$NetBSD: ddb.4,v 1.5 1994/11/30 16:22:09 jtc Exp $
Mach Operating System
Copyright (c) 1991,1990 Carnegie Mellon University
All Rights Reserved.
Permission to use, copy, modify and distribute this software and its
documentation is hereby granted, provided that both the copyright
notice and this permission notice appear in all copies of the
software, derivative works or modified versions, and any portions
thereof, and that both notices appear in supporting documentation.
CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
Carnegie Mellon requests users of this software to return to
Software Distribution Coordinator or Software.Distribution@CS.CMU.EDU
School of Computer Science
Carnegie Mellon University
Pittsburgh PA 15213-3890
any improvements or extensions that they make and grant Carnegie Mellon
the rights to redistribute these changes.
.Dd $Mdocdate: September 19 2023 $ .Dt DDB 4 .Os .Sh NAME .Nm ddb .Nd kernel debugger .Sh DESCRIPTION The .Nm debugger provides a means for debugging the kernel, and analysing the kernel after a system crash ("panic"), with a .Xr gdb 1 Ns -like syntax.
p .Nm is invoked upon a kernel panic when the .Xr sysctl 8 .Va ddb.panic is set to 1. It may be invoked from the console when the sysctl .Va ddb.console is set to 1, using any of the following methods: l -dash -offset 3n t Using the key sequence .Li Ctrl-Alt-Esc . t Sending a .Dv BREAK when using a serial console. t Writing to the sysctl .Va ddb.trigger . t For i386 and amd64 architectures, using the key sequence .Li Ctrl-Alt-Delete when the sysctl .Va machdep.kbdreset is set to 2. .El
p .Nm prompts for commands on the console with:
p .Dl ddb>
p The general syntax of a .Nm command is: d -ragged -offset indent .Ar command .Oo Ic / Ns Ar modifiers Oc " " .Oo Ar address Oc Ns .Op Ic , Ns Ar count .Ed
p
To save typing,
.Nm
makes use of a context inferred from previous commands.
In this context,
the current location is called
.Va dot .
The
.Va dot
is displayed with
a hexadecimal format at a prompt.
The
c examine , c search , c show struct , and
c write commands update
.Va dot
to be that of the last address
examined or the last location modified, and
have intuitive effects on
.Va next
and
.Va prev .
All the other commands do not change
.Va dot ,
and set
.Va next
to be the same.
(See
.Sx VARIABLES . )
p
Specifying
.Ar address
in a command sets
.Va dot .
An expression can be used in place of
.Ar address
(see
.Sx EXPRESSIONS ) .
Omitting
.Ar address
in a command uses the last value of
.Va dot .
A missing
.Ar count
is taken to be 1 for printing commands or \*(If
for stack traces.
Entering a blank line causes the last command to be repeated using
.Va next
in place of
.Ar address ,
a
.Ar count
of 1, and no modifiers.
p .Nm has a feature like .Xr more 1 for the output. If the number of lines output in response to one command exceeds the number set in the .Va $lines variable, it displays the message .Ql "--db_more--" and waits for a response.
p The valid responses are:
p l -tag -width 10n -offset indent -compact t Aq Ic space One more page. t Aq Ic return One more line. t Ic q Abort the current command, and return to the command input mode. .El
p The following command line editing keys are provided:
p
l -tag -width 10n -offset indent -compact t Ic ^b back one character
t Ic ^f forward one character
t Ic ^a beginning of line
t Ic ^e end of line
t Ic ^w erase word back
t Ic ^h | Aq Ic del erase previous character
t Ic ^d erase next character
t Ic ^k delete to end of line
t Ic ^u delete line
t Ic ^p previous in command history
t Ic ^n next in command history
t Ic ^r redraw line
t Ic ^t exchange the two characters to the left of the cursor
.El
.Sh COMMANDS
The following commands may be typed at the
.Ql ddb>
prompt.
Some commands consist of more than one word, and if only the first word
or words are entered, the possible alternatives to complete the command
are displayed and no other action is performed.
l -tag -width 10n --------------------
t Ic help List the available commands.
--------------------
.Tg examine
t Xo .Oo Ic e Oc Ns
c x Ns Op Ic amine .Op Cm /bhlqaAxzodurcsmiI
.Op Ar addr Ns
.Op Ic , Ns Ar count
.Xc
Display the contents at address
.Ar addr
according to the formats in the modifier.
If no format is specified, the last formats specified for this command
are used.
p The format characters are:
p l -tag -width 4n -compact t Cm /b look at by bytes (8 bits) t Cm /h look at by half words (16 bits) t Cm /l look at by long words (32 bits) (default) t Cm /q look at by long longs (64 bits) (only available on 64-bit platforms) t Cm /a print the location being displayed t Cm /A print the location with a line number if possible t Cm /x display in unsigned hex t Cm /z display in signed hex t Cm /o display in unsigned octal t Cm /d display in signed decimal t Cm /u display in unsigned decimal t Cm /r display in current radix, signed t Cm /c display low 8 bits as a character. Non-printing characters are displayed as an octal escape code (e.g., '\e000'). t Cm /s display the null-terminated string at the location. Non-printing characters are displayed as octal escapes. t Cm /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. t Cm /i display as an instruction t Cm /I display as an alternate format instruction depending on the machine:
p l -tag -width powerpc_ -compact t alpha Print affected register contents for every instruction. t amd64 , t i386 Do not skip padding to the next long word boundary for unconditional jumps. t m88k Decode instructions for the opposite CPU model (e.g. m88110 when running on an m88100 processor). .El .El
p
The value of
.Va next
is set to the
.Ar addr
plus the size of the data examined.
--------------------
.Tg print
t Xo c p Ns Op Ic rint .Op Cm /axzodurc
.Op Ar addr
.Xc
Print
.Ar addr
according to the modifier character.
The valid modifiers are a subset of those from the
c examine command, and act as described there.
If no modifier is specified, the last one specified in a
previous use of
c print is used.
p For example, d -literal -offset indent print/x $eax .Ed
p
will print something like this:
d -literal -offset indent xxxxxx
.Ed
--------------------
.Tg pprint
t Xo c pp Ns Op Ic rint .Op Ar addr
.Xc
Pretty-print
.Ar addr
using CTF debug symbols included in the kernel binary image.
The CTF section is normally added by running
.Xr ctfstrip 1
as part of building a new kernel.
--------------------
.It Xo Ic w Ns Op Cm /bhl
.Op Ar addr
.Ar expr Op expr ...
.Xc
.Tg write
t Xo c w Ns Op Ic rite .Op Cm /bhl
.Op Ar addr
.Ar expr Op Ar expr ...
.Xc
Write the value of each
.Ar expr
expression at succeeding locations start at
.Ar addr .
The write unit size can be specified using one of the modifiers:
p l -tag -width 4n -offset indent -compact t Cm /b byte (8 bits) t Cm /h half word (16 bits) t Cm /l long word (32 bits) (default) .El
p The value of .Va next is set to .Ar addr plus the size of values written.
p
.Sy Warning :
since there is no delimiter between expressions, the
command may not parse as you expect.
It is best to enclose each expression in parentheses.
--------------------
t Xo Ic set c $ Ns Ar name .Op Ic =
.Ar expr
.Xc
Set the named variable or register with the value of
.Ar expr .
For valid variable names,
see
.Sx VARIABLES .
--------------------
t Ic boot Ar how Reboot the machine depending on
.Ar how :
p
l -tag -width "boot poweroff" -compact t Ic boot sync Sync disks and reboot.
t Ic boot crash Dump core and reboot.
t Ic boot dump Sync disks, dump core and reboot.
t Ic boot halt Just halt.
t Ic boot reboot Just reboot.
t Ic boot poweroff Power down the machine whenever possible; if it fails, just halt.
.El
--------------------
t Xo c break .Op Ar addr Ns
.Op Ic , Ns Ar count
.Xc
Set a break point at
.Ar addr .
If
.Ar count
is supplied,
.Nm
allows the breakpoint to be silently hit
.Ar ( count No - 1 )
times before stopping at the
break point.
--------------------
.Tg delete
t Xo c d Ns Op Ic elete .Op Ar addr
.Xc
Delete the break point set with the
c break command.
--------------------
.It Xo Ic s Ns Op Cm /p
.Op Ic , Ns Ar count
.Xc
.Tg step
t Xo c s Ns Op Ic tep .Op Cm /p
.Op Ic , Ns Ar count
.Xc
Single step
.Ar count
times.
If the
.Cm /p
modifier is specified, print each instruction at each step.
Otherwise, only print the last instruction.
p
.Sy Warning :
depending on machine type, it may not be possible to
single-step through some low-level code paths.
On machines with software-emulated single-stepping (e.g., alpha),
stepping through code executed by interrupt handlers will probably
do the wrong thing.
--------------------
t Ic call Ar name Ns Xo c ( Ns Ar expr .Op Ic , Ar expr ...
c ) .Xc
Call the function named by
.Ar name
with the argument(s) listed in parentheses.
Parentheses may be omitted if the function takes no arguments.
The number of arguments is currently limited to 10.
--------------------
.It Ic c Ns Op Cm /c
.Tg continue
t Xo c c Ns Op Ic ontinue .Op Cm /c
.Xc
Continue execution until a breakpoint or watchpoint.
If the
.Cm /c
modifier is given, instructions are counted while executing.
p
.Sy Warning :
when counting with
.Cm /c ,
.Nm
is really silently single-stepping.
This means that single-stepping on low-level code may cause strange
behavior.
--------------------
t Xo c watch .Ar addr
.Op Ic , Ns Ar size
.Xc
Set a watchpoint for the region starting at
.Ar addr .
Execution stops and control returns to
.Nm
when an attempt is made to modify a watched region.
The
.Ar size
argument defaults to 4.
p If you specify a wrong space address, the request is rejected with an error message.
p
.Sy Warning :
attempts to watch wired kernel memory
may cause an unrecoverable error on some systems (e.g., i386).
--------------------
t Ic dwatch Ar addr Delete the watchpoint at address
.Ar addr
that was previously set with a
c watch command.
--------------------
t Xo c hangman .Op Cm /s Ns Op Ic 0-9
.Xc
This is a tiny and handy tool for random kernel hangs analysis, of which its
depth is controlled by the optional argument of the default value of five.
It uses some sophisticated heuristics to spot the global symbol that
caused the hang.
Since the discovering algorithm is a probabilistic one,
you may spend substantial time to figure the exact symbol name.
This smart thing requires a little of your attention, the input it accepts
is mostly of the same format as that of the famous
.Xr hangman 6
game, to which it, apparently, is obliged by the name.
Hint: the
.Xr nm 1
utility might help.
--------------------
t Ic until Op Cm /p Stop at the next
.Qq call
or
.Qq return
instruction.
If the
.Cm /p
modifier is specified,
.Nm
prints the call nesting depth and the
cumulative instruction count at each call or return.
Otherwise, it stays silent until the matching return is hit.
--------------------
t Ic match Op Cm /p Stop at the next matching return instruction.
If the
.Cm /p
modifier is specified,
.Nm
prints the call nesting depth and the
cumulative instruction count at each call or return.
Otherwise, it remains mostly quiet.
--------------------
t Ic next Op Cm /p The
c next command is a synonym for
c match . --------------------
t Ic kill Ar pid Send an uncatchable
.Dv SIGABRT
signal to the process specified by the
.Ar pid
argument.
--------------------
t Xo c trace .Op Cm /tu
.Op Ar frameaddr Ns
.Op Ic , Ns Ar count
.Xc
Show the stack trace.
The
.Cm /t
modifier interprets the
.Ar frameaddr
argument as the TID of a process and shows the stack trace of
that process.
.Ar frameaddr
is subject to the radix; use the 0t prefix to enter a decimal TID.
The
.Cm /t
modifier is not supported on all platforms.
The
.Cm /u
modifier shows the stack trace of user space;
if omitted, the kernel stack is traced instead.
The
.Ar count
argument is the limit on the number of frames to be followed.
If
.Ar count
is omitted, all frames are printed.
p
.Sy Warning :
user space stack trace is valid
only if the machine dependent code supports it.
--------------------
t Xo c search .Op Cm /bhl
.Op Ar addr
.Ar value
.Op Ar mask
.Op Ic , Ns Ar count
.Xc
Search memory for a value beginning at
.Ar addr .
This command might fail in interesting
ways if it doesn't find the searched-for value.
This is because
.Nm
doesn't always recover from touching bad memory.
The optional
.Ar count
argument limits the search.
The modifiers are the same as those of the
c write command.
p
The
.Va next
address is set to the address where
.Ar value
is found, or just after where the search area finishes.
--------------------
t Ic reboot Shortcut for
c boot reboot --------------------
t Ic show Ar what Displays various things, depending on
.Ar what :
l -tag -width 4n --------------------
t Ic show bcstats Prints the buffer cache statistics.
--------------------
t Ic show breaks Prints a list of all breakpoints that have been set with the
c break command.
--------------------
t Ic show buf Ar addr Prints the
.Vt struct buf
at
.Ar addr .
--------------------
t Ic show extents Prints a detailed list of all extents.
--------------------
t Ic show locks Op Ar addr Prints the list of locks held by a thread.
If an optional address is not specified,
.Li curproc
is assumed.
The
.Cm option WITNESS
is required for this command to be available.
--------------------
t Ic show malloc Op Ar addr Prints malloc debugging information if available.
If an optional address is specified, only information about that address
is printed.
--------------------
t Xo c show map .Op Cm /f
.Ar addr
.Xc
Prints the
.Li vm_map
at
.Ar addr .
If the
.Cm /f
modifier is specified, the complete map is printed.
--------------------
t Ic show mbuf Ar addr Prints the
.Vt struct mbuf
header at
.Ar addr .
Depending on the mbuf flags
.Vt struct pkthdr
and
.Vt struct m_ext
are printed as well.
--------------------
t Xo c show mount .Op Cm /f
.Ar addr
.Xc
Prints the
.Vt struct mount
at
.Ar addr .
If the
.Cm /f
modifier is specified, prints out all
.Li vnodes
(see also
.Cm show vnode )
and also all
.Li bufs
(see also
.Cm show buf )
on all those vnodes.
--------------------
t Xo c show nfsnode .Op Cm /f
.Ar addr
.Xc
Prints the
.Vt struct nfsnode
at
.Ar addr .
If the
.Cm /f
modifier is specified, prints out additional
information as well.
--------------------
t Xo c show nfsreq .Op Cm /f
.Ar addr
.Xc
Prints the
.Vt struct nfsreq
at
.Ar addr .
If the
.Cm /f
modifier is specified, prints out additional
information as well.
--------------------
t Xo c show object .Op Cm /f
.Ar addr
.Xc
Prints the
.Li vm_object
at
.Ar addr .
If the
.Cm /f
modifier is specified, the complete object is printed.
--------------------
t Xo c show page .Op Cm /f
.Ar addr
.Xc
Prints the
.Li vm_page
at
.Ar addr .
If the
.Cm /f
modifier is specified, the complete page is printed.
--------------------
t Ic show panic Prints the panic string.
--------------------
t Xo c show pool .Op Cm /p
.Ar addr
.Xc
Prints the
.Li pool
at
.Ar addr .
Valid modifiers:
l -tag -width xxx -compact t Cm /p Print the pagelist for this pool.
.El
--------------------
t Xo c show proc .Op Cm /t
.Op Ar addr
.Xc
Prints the
.Vt struct proc
at
.Ar addr .
If an optional address is not specified,
.Li curproc
is assumed.
The
.Cm /t
modifier interprets the
.Ar addr
argument as the TID of a proc.
--------------------
t Ic show registers Op Cm /u Display the register set.
If the
.Cm /u
modifier is specified, it displays user registers (or the currently
saved registers) instead of the kernel's.
Note: The
.Cm /u
modifier is not supported on every machine, in which case
incorrect information may be displayed.
--------------------
t Ic show route Ar addr Prints the
.Vt struct rtentry
at
.Ar addr .
--------------------
t Ic show socket Ar addr Prints the
.Vt struct socket
at
.Ar addr .
If the socket is spliced, the
.Vt struct sosplice
associated with the socket is printed as well.
--------------------
t Xo c show struct .Ar name
.Op addr
.Xc
Prints the content of the memory at
.Ar addr
as a struct
.Ar name .
Nested structures and bit fields are not printed.
Character arrays are printed as bytes.
--------------------
t Ic show swap Prints a detailed list of all swaps.
--------------------
t Xo c show tdb .Op Cm /f
.Ar addr
.Xc
Prints the
.Vt struct tdb
at
.Ar addr .
If the
.Cm /f
modifier is specified, prints out all fields of this IPsec SA.
--------------------
t Ic show uvmexp Displays a selection of uvm counters and statistics.
--------------------
t Xo c show vnode .Op Cm /f
.Ar addr
.Xc
Prints the
.Vt struct vnode
at
.Ar addr .
If the
.Cm /f
modifier is specified, prints out all
.Li bufs
(see also
.Cm show buf )
currently attached to this
.Li vnode .
--------------------
t Ic show watches Displays all watchpoints set with the
c watch command.
--------------------
t Ic show witness Op Cm /b Prints the current order list.
If the
.Cm /b
modifier is specified, the list of found lock order violations is printed
instead.
The
.Cm option WITNESS
is required for this command to be available.
--------------------
t Ic show all procs Op Cm /anow Display information on all processes.
p
l -tag -width foo -compact t Cm /n (Default) Show process information in a
.Xr ps 1 Ns -like
format.
Information printed includes process ID, thread ID, parent
process ID, UID, process status, process flags, process
wait channel message and process command name.
t Cm /a Shows the kernel virtual addresses of each process'
proc structure, u-area, and vmspace structure.
The vmspace address is also the address of the process'
.Li vm_map
structure
and can be used in the
c show map command.
t Cm /o Shows non-idle threads that were on CPU when ddb was entered.
Information printed includes thread ID, process ID, UID, process flags,
thread flags, current CPU, and command name.
t Cm /w Shows each thread's ID, command, process group,
wait channel address, and wait channel message.
.El
--------------------
t Ic show all bufs Op Cm /f Display information about all buffers in the system.
p
l -tag -width foo -compact t Cm /f For each buffer, print a more detailed output.
See the
c show buf command for more information.
.El
--------------------
t Ic show all callout Display the contents of the callout table.
--------------------
t Ic show all pools Op Cm /a Display information about all system pools in a format similar to
.Xr vmstat 8 .
p
l -tag -width foo -compact t Cm /a Displays
.Dq interesting
address information.
.El
--------------------
t Ic show all locks Prints the list of locks held by all threads in the system.
The
.Cm option WITNESS
is required for this command to be available.
--------------------
t Ic show all mounts Op Cm /f Display information on all mounted filesystems.
p
l -tag -width foo -compact t Cm /f For each filesystem, list all its struct vnode addresses.
These addresses can be used in the
c show vnode command.
.El
--------------------
t Ic show all nfsnodes Op Cm /f Display information about all nfsnodes in the system.
p
l -tag -width foo -compact t Cm /f For each nfsnode, print a more detailed output.
See the
c show nfsnode command for more information.
.El
--------------------
t Ic show all nfsreqs Op Cm /f Display information for all outstanding NFS requests.
p
l -tag -width foo -compact t Cm /f For each NFS requests, print a more detailed output.
See the
c show nfsreq command for more information.
.El
--------------------
t Xo c show all routes .Op Cm /iI
.Op Ar rtableid
.Op Ic , Ns Ar count
.Xc
Show internet routing tables.
Default for
.Ar rtableid
is 0 and
.Ar count
is 1.
p
l -tag -width foo -compact t Cm /i Restrict to AF_INET.
t Cm /I Restrict to AF_INET6.
.El
--------------------
t Ic show all tdbs Op Cm /f Display information about all IPsec SAs in the system.
p
l -tag -width foo -compact t Cm /f For each tdb, print a more detailed output.
See the
c show tdb command for more information.
.El
--------------------
t Ic show all vnodes Op Cm /f Display information about all vnodes in the system.
p
l -tag -width foo -compact t Cm /f For each vnode, print a more detailed output.
See the
c show vnode command for more information.
.El
.El
--------------------
t Ic callout A synonym for the
c show all callout command.
--------------------
t Ic ps Op Cm /anow A synonym for
c show all procs . --------------------
.Tg machine
t Xo c mac Ns Op Ic hine .Ar subcommand Op Ar arg ...
.Xc
Perform a platform-specific command.
p The following commands are supported by multiprocessor kernels on these platforms: amd64, i386, macppc, mips64, and sparc64.
p l -tag -width "searchframe [addr]" -compact t Ic cpuinfo Display the state of each CPU. t Ic ddbcpu Ar N Stop the current CPU and start handling .Nm on the selected CPU. t Ic startcpu Op Ar N Resume normal processing on the selected CPU, or all CPUs if none is specified. t Ic stopcpu Op Ar N Stop normal processing on the selected CPU, or all CPUs (except the one handling .Nm ) if none is specified. .El
p
Other platform-specific commands:
.Pp
amd64:
p arm: l -tag -width "searchframe [addr]" -compact t Ic frame Ar addr Display the trapframe at .Ar addr . .El
p
i386:
l -tag -width "searchframe [addr]" -compact .It Ic acpi disasm Ar ??
.It Ic acpi showval Ar ??
.It Ic acpi tree Ar ??
.It Ic acpi trace Ar ??
Evil ACPI debugging magic.
t Ic sysregs Display the contents of the privileged registers:
.Va IDTR ,
.Va GDTR ,
.Va LDTR ,
.Va TR ,
.Va CR0 ,
.Va CR2 ,
.Va CR3 ,
and
.Va CR4 .
(i386-only)
.El
p m88k: l -tag -width "searchframe [addr]" -compact t Ic ddbcpu Ar N Stop the current CPU and start handling .Nm on the selected CPU. t Ic frame Ar addr Display the trapframe at .Ar addr . t Ic regs Display the registers from when .Nm was entered. t Ic searchframe Op Ar addr Search for and display stack exception frames, starting from .Ar addr if given, else the address in register .Va r31 , and stopping at the next 8k boundary. t Ic where Display where the current CPU was stopped. .El
p mips64: l -tag -width "searchframe [addr]" -compact t Xo c tlb Op Cm /p Ar asid .Op Cm /c .Op Ar tlb .Xc t Ic trap Ar ?? .El
p
sh:
l -tag -width "searchframe [addr]" -compact t Ic cache Op Ar addr Display the cache, starting from
.Ar addr ,
defaulting to 0.
t Ic frame Display the switch and trap frames.
.It Ic stack
t Ic tlb Display the TLB.
.El
p
sparc64:
l -tag -width "searchframe [addr]" -compact t Ic ctx Display the context addresses for all threads.
.It Ic dtlb Ar 0,2
.It Ic dtsb
.It Ic esp
.It Ic fpstate Op Ar addr
.It Ic itlb Ar 0,1,2
.It Ic kmap Op Ar addr
.It Ic pcb Op Ar addr
.It Ic pctx Ar pid
.It Ic page Ar addr
.It Ic phys Op Ar addr
.It Ic pmap Op Ar addr
.It Ic proc Op Ar addr
.It Ic prom Ar ??
.It Ic pv Ar addr
.It Ic stack Op Ar addr
.It Ic tf Op Ar addr
.It Ic ts
.It Ic traptrace Op Ar addr
.It Ic watch Op Ar addr
.It Ic window Op Ar winnum
.It Ic xir Op Ar addr
.El
--------------------
.El
.Sh VARIABLES
.Nm
denotes registers and variables by
c $ Ns Va name . Register names can be found with the
c show registers command.
p Some variable names are suffixed with numbers, and some may have a modifier following a colon immediately after the variable name. For example, register variables can have the .Ql :u modifier to indicate a user register (e.g., .Ql $eax:u ) .
p Built-in debugger variables currently supported are: l -tag -width 10n -compact -offset indent t Va $radix Input and output radix. t Va $maxoff Addresses are printed as .Ar symbol Ns Li + Ns Ar offset unless .Ar offset is greater than .Va $maxoff . t Va $maxwidth The width of the displayed lines. t Va $lines The number of lines to page. This is used by the .Dq more feature. t Va $tabstops Tab stop width. t Va $log Controls whether the output of .Nm will also appear in the system message buffer. .El
p These variables can also be controlled outside .Nm via the .Sq ddb .Xr sysctl 8 hierarchy. .Sh EXPRESSIONS Almost all expression operators in C are supported except for .Ql ~ , .Ql ^ , and unary .Ql & . Special rules for expressions in .Nm are: l -tag -width 15n -compact -offset indent t Ar identifier The name of a symbol. It is translated to the address (or value) of the symbol. .Ql . and .Ql : can be used in the identifier. The following can be accepted as an identifier, if supported by an object format dependent routine: l -item -offset indent -compact t .Sm off .Oo Ar filename Li : Oc Ar func .Op : Ar linenumber t .Op Ar filename : .Ar variable t .Ar filename .Op : Ar linenumber .Sm on .El The symbol may be prefixed with .Sq Ar symboltablename Ns :: (e.g., .Ql emulator::mach_msg_trap ) to specify other than kernel symbols. t Ar number The radix is determined by the first two letters: .Ql 0x : hex, .Ql 0o : octal, .Ql 0t : decimal, otherwise, the value of .Va $radix is used. t Li . .Va dot : the current address. t Li + .Va next : the next address. t Li .. The address of the start of the last line examined. Unlike .Va dot or .Va next , this is only changed by the c examine or c write command. t Li ' The last address explicitly specified. t Li $ Ns Ar variable The value of a register or variable. The name may be followed by a .Ql : and modifiers as described above with .Ar identifier . t Ar expr Li # Ar expr A binary operator which rounds up the left hand side to the next multiple of right hand side. t Li * Ns Ar expr Indirection. It may be followed by a ':' and modifiers as described above. .El .Sh SEE ALSO .Xr ctfstrip 1 , .Xr gdb 1 , .Xr nm 1 , .Xr witness 4 , .Xr sysctl.conf 5 , .Xr hangman 6 , .Xr crash 8 , .Xr sysctl 8 , .Xr extent 9 , .Xr pool 9 , .Xr uvm_init 9 .Sh HISTORY This kernel facility first appeared in the MACH 2 operating system developed by CMU. Hangman (which stands for "hangs maniacal analyzer") first appeared in .Ox 1.2 .