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ddb - interactive kernel debugger


See Also


.Cd options DDB

To prevent activation of the debugger on kernel panic(9):


The ddb kernel debugger has most of the features of the old kdb, but with a more rational syntax inspired by gdb(1). If linked into the running kernel, it can be invoked locally with the ‘debugkeymap(5) action. The debugger is also invoked on kernel panic(9) if the debug.debugger_on_panic sysctl(8) MIB variable is set non-zero, which is the default unless the KDB_UNATTENDED option is specified.

The current location is called ‘dot’. The ‘dot’ is displayed with a hexadecimal format at a prompt. Examine and write commands update ‘dot’ to the address of the last line examined or the last location modified, and set ‘next’ to the address of the next location to be examined or changed. Other commands do not change ‘dot’, and set ‘next’ to be the same as ‘dot’.

The general command syntax is: command [/ modifier] address [,count]

A blank line repeats the previous command from the address ‘next’ with count 1 and no modifiers. Specifying address sets ‘dot’ to the address. Omitting address uses ‘dot’. A missing count is taken to be 1 for printing commands or infinity for stack traces.

The ddb debugger has a feature like the more(1) command for the output. If an output line exceeds the number set in the $lines variable, it displays "--db_more--" and waits for a response. The valid responses for it are:

SPC one more page
RET one more line
q abort the current command, and return to the command input mode

Finally, ddb provides a small (currently 10 items) command history, and offers simple emacs-style command line editing capabilities. In addition to the emacs control keys, the usual ANSI arrow keys might be used to browse through the history buffer, and move the cursor within the current line.


Display the addressed locations according to the formats in the modifier. Multiple modifier formats display multiple locations. If no format is specified, the last formats specified for this command is used.

The format characters are:

b look at by bytes (8 bits)
h look at by half words (16 bits)
l look at by long words (32 bits)
a print the location being displayed
A print the location with a line number if possible
x display in unsigned hex
z display in signed hex
o display in unsigned octal
d display in signed decimal
u display in unsigned decimal
r display in current radix, signed
c display low 8 bits as a character. Non-printing characters are displayed as an octal escape code (e.g., ‘\000’).
s display the null-terminated string at the location. Non-printing characters are displayed as octal escapes.
m display in unsigned hex with character dump at the end of each line. The location is also displayed in hex at the beginning of each line.
i display as an instruction
I display as an instruction with possible alternate formats depending on the machine:
alpha Show the registers of the instruction.
amd64 No alternate format.
i386 No alternate format.
ia64 No alternate format.
powerpc No alternate format.
sparc64 No alternate format.
xf Examine forward: Execute an examine command with the last specified parameters to it except that the next address displayed by it is used as the start address.
xb Examine backward: Execute an examine command with the last specified parameters to it except that the last start address subtracted by the size displayed by it is used as the start address.
print [/acdoruxz]
Print addr s according to the modifier character (as described above for examine). Valid formats are: a, x, z, o, d, u, r, and c. If no modifier is specified, the last one specified to it is used. addr can be a string, in which case it is printed as it is. For example:
print/x "eax = " $eax "\necx = " $ecx "\n"

will print like:
eax = xxxxxx
ecx = yyyyyy

write [/bhl] addr expr1 ["expr2 ..."]
Write the expressions specified after addr on the command line at succeeding locations starting with addr The write unit size can be specified in the modifier with a letter b (byte), h (half word) or l (long word) respectively. If omitted, long word is assumed.

Warning: since there is no delimiter between expressions, strange things may happen. It is best to enclose each expression in parentheses.

set $ variable [=] expr
Set the named variable or register with the value of expr. Valid variable names are described below.
break [/u] Set a break point at addr. If count is supplied, continues count - 1 times before stopping at the break point. If the break point is set, a break point number is printed with '#'. This number can be used in deleting the break point or adding conditions to it.

If the u modifier is specified, this command sets a break point in user space address. Without the u option, the address is considered in the kernel space, and wrong space address is rejected with an error message. This modifier can be used only if it is supported by machine dependent routines.

Warning: If a user text is shadowed by a normal user space debugger, user space break points may not work correctly. Setting a break point at the low-level code paths may also cause strange behavior.

delete addr
delete # number
Delete the break point. The target break point can be specified by a break point number with #, or by using the same addr specified in the original break command.
step [/p] Single step count times (the comma is a mandatory part of the syntax). If the p modifier is specified, print each instruction at each step. Otherwise, only print the last instruction.

Warning: depending on machine type, it may not be possible to single-step through some low-level code paths or user space code. On machines with software-emulated single-stepping (e.g., pmax), stepping through code executed by interrupt handlers will probably do the wrong thing.

continue [/c]
Continue execution until a breakpoint or watchpoint. If the c modifier is specified, count instructions while executing. Some machines (e.g., pmax) also count loads and stores.

Warning: when counting, the debugger is really silently single-stepping. This means that single-stepping on low-level code may cause strange behavior.

until [/p] Stop at the next call or return instruction. If the p modifier is specified, print the call nesting depth and the cumulative instruction count at each call or return. Otherwise, only print when the matching return is hit.
next [/p]
match [/p]
Stop at the matching return instruction. If the p modifier is specified, print the call nesting depth and the cumulative instruction count at each call or return. Otherwise, only print when the matching return is hit.
trace [/u] [frame] [,count]
Stack trace. The u option traces user space; if omitted, trace only traces kernel space. count is the number of frames to be traced. If count is omitted, all frames are printed.

Warning: User space stack trace is valid only if the machine dependent code supports it.

search [/bhl] addr value [mask] [,count]
Search memory for value. This command might fail in interesting ways if it does not find the searched-for value. This is because ddb does not always recover from touching bad memory. The optional count argument limits the search.
show all procs [/m]
ps [/m]
Display all process information. The process information may not be shown if it is not supported in the machine, or the bottom of the stack of the target process is not in the main memory at that time. The m modifier will alter the display to show VM map addresses for the process and not show other info.
show registers [/u]
Display the register set. If the u option is specified, it displays user registers instead of kernel or currently saved one.

Warning: The support of the u modifier depends on the machine. If not supported, incorrect information will be displayed.

show map [/f] addr
Prints the VM map at addr. If the f modifier is specified the complete map is printed.
show object [/f] addr
Prints the VM object at addr. If the f option is specified the complete object is printed.
"show watches"
Displays all watchpoints.
reset Hard reset the system.
watch addr , size
Set a watchpoint for a region. Execution stops when an attempt to modify the region occurs. The size argument defaults to 4. If you specify a wrong space address, the request is rejected with an error message.

Warning: Attempts to watch wired kernel memory may cause unrecoverable error in some systems such as i386. Watchpoints on user addresses work best.

hwatch addr , size
Set a hardware watchpoint for a region if supported by the architecture. Execution stops when an attempt to modify the region occurs. The size argument defaults to 4.

Warning: The hardware debug facilities do not have a concept of separate address spaces like the watch command does. Use hwatch for setting watchpoints on kernel address locations only, and avoid its use on user mode address spaces.

dhwatch addr , size
Delete specified hardware watchpoint.
gdb Toggles between remote GDB and DDB mode. In remote GDB mode, another machine is required that runs gdb(1) using the remote debug feature, with a connection to the serial console port on the target machine. Currently only available on the i386 and Alpha architectures.
help Print a short summary of the available commands and command abbreviations.


The debugger accesses registers and variables as $ name. Register names are as in the "show registers" command. Some variables are suffixed with numbers, and may have some modifier following a colon immediately after the variable name. For example, register variables can have a u modifier to indicate user register (e.g., $eax:u).

Built-in variables currently supported are:

radix Input and output radix
maxoff Addresses are printed as ’symbol’+offset unless offset is greater than maxoff.
maxwidth The width of the displayed line.
lines The number of lines. It is used by "more" feature.
tabstops Tab stop width.
work xx Work variable. xx can be 0 to 31.


Almost all expression operators in C are supported except '~', '^', and unary '&'. Special rules in ddb are:
Identifiers The name of a symbol is translated to the value of the symbol, which is the address of the corresponding object. '.' and ':' can be used in the identifier. If supported by an object format dependent routine,
.Sm off [filename:func: lineno],
.Sm on [filename:variable], and [filename:lineno] can be accepted as a symbol.
Numbers Radix is determined by the first two letters: 0x: hex, 0o: octal, 0t: decimal; otherwise, follow current radix.
. ‘dot’
+ ‘next’
.. address of the start of the last line examined. Unlike ‘dot’ or ‘next’, this is only changed by "examine" or "write" command.
last address explicitly specified.
$ variable Translated to the value of the specified variable. It may be followed by a : and modifiers as described above.
a # b a binary operator which rounds up the left hand side to the next multiple of right hand side.
* expr indirection. It may be followed by a '': and modifiers as described above.


On machines with an ISA expansion bus, a simple NMI generation card can be constructed by connecting a push button between the A01 and B01 (CHCHK# and GND) card fingers. Momentarily shorting these two fingers together may cause the bridge chipset to generate an NMI, which causes the kernel to pass control to ddb. Some bridge chipsets do not generate a NMI on CHCHK#, so your mileage may vary. The NMI allows one to break into the debugger on a wedged machine to diagnose problems. Other bus’ bridge chipsets may be able to generate NMI using bus specific methods.




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