ddb(4)

NAME

ddb - interactive kernel debugger

SYNOPSIS

options KDB
options DDB
To prevent activation of the debugger on kernel panic(9):
options KDB_UNATTENDED

DESCRIPTION

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 `debug' keymap(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 dis

played with a hexadecimal format at a prompt. Examine and write commands up

date `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 exam

ined 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 vari

able, 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 com

mand 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 with

in the current
line.

COMMANDS

examine

x
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., ` 00').

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 depend

ing 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 sub

tracted by the
size displayed by it is used as the start address.

print[/acdoruxz]
Print addrs according to the modifier character (as de

scribed 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 "0cx = " $ecx "0

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 paren

theses.

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 consid

ered 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 spec

ified by a
break point number with #, or by using the same addr speci

fied 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 ma

chines (e.g.,
pmax) also count loads and stores.

Warning: when counting, the debugger is really silently sin

gle-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 modi

fier is specified, print the call nesting depth and the cumulative in

struction 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 re

turn 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 inter

esting 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 ma

chine. 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 at

tempt 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 un

recoverable
error in some systems such as i386. Watchpoints on user ad

dresses 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 ma

chine. Currently
only available on the i386 and Alpha architectures.

help
Print a short summary of the available commands and command

abbreviations.

VARIABLES

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 un

less 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.
workxx Work variable. xx can be 0 to 31.

EXPRESSIONS

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 corre

sponding
object. `.' and `:' can be used in the

identifier. If
supported by an object format dependent

routine,
[filename:]func:lineno,

[filename:]variable, and
[filename:]lineno can be accepted as a sym

bol.

Numbers Radix is determined by the first two let

ters: 0x: hex,

0o: octal, 0t: decimal; otherwise, follow

current radix.

. `dot'

+ `next'

.. address of the start of the last line exam

ined. 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 de

scribed 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.

HINTS

On machines with an ISA expansion bus, a simple NMI genera

tion 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 spe

cific methods.

SEE ALSO

gdb(1)

HISTORY

The ddb debugger was developed for Mach, and ported to

386BSD 0.1. This
manual page translated from -man macros by Garrett Wollman.

BSD February 27, 2006

BSD