man/man4/ddb.4

.\"	$NetBSD: ddb.4,v 1.66 2003/05/18 09:04:52 wiz Exp $
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.Dd May 15, 2003
.Dt DDB 4
.Os
.Sh NAME
.Nm ddb
.Nd in-kernel debugger
.Sh SYNOPSIS
.Cd options DDB
.Pp
To enable history editing:
.Cd options DDB_HISTORY_SIZE=integer
.Pp
To disable entering
.\" XXX: hack; .Nm automatically introduces newline in SYNOPSIS
.Ic ddb
upon kernel panic:
.Cd options DDB_ONPANIC=0
.Sh DESCRIPTION
.Nm
is the in-kernel debugger.
It may be entered at any time via a special key sequence, and
optionally may be invoked when the kernel panics.
.Sh ENTERING THE DEBUGGER
Unless
.Dv DDB_ONPANIC
is set to 0,
.Nm
will be activated whenever the kernel would otherwise panic.
.Pp
.Nm
may also be activated from the console.
In general, sending a break on a serial console will activate
.Nm .
There are also key sequences for each port that will activate
.Nm
from the keyboard:
.Bl -tag -offset indent -width "mvme68k" -compact
.It alpha
\*[Lt]Ctrl\*[Gt]-\*[Lt]Alt\*[Gt]-\*[Lt]Esc\*[Gt] on PC style keyboards.
.It amiga
\*[Lt]LAlt\*[Gt]-\*[Lt]LAmiga\*[Gt]-\*[Lt]F10\*[Gt]
.It atari
\*[Lt]Alt\*[Gt]-\*[Lt]LeftShift\*[Gt]-\*[Lt]F9\*[Gt]
.It hp300
\*[Lt]Shift\*[Gt]-\*[Lt]Reset\*[Gt]
.It hpcmips
\*[Lt]Ctrl\*[Gt]-\*[Lt]Alt\*[Gt]-\*[Lt]Esc\*[Gt]
.It hpcsh
\*[Lt]Ctrl\*[Gt]-\*[Lt]Alt\*[Gt]-\*[Lt]Esc\*[Gt]
.It i386
\*[Lt]Ctrl\*[Gt]-\*[Lt]Alt\*[Gt]-\*[Lt]Esc\*[Gt]
.br
\*[Lt]Break\*[Gt] on serial console.
.It mac68k
\*[Lt]Command\*[Gt]-\*[Lt]Power\*[Gt], or the Interrupt switch.
.It macppc
Some models:
\*[Lt]Command\*[Gt]-\*[Lt]Option\*[Gt]-\*[Lt]Power\*[Gt]
.It mvme68k
Abort switch on CPU card.
.It pmax
\*[Lt]Do\*[Gt] on
.Tn LK-201
rcons console.
.br
\*[Lt]Break\*[Gt] on serial console.
.It sparc
\*[Lt]L1\*[Gt]-A, or \*[Lt]Stop\*[Gt]-A on a
.Tn Sun
keyboard.
.br
\*[Lt]Break\*[Gt] on serial console.
.It sun3
\*[Lt]L1\*[Gt]-A, or \*[Lt]Stop\*[Gt]-A on a
.Tn Sun
keyboard.
.br
\*[Lt]Break\*[Gt] on serial console.
.It sun3x
\*[Lt]L1\*[Gt]-A, or \*[Lt]Stop\*[Gt]-A on a
.Tn Sun
keyboard.
.br
\*[Lt]Break\*[Gt] on serial console.
.It x68k
Interrupt switch on the body.
.El
.Pp
In addition,
.Nm
may be explicitly activated by the debugging code in the kernel
if
.Cm DDB
is configured.
.Sh COMMAND SYNTAX
The general command syntax is:
.Bd -ragged -offset indent
.Ic command Ns Op Cm / Ns Ar modifier
.Ar address
.Op Cm , Ns Ar count
.Ed
.Pp
The current memory location being edited is referred to as
.Ar dot ,
and the next location is
.Ar next .
They are displayed as hexadecimal numbers.
.Pp
Commands that examine and/or modify memory update
.Ar dot
to the address of the last line examined or the last location
modified, and set
.Ar next
to the next location to be examined or modified.
Other commands don't change
.Ar dot ,
and set
.Ar next
to be the same as
.Ar dot .
.Pp
A blank line repeats the previous command from the address
.Ar next
with the previous
.Cm count
and no modifiers.
Specifying
.Cm address
sets
.Em dot
to the address.
If
.Cm address
is omitted,
.Em dot
is used.
A missing
.Cm count
is taken to be 1 for printing commands, and infinity for stack traces.
.Pp
The syntax:
.Bd -ragged -offset indent
.Ic Cm , Ns Ar count
.Ed
.Pp
repeats the previous command, just as a blank line does, but with
the specified
.Cm count .
.Pp
.Nm
has a
.Xr more 1 -like
functionality; if a number of lines in a command's output exceeds the number
defined in the
.Va lines
variable, then
.Nm
displays
.Dq "--db more--"
and waits for a response, which may be one of:
.Bl -tag -offset indent -width "\*[Lt]return\*[Gt]"
.It \*[Lt]return\*[Gt]
one more line.
.It \*[Lt]space\*[Gt]
one more page.
.It Ic q
abort the current command, and return to the command input mode.
.El
.Pp
If
.Nm
history editing is enabled (by defining the
.D1 Cd options DDB_HISTORY_SIZE=num
kernel option), then a history of the last
.Cm num
commands is kept.
The history can be manipulated with the following key sequences:
.Bl -tag -offset indent -width "\*[Lt]Ctrl\*[Gt]-P"
.It \*[Lt]Ctrl\*[Gt]-P
retrieve previous command in history (if any).
.It \*[Lt]Ctrl\*[Gt]-N
retrieve next command in history (if any).
.El
.Sh COMMANDS
.Nm
supports the following commands:
.Bl -tag -width 5n
.It Xo
.Sm off
.Ic !
.Ar address
.Oo
.Cm ( Ar expression
.Op Ar ,...
.Cm )
.Oc
.Sm on
.Xc
A synonym for
.Ic call .
.It Xo
.Ic break Ns Op Cm /u
.Ar address Ns Op Cm , Ns Ar count
.Xc
Set a breakpoint at
.Ar address .
If
.Ar count
is supplied, continues
.Pq Ar count Ns -1
times before stopping at the breakpoint.
If the breakpoint is set, a breakpoint number is printed with
.Sq \&# .
This number can be used to
.Ic delete
the breakpoint, or to add
conditions to it.
.Pp
If
.Cm /u
is specified,
set a breakpoint at a user-space address.
Without
.Cm /u ,
.Ar address
is considered to be in the kernel-space, and an address in the wrong
space will be rejected, and an error message will be emitted.
This modifier may only be used if it is supported by machine dependent
routines.
.Pp
Warning: if a user text is shadowed by a normal user-space debugger,
user-space breakpoints may not work correctly.
Setting a breakpoint at the low-level code paths may also cause
strange behavior.
.It Xo
.Ic bt Ns Op Cm /u
.Sm off
.Op Ar frame-address
.Op Cm , Ar count
.Sm on
.Xc
A synonym for
.Ic trace .
.It Xo
.Ic bt/t
.Sm off
.Op Ar pid
.Op Cm , Ar count
.Sm on
.Xc
A synonym for
.Ic trace .
.It Xo
.Ic call
.Sm off
.Ar address
.Oo
.Cm ( Ar expression
.Op Ar ,...
.Cm )
.Oc
.Sm on
.Xc
Call the function specified by
.Ar address
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 continue Ns Op Cm /c
Continue execution until a breakpoint or watchpoint.
If
.Cm /c
is specified, count instructions while executing.
Some machines (e.g., pmax) also count loads and stores.
.Pp
Warning: when counting, the debugger is really silently
single-stepping.
This means that single-stepping on low-level may cause strange
behavior.
.It Xo
.Ic delete
.Ar "address" |
.Cm # Ns Ar number
.Xc
Delete a breakpoint.
The target breakpoint may be specified by
.Ar address ,
as per
.Ic break ,
or by the breakpoint number returned by
.Ic break
if it's prefixed with
.Sq Cm \&# .
.It Xo
.Ic dmesg
.Xc
Prints the contents of the kernel message buffer.
.It Xo
.Ic dwatch
.Ar address
.Xc
Delete the watchpoint at
.Ar address
that was previously set with
.Ic watch
command.
.It Xo
.Ic examine Ns Op Cm / Ns Ar modifier
.Ar address Ns Op Cm , Ns Ar count
.Xc
Display the address locations according to the format in
.Ar modifier .
Multiple modifier formats display multiple locations.
If
.Ar modifier
isn't specified, the modifier from the last use of
.Ic examine
is used.
.Pp
The valid format characters for
.Ar modifier
are:
.Bl -tag -offset indent -width 2n -compact
.It Cm b
examine bytes (8 bits).
.It Cm h
examine half-words (16 bits).
.It Cm l
examine words (legacy
.Dq long ,
32 bits).
.It Cm L
examine long words (implementation dependent)
.It Cm a
print the location being examined.
.It Cm A
print the location with a line number if possible.
.It Cm x
display in unsigned hex.
.It Cm z
display in signed hex.
.It Cm o
display in unsigned octal.
.It Cm d
display in signed decimal.
.It Cm u
display in unsigned decimal.
.It Cm r
display in current radix, signed.
.It Cm c
display low 8 bits as a character.
Non-printing characters as displayed as an octal escape code
(e.g.,
.Sq \e000 ) .
.It Cm s
display the NUL terminated string at the location.
Non-printing characters are displayed as octal escapes.
.It Cm m
display in unsigned hex with a character dump at the end of each line.
The location is displayed as hex at the beginning of each line.
.It Cm i
display as a machine instruction.
.It Cm I
display as a machine instruction, with possible alternative formats
depending upon the machine:
.Bl -tag -offset indent -width "sparc" -compact
.It alpha
print register operands
.It m68k
use Motorola syntax
.It pc532
print instruction bytes in hex
.It vax
don't assume that each external label is a procedure entry mask
.El
.El
.It Xo
.Ic kill
.Ar pid Ns Op Cm , Ns Ar signal_number
.Xc
Send a signal to the process specified by the
.Ar pid .
Note that
.Ar pid
is interpreted using the current radix (see
.Cm trace/t
command for details).
If
.Ar signal_number
isn't specified, the SIGTERM signal is sent.
.It Ic match Ns Op Cm /p
A synonym for
.Ic next .
.It Ic next Ns Op Cm /p
Stop at the matching return instruction.
If
.Cm /p
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.
.It Xo
.Ic print Ns Op Cm /axzodurc
.Ar address Op Ar address ...
.Xc
Print addresses
.Ar address
according to the modifier character, as per
.Ic examine .
Valid modifiers are:
.Cm /a ,
.Cm /x ,
.Cm /z ,
.Cm /o ,
.Cm /d ,
.Cm /u ,
.Cm /r ,
and
.Cm /c
(as per
.Ic examine ) .
If no modifier is specified, the most recent one specified is used.
.Ar address
may be a string, and is printed
.Dq as-is .
For example:
.Bd -literal -offset indent
print/x "eax = " $eax "\enecx = " $ecx "\en"
.Ed
.Pp
will produce:
.Bd -literal -offset indent
eax = xxxxxx
ecx = yyyyyy
.Ed
.It Xo
.Sm off
.Ic ps
.Op Cm /a
.Op Cm /n
.Op Cm /w
.Sm on
.Xc
A synonym for
.Ic "show all procs" .
.It Ic reboot Op Ar flags
Reboot, using the optionally supplied boot
.Ar flags .
.Pp
Note: Limitations of the command line interface preclude
specification of a boot string.
.It Xo
.Ic search Ns Op Cm /bhl
.Ar address
.Ar value
.Op Ar mask
.Op Cm , Ns Ar count
.Xc
Search memory from
.Ar address
for
.Ar value .
The unit size is specified with a modifier character, as per
.Ic examine .
Valid modifiers are:
.Cm /b ,
.Cm /h ,
and
.Cm /l .
If no modifier is specified,
.Cm /l
is used.
.Pp
This command might fail in interesting ways if it doesn't find
.Ar value .
This is because
.Nm
doesn't always recover from touching bad memory.
The optional
.Ar count
limits the search.
.It Xo
.Ic set
.Cm $ Ns Ar variable
.Op Cm =
.Ar expression
.Xc
Set the named variable or register to the value of
.Ar expression .
Valid variable names are described in
.Sx VARIABLES .
.It Xo
.Sm off
.Ic "show\ all\ procs"
.Op Cm /a
.Op Cm /n
.Op Cm /w
.Sm on
.Xc
Display all process information.
Valid modifiers:
.Bl -tag -width 3n
.It Cm /n
show process information in a
.Xr ps 1
style format (this is the default).
Information printed includes: process ID, parent process ID,
process group, UID, process status, process flags, process
command name, and process wait channel message.
.It Cm /a
show the kernel virtual addresses of each process'
proc structure, u-area, and vmspace structure.
The vmspace address is also the address of the process'
vm_map structure, and can be used in the
.Ic "show map"
command.
.It Cm /w
show each process' PID, command, system call emulation, wait channel
address, and wait channel message.
.El
.It Ic "show breaks"
Display all breakpoints.
.It Xo
.Ic "show buf" Ns Op Cm /f
.Ar address
.Xc
Print the struct buf at
.Ar address .
The
.Cm /f
does nothing at this time.
.It Xo
.Ic "show event" Ns Op Cm /f
.Xc
Print all the non-zero
.Xr evcnt 9
event counters.
If
.Cm /f
is specified, all event counters with a count of zero are printed as well.
.It Xo
.Ic "show map" Ns Op Cm /f
.Ar address
.Xc
Print the vm_map at
.Ar address .
If
.Cm /f
is specified, the complete map is printed.
.It Ic "show ncache" Ar address
Dump the namecache list associated with vnode at
.Ar address .
.It Xo
.Ic "show object" Ns Op Cm /f
.Ar address
.Xc
Print the vm_object at
.Ar address .
If
.Cm /f
is specified, the complete object is printed.
.It Xo
.Ic "show page" Ns Op Cm /f
.Ar address
.Xc
Print the vm_page at
.Ar address .
If
.Cm /f
is specified, the complete page is printed.
.It Xo
.Ic "show pool" Ns Op Cm /clp
.Ar address
.Xc
Print the pool at
.Ar address .
Valid modifiers:
.Bl -tag -width 4n -compact
.It Cm /c
Print the cachelist and its statistics for this pool.
.It Cm /l
Print the log entries for this pool.
.It Cm /p
Print the pagelist for this pool.
.El
.It Ic "show registers" Ns Op Cm /u
Display the register set.
If
.Cm /u
is specified, display user registers instead of kernel registers
or the currently save one.
.Pp
Warning: support for
.Cm /u
is machine dependent.
If not supported, incorrect information will be displayed.
.It Ic "show uvmexp"
Print a selection of UVM counters and statistics.
.It Xo
.Ic "show vnode" Ns Op Cm /f
.Ar address
.Xc
Print the vnode at
.Ar address .
If
.Cm /f
is specified, the complete vnode is printed.
.It Ic "show watches"
Display all watchpoints.
.It Xo
.Ic sifting Ns Op Cm /F
.Ar string
.Xc
Search the symbol tables for all symbols of which
.Ar string
is a substring, and display them.
If
.Cm /F
is specified, a character is displayed immediately after each symbol
name indicating the type of symbol.
.Pp
For
.Ns Xr a.out 5 -format
symbol tables,
absolute symbols display
.Sy @ ,
text segment symbols display
.Sy * ,
data segment symbols display
.Sy + ,
.Tn BSS
segment symbols display
.Sy - ,
and filename symbols display
.Sy / .
For
.Tn ELF Ns -format
symbol tables,
object symbols display
.Sy + ,
function symbols display
.Sy * ,
section symbols display
.Sy \*[Am] ,
and file symbols display
.Sy / .
.Pp
To sift for a string beginning with a number, escape the first
character with a backslash as:
.Bd -literal -offset indent
sifting \\386
.Ed
.It Xo
.Ic step Ns Op Cm /p
.Op Cm , Ns Ar count
.Xc
Single-step
.Ar count
times.
If
.Cm /p
is specified, print each instruction at each step.
Otherwise, only print the last instruction.
.Pp
Warning: depending on the 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.
.It Ic sync
Force a crash dump, and then reboot.
.It Xo
.Ic trace
.Ic Ns Op Cm /u Ns Op Cm l
.Sm off
.Op Ar frame-address
.Op Cm , Ar count
.Sm on
.Xc
Stack trace from
.Ar frame-address .
If
.Cm /u
is specified, trace user-space, otherwise trace kernel-space.
.Ar count
is the number of frames to be traced.
If
.Ar count
is omitted, all frames are printed.
If
.Cm /l
is specified, the trace is printed and also stored in the kernel
message buffer.
.Pp
Warning: user-space stack trace is valid only if the machine dependent
code supports it.
.It Xo
.Ic trace/t Ns Op Cm l
.Sm off
.Op Ar pid
.Op Cm , Ar count
.Sm on
.Xc
Stack trace by
.Dq thread
(process, on
.Nx )
rather than by stack frame address.
Note that
.Ar pid
is interpreted using the current radix, whilst
.Ic ps
displays pids in decimal; prefix
.Ar pid
with
.Sq 0t
to force it to be interpreted as decimal (see
.Sx VARIABLES
section for radix).
If
.Cm /l
is specified, the trace is printed and also stored in the kernel
message buffer.
.Pp
Warning: trace by pid is valid only if the machine dependent code
supports it.
.It Ic until Ns Op Cm /p
Stop at the next call or return instruction.
If
.Cm /p
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.
.It Xo
.Ic watch
.Sm off
.Ar address
.Op Cm , Ar size
.Sm on
.Xc
Set a watchpoint for a region.
Execution stops when an attempt to modify the region occurs.
.Ar size
defaults to 4.
.Pp
If you specify a wrong space address, the request is
rejected with an error message.
.Pp
Warning: attempts to watch wired kernel memory may cause
an unrecoverable error in some systems such as i386.
Watchpoints on user addresses work the best.
.It Xo
.Ic write Ns Op Cm /bhl
.Ar address
.Ar expression Op Ar expression ...
.Xc
Write the
.Ar expression Ns s
at succeeding locations.
The unit size is specified with a modifier character, as per
.Ic examine .
Valid modifiers are:
.Cm /b ,
.Cm /h ,
and
.Cm /l .
If no modifier is specified,
.Cm /l
is used.
.Pp
Warning: since there is no delimiter between
.Ar expression Ns s ,
strange things may occur.
It's best to enclose each
.Ar expression
in parentheses.
.It Xo
.Ic x Ns Op Cm / Ns Ar modifier
.Ar address Ns Op Cm , Ns Ar count
.Xc
A synonym for
.Ic examine .
.\" XXX - these commands aren't implemented; jhawk 19 May 2000
.\" .It Ic xf
.\" Examine forward.
.\" .Ic xf
.\" re-executes the most recent
.\" .Ic execute
.\" command with the same parameters except that
.\" .Ar address
.\" is set to
.\" .Ar next .
.\" .It Ic xb
.\" Examine backward.
.\" .Ic xb
.\" re-executes the most recent
.\" .Ic execute
.\" command with the same parameters, except that
.\" .Ar address
.\" is set to the last start address minus its size.
.El
.Sh MACHINE-SPECIFIC COMMANDS
The "glue" code that hooks
.Nm
into the
.Nx
kernel for any given port can also add machine specific commands
to the
.Nm
command parser.
All of these commands are preceded by the command word
.Em machine
to indicate that they are part of the machine-specific command
set (e.g.
.Ic "machine reboot" ) .
Some of these commands are:
.Ss ALPHA
.Bl -tag -width "intrchain" -compact
.It Ic halt
Call the PROM monitor to halt the CPU.
.It Ic reboot
Call the PROM monitor to reboot the CPU.
.El
.Ss ARM32
.Bl -tag -width "intrchain" -compact
.It Ic vmstat
Equivalent to
.Xr vmstat 1
output with "-s" option (statistics).
.It Ic vnode
Print out a description of a vnode.
.It Ic intrchain
Print the list of IRQ handlers.
.It Ic panic
Print the current "panic" string.
.It Ic frame
Given a trap frame address, print out the trap frame.
.El
.Ss MIPS
.Bl -tag -width "intrchain" -compact
.It Ic kvtop
Print the physical address for a given kernel virtual address.
.It Ic tlb
Print out the Translation Lookaside Buffer (TLB).
Only works in
.Nx
kernels compiled with
.Dv DEBUG
option.
.El
.Ss SH3
.Bl -tag -width "intrchain" -compact
.It Ic tlb
Print TLB entries
.It Ic cache
Print cache entries
.It Ic frame
Print switch frame and trap frames.
.It Ic stack
Print kernel stack usage.
Only works in NetBSD kernels compiled with the KSTACK_DEBUG option.
.El
.Ss SPARC
.Bl -tag -width "intrchain" -compact
.It Ic prom
Exit to the Sun PROM monitor.
.El
.Ss SPARC64
.Bl -tag -width "intrchain" -compact
.It Ic buf
Print buffer information.
.It Ic ctx
Print process context information.
.It Ic dtlb
Print data translation look-aside buffer context information.
.It Ic dtsb
Display data translation storage buffer information.
.It Ic kmap
Display information about the listed mapping in the kernel pmap.
Use the
.Dq f
modifier to get a full listing.
.It Ic pcb
Display information about the
.Dq struct pcb
listed.
.It Ic pctx
Attempt to change process context.
.It Ic page
Display the pointer to the
.Dq struct vm_page
for this physical address.
.It Ic phys
Display physical memory.
.It Ic pmap
Display the pmap.
Use the
.Dq f
modifier to get a fuller listing.
.It Ic proc
Display some information about the process pointed to, or curproc.
.It Ic prom
Enter the OFW PROM.
.It Ic pv
Display the
.Dq struct pv_entry
pointed to.
.It Ic stack
Dump the window stack.
Use the
.Dq u
modifier to get userland information.
.It Ic tf
Display full trap frame state.
This is most useful for inclusion with bug reports.
.It Ic ts
Display trap state.
.It Ic traptrace
Display or set trap trace information.
Use the
.Dq r
and
.Dq f
modifiers to get reversed and full information, respectively.
.It Ic uvmdump
Dumps the UVM histories.
.It Ic watch
Set or clear a physical or virtual hardware watchpoint.
Pass the address to be watched, or
.Dq 0
to clear the watchpoint.
Append
.Dq p
to the watch point to use the physical watchpoint registers.
.It Ic window
Print register window information about given address.
.El
.Ss SUN3 and SUN3X
.Bl -tag -width "intrchain" -compact
.It Ic abort
Drop into monitor via abort (allows continue).
.It Ic halt
Exit to Sun PROM monitor as in
.Xr halt 8 .
.It Ic reboot
Reboot the machine as in
.Xr reboot 8 .
.It Ic pgmap
Given an address, print the address, segment map, page map, and
Page Table Entry (PTE).
.El
.Sh VARIABLES
.Nm
accesses registers and variables as
.Cm $ Ns Ar name .
Register names are as per the
.Ic "show registers"
command.
Some variables are suffixed with numbers, and may have a modifier
following a colon immediately after the variable name.
For example, register variables may have a
.Sq u
modifier to indicate user register
(e.g.,
.Li "$eax:u" ) .
.Pp
Built-in variables currently supported are:
.Bl -tag -offset indent -width "maxwidth" -compact
.It Va lines
The number of lines.
This is used by the
.Ic more
feature.
.It Va maxoff
Addresses are printed as
.Li 'symbol'+offset
unless
.Li offset
is greater than
.Va maxoff .
.It Va maxwidth
The width of the displayed line.
.It Va onpanic
If non-zero (the default),
.Nm
will be invoked when the kernel panics.
If the kernel configuration option
.D1 Cd options DDB_ONPANIC=0
is used,
.Va onpanic
will be initialized to off.
.It Va fromconsole
If non-zero (the default),
the kernel allows to enter
.Nm
from the console (by break signal or special key sequence).
If the kernel configuration option
.D1 Cd options DDB_FROMCONSOLE=0
is used,
.Va fromconsole
will be initialized to off.
.It Va radix
Input and output radix.
.It Va tabstops
Tab stop width.
.\" .It Va work Ns Sy xx
.\" Temporary work variable.
.\" .Sq Sy xx
.\" is between 0 and 31.
.El
.Pp
All built-in variables are accessible via
.Xr sysctl 3 .
.Sh EXPRESSIONS
Almost all expression operators in C are supported, except
.Sq \&~ ,
.Sq \&^ ,
and unary
.Sq \&\*[Am] .
Special rules in
.Nm
are:
.Bl -tag -offset indent -width "identifier"
.It Ar identifier
name of a symbol.
It is translated to the address (or value) of it.
.Sq \&.
and
.Sq \&:
can be used in the identifier.
If supported by an object format dependent routine,
.Sm off
.Xo
.Oo Ar filename : Oc
.Ar function
.Oo : Ar "line number" Oc ,
.Xc
.Xo
.Oo Ar filename : Oc
.Ar variable ,
.Xc
and
.Xo
.Ar filename
.Oo : Ar "line number" Oc ,
.Xc
.Sm on
can be accepted as a symbol.
The symbol may be prefixed with
.Ar symbol_table_name : :
(e.g.,
.Li emulator::mach_msg_trap )
to specify other than kernel symbols.
.It Ar number
number.
Radix is determined by the first two characters:
.Sq 0x
- hex,
.Sq 0o
- octal,
.Sq 0t
- decimal,
otherwise follow current radix.
.It Cm .
.Ar dot
.It Cm +
.Ar next
.It Cm ..
address of the start of the last line examined.
Unlike
.Ar dot
or
.Ar next ,
this is only changed by the
.Ic examine
or
.Ic write
commands.
.It Cm \&"
last address explicitly specified.
.It Cm $ Ns Ar name
register name or variable.
It is translated to the value of it.
It may be followed by a
.Sq \&:
and modifiers as described above.
.It Cm a
multiple of right-hand side.
.It Cm * Ns Ar expr
expression indirection.
It may be followed by a
.Sq \&:
and modifiers as described above.
.El
.Sh SEE ALSO
.Xr options 4 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
kernel debugger was written as part of the MACH project at
Carnegie-Mellon University.