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ddb - interactive kernel debugger
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 ‘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 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
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
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
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.
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.
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 addrs 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.
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
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 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.
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 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
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.
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]
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.
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.
Displays all watchpoints.
Hard reset the system.
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
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.
Delete specified hardware watchpoint.
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.
Print a short summary of the available commands and command
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.
workxx 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, [filename:]func:lineno,
[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.
.. address of the start of the last line examined. Unlike
‘dot’ or ‘next’, this is only changed by “examine” or
’ 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
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.
The ddb debugger was developed for Mach, and ported to 386BSD 0.1. This
manual page translated from -man macros by Garrett Wollman.