELEASE_PPC for non-debug builds), not the bootable kernel that you copied onto the debug target. Otherwise most of the gdb macros will fail. The correct kernel should be several times as large as a normal kernel.

You must do the p proc0 command and source the .gdbinit file (from the appropriate kernel sources) twice to work around a bug in gdb. Of course, if you do not need any of the macros in .gdbinit, you can skip those two instructions. The macros are mostly of interest to people debugging aspects of Mach, though they also provide ways of obtaining information about currently loaded KEXTs.

If you are debugging a kernel module, you need to do some additional work to get debugging symbol information about the module. First, you need to know the load address for the module. You can get this information by running kextstat (kmodstat on systems running OS X v10.1 or earlier) as root on the target.

If you are already in the debugger, then assuming the target did not panic, you should be able to use the continue function in gdb to revive the target, get this information, then trigger another NMI to drop back into the debugger.

If the target is no longer functional, and if you have a fully symbol–laden kernel file on your debug host that matches the kernel on your debug target, you can use the showallkmods macro to obtain this information. Obtaining a fully symbol–laden kernel generally requires compiling the kernel yourself.

Once you have the load address of the module in question, you need to create a symbol file for the module. You do this in different ways on different versions of OS X.

For versions 10.1 and earlier, you use the kmodsyms program to create a symbol file for the module. If your KEXT is called mykext and it is loaded at address 0xf7a4000, for example, you change directories to mykext.kext/Contents/MacOS and type:

kmodsyms -k path/to/mach_kernel -o mykext.sym mykext@0xf7a4000

Be sure to specify the correct path for the mach kernel that is running on your target (assuming it is not the same as the kernel running on your debug host).

For versions after 10.1, you have two options. If your KEXT does not crash the computer when it loads, you can ask kextload to generate the symbols at load time by passing it the following options:

kextload -s symboldir mykext.kext

It will then write the symbols for your kernel extension and its dependencies into files within the directory you specified. Of course, this only works if your target doesn’t crash at or shortly after load time.

Alternately, if you are debugging an existing panic, or if your KEXT can’t be loaded without causing a panic, you can generate the debugging symbols on your debug host. You do this by typing:

kextload -n -s symboldir mykext.kext

If will then prompt you for the load address of the kernel extension and the addresses of all its dependencies. As mentioned previously, you can find the addresses with kextstat (or kmodstat) or by typing showallkmods inside gdb.

You should now have a file or files containing symbolic information that gdb can use to determine address–to–name mappings within the KEXT. To add the symbols from that KEXT, within gdb on your debug host, type the command

add-symbol-file mykext.sym

for each symbol file. You should now be able to see a human-readable representation of the addresses of functions, variables, and so on.

Special gdb I/O Addressing Issues

As described in “Address Spaces,” some Macintosh hardware has a third addressing mode called I/O addressing which differs from both physical and virtual addressing modes. Most developers will not need to know about these modes in any detail.

Where some developers may run into problems is debugging PCI device drivers and attempting to access device memory/registers.

To allow I/O-mapped memory dumping, do the following:

set kdp_read_io=1

To dump in physical mode, do the following:

set kdp_trans_off=1

For example:

(gdb) x/x 0xf8022034
0xf8022034: Cannot access memory at address 0xf8022034
(gdb) set kdp_trans_off=1
(gdb) x/x 0xf8022034
0xf8022034: Cannot access memory at address 0xf8022034
(gdb) set kdp_read_io=1
(gdb) x/x 0xf8022034
0xf8022034: 0x00000020

If you experience problems accessing I/O addresses that are not corrected by this procedure, please contact Apple Developer Technical Support for additional assistance.

Using ddb for Kernel Debugging

When doing typical debugging, gdb is probably the best solution. However, there are times when gdb cannot be used or where gdb can easily run into problems. Some of these include

  • drivers for built-in Ethernet hardware

  • interrupt handlers (the hardware variety, not handler threads)

  • early bootstrap before the network hardware is initialized

When gdb is not practical (or if you’re curious), there is a second debug mechanism that can be compiled into OS X. This mechanism is called ddb, and is similar to the kdb debugger in most BSD UNIX systems. It is not quite as easy to use as gdb, mainly because of the hardware needed to use it.

Unlike gdb (which uses Ethernet for communication with a kernel stub), ddb is built into the kernel itself, and interacts directly with the user over a serial line. Also unlike gdb, using ddb requires building a custom kernel using the DEBUG configuration. For more information on building this kernel, see “Building Your First Kernel.”

If your target computer has two serial ports, ddb uses the modem port (SCC port 0). However, if your target has only one serial port, that port is probably attached to port 1 of the SCC cell, which means that you have to change the default port if you want to use ddb. To use this port (SCC port 1), change the line:

const int console_unit=0;

in osfmk/ppc/serial_console.c to read:

const int console_unit=1;

and recompile the kernel.

Once you have a kernel with ddb support, it is relatively easy to use. First, you need to set up a terminal emulator program on your debug host. If your debug host is running Mac OS 9, you might use ZTerm, for example. For OS X computers, or for computers running Linux or UNIX, minicom provides a good environment. Setting up these programs is beyond the scope of this document.

Once you boot a kernel with ddb support, a panic will allow you to drop into the debugger, as will a call to PE_enter_debugger. If the DB_KDB flag is not set, you will have to press the D key on the keyboard to use ddb. Alternately, if both DB_KDB and DB_NMI are set, you should be able to drop into ddb by generating a nonmaskable interrupt (NMI). See “Setting Debug Flags in Open Firmware” for more information on debug flags.

To generate a nonmaskable interrupt, hold down the command, option, control, and shift keys and hit escape (OS X v10.4 and newer), hold down the command key while pressing the power key on your keyboard (on hardware with a power key), or press the interrupt button on your target computer. At this point, the system should hang, and you should see ddb output on the serial terminal. If you do not, check your configuration and verify that you have specified the correct serial port on both computers.

Commands and Syntax of ddb

The ddb debugger is much more gdb-like than previous versions, but it still has a syntax that is very much its own (shared only with other ddb and kdb debuggers). Because ddb is substantially different from what most developers are used to using, this section outlines the basic commands and syntax.

The commands in ddb are generally in this form:

command[/switch] address[,count]

The switches can be one of those shown in Table 20-2.

Table 20-2  Switch options in ddb




Print the location with line number if possible


Display as instruction with possible alternate machine-dependent format


Print the location being displayed


Display or process by bytes


Display low 8 bits as a character (nonprinting characters as octal) or count instructions while executing (depends on instruction)


Display as signed decimal


Display or process by half word (16 bits)


Display as an instruction


Display or process by long word (32 bits)


Display as unsigned hex with character dump for each line


Display in unsigned octal


Print cumulative instruction count and call tree depth at each call or return statement


Display in current radix, signed


Display the null-terminated string at address (nonprinting as octal).


Display in unsigned decimal or set breakpoint at a user space address (depending on command).


Display in unsigned hex


Display in signed hex

The ddb debugger has a rich command set that has grown over its lifetime. Its command set is similar to that of ddb and kdb on other BSD systems, and their manual pages provide a fairly good reference for the various commands. The command set for ddb includes the following commands:

break[/u] addr

Set a breakpoint at the address specified by addr. Execution will stop when the breakpoint is reached. The /u switch means to set a breakpoint in user space.

c or continue[/c]

Continue execution after reaching a breakpoint. The /c switch means to count instructions while executing.


Call a function.


Set condition breakpoints. This command is not supported on PowerPC.

cpu cpunum

Causes ddb to switch to run on a different CPU.

d or delete [addr|#]

Delete a breakpoint. This takes a single argument that can be either an address or a breakpoint number.


Equivalent to running kextstat while the target computer is running. This lists loaded KEXTs, their load addresses, and various related information.

dl vaddr

Dumps a range of memory starting from the address given. The parameter vaddr is a kernel virtual address. If vaddr is not specified, the last accessed address is used. See also dr, dv.


Displays mapping information for the last address accessed.

dmacro name

Delete the macro called name. See macro.


Displays the currently active page table.

dr addr

Dumps a range of memory starting from the address given. The parameter address is a physical address. If addr is not specified, the last accessed address is used. See also dl, dv.


Dumps save areas of all Mach tasks.

dv [addr [vsid]]

Dumps a range of memory starting from the address given. The parameter addr is a virtual address in the address space indicated by vsid. If addr is not specified, the last accessed address is used. Similarly, if vsid is not specified, the last vsid is used. See also dl, dr.

dwatch addr

Delete a watchpoint. See watch.


Displays CPU registers.


See print.


Switches to gdb mode, allowing gdb to attach to the computer.


On PowerPC only: Dumps the PowerPC exception trace table.

macro name command [ ; command .. ]

Create a macro called name that executes the listed commands. You can show a macro with the command show macro name or delete it with dmacro name.


Stop at the matching return instruction. If the /p switch is not specified, summary information is printed only at the final return.

print[/AIabcdhilmorsuxz] addr1 [addr2 ...]

Print the values at the addresses given in the format specified by the switch. If no switch is given, the last used switch is assumed. Synonymous with examine and x. Note that some of the listed switches may work for examine and not for print.


Reboots the computer. Immediately. Without doing any file-system unmounts or other cleanup. Do not do this except after a panic.

s or step

Single step through instructions.

search[/bhl] addr value [mask[,count]]

Search memory for value starting at addr. If the value is not found, this command can wreak havoc. This command may take other formatting values in addition to those listed.

set $name [=] expr

Sets the value of the variable or register named by name to the value indicated by expr.


Display system data. For a list of information that can be shown, type the show command by itself. Some additional options are available for certain options, particularly show all. For those suboptions, type show all by itself.


Prints a stack backtrace. If the /u flag is specified, the stack trace extends to user space if supported by architecture-dependent code.


Stop at the next call or return.

w or write[/bhl] addr expr1 [expr2 ... ]

Writes the value of expr1 to the memory location stored at addr in increments of a byte, half word, or long word. If additional expressions are specified, they are written to consecutive bytes, half words, or long words.

watch addr[,size]

Sets a watchpoint on a particular address. Execution stops when the value stored at that address is modified. Watch points are not supported on PowerPC.

Warning  Watching addresses in wired kernel memory may cause unrecoverable errors on i386.


Short for examine. See print.


Examine backward. Execute the last examine command, but use the address previous to the last one used (jumping backward by increments of the last width displayed).


Examine forward. Execute the last examine command, but use the address following the last one used (jumping by increments of the last width displayed).

The ddb debugger should seem relatively familiar to users of gdb, and its syntax was changed radically from its predecessor, kdb, to be more gdb-like. However, it is still sufficiently different that you should take some time to familiarize yourself with its use before attempting to debug something with it. It is far easier to use ddb on a system whose memory hasn’t been scribbled upon by an errant DMA request, for example.