.\"
.\" Mach Operating System
.\" Copyright (c) 1991,1990 Carnegie Mellon University
.\" Copyright (c) 2007 Robert N. M. Watson
.\" 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.
.\"
.\" changed a \# to #, since groff choked on it.
.\"
.\" HISTORY
.\" ddb.4,v
.\" Revision 1.1  1993/07/15  18:41:02  brezak
.\" Man page for DDB
.\"
.\" Revision 2.6  92/04/08  08:52:57  rpd
.\" 	Changes from OSF.
.\" 	[92/01/17  14:19:22  jsb]
.\" 	Changes for OSF debugger modifications.
.\" 	[91/12/12            tak]
.\"
.\" Revision 2.5  91/06/25  13:50:22  rpd
.\" 	Added some watchpoint explanation.
.\" 	[91/06/25            rpd]
.\"
.\" Revision 2.4  91/06/17  15:47:31  jsb
.\" 	Added documentation for continue/c, match, search, and watchpoints.
.\" 	I've not actually explained what a watchpoint is; maybe Rich can
.\" 	do that (hint, hint).
.\" 	[91/06/17  10:58:08  jsb]
.\"
.\" Revision 2.3  91/05/14  17:04:23  mrt
.\" 	Correcting copyright
.\"
.\" Revision 2.2  91/02/14  14:10:06  mrt
.\" 	Changed to new Mach copyright
.\" 	[91/02/12  18:10:12  mrt]
.\"
.\" Revision 2.2  90/08/30  14:23:15  dbg
.\" 	Created.
.\" 	[90/08/30            dbg]
.\"
.\" $FreeBSD$
.\"
.Dd January 15, 2016
.Dt DDB 4
.Os
.Sh NAME
.Nm ddb
.Nd interactive kernel debugger
.Sh SYNOPSIS
In order to enable kernel debugging facilities include:
.Bd -ragged -offset indent
.Cd options KDB
.Cd options DDB
.Ed
.Pp
To prevent activation of the debugger on kernel
.Xr panic 9 :
.Bd -ragged -offset indent
.Cd options KDB_UNATTENDED
.Ed
.Pp
In order to print a stack trace of the current thread on the console
for a panic:
.Bd -ragged -offset indent
.Cd options KDB_TRACE
.Ed
.Pp
To print the numerical value of symbols in addition to the symbolic
representation, define:
.Bd -ragged -offset indent
.Cd options DDB_NUMSYM
.Ed
.Pp
To enable the
.Xr gdb 1
backend, so that remote debugging with
.Xr kgdb 1
is possible, include:
.Bd -ragged -offset indent
.Cd options GDB
.Ed
.Sh DESCRIPTION
The
.Nm
kernel debugger is an interactive debugger with a syntax inspired by
.Xr gdb 1 .
If linked into the running kernel,
it can be invoked locally with the
.Ql debug
.Xr keymap 5
action.
The debugger is also invoked on kernel
.Xr panic 9
if the
.Va debug.debugger_on_panic
.Xr sysctl 8
MIB variable is set non-zero,
which is the default
unless the
.Dv KDB_UNATTENDED
option is specified.
.Pp
The current location is called
.Va dot .
The
.Va dot
is displayed with
a hexadecimal format at a prompt.
The commands
.Ic examine
and
.Ic write
update
.Va dot
to the address of the last line
examined or the last location modified, and set
.Va next
to the address of
the next location to be examined or changed.
Other commands do not change
.Va dot ,
and set
.Va next
to be the same as
.Va dot .
.Pp
The general command syntax is:
.Ar command Ns Op Li / Ns Ar modifier
.Ar address Ns Op Li , Ns Ar count
.Pp
A blank line repeats the previous command from the address
.Va next
with
count 1 and no modifiers.
Specifying
.Ar address
sets
.Va dot
to the address.
Omitting
.Ar address
uses
.Va dot .
A missing
.Ar count
is taken
to be 1 for printing commands or infinity for stack traces.
.Pp
The
.Nm
debugger has a pager feature (like the
.Xr more 1
command)
for the output.
If an output line exceeds the number set in the
.Va lines
variable, it displays
.Dq Li --More--
and waits for a response.
The valid responses for it are:
.Pp
.Bl -tag -compact -width ".Li SPC"
.It Li SPC
one more page
.It Li RET
one more line
.It Li q
abort the current command, and return to the command input mode
.El
.Pp
Finally,
.Nm
provides a small (currently 10 items) command history, and offers
simple
.Nm emacs Ns -style
command line editing capabilities.
In addition to
the
.Nm emacs
control keys, the usual
.Tn ANSI
arrow keys may be used to
browse through the history buffer, and move the cursor within the
current line.
.Sh COMMANDS
.Bl -tag -width indent -compact
.It Ic examine
.It Ic 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 format specified for this command
is used.
.Pp
The format characters are:
.Bl -tag -compact -width indent
.It Cm b
look at by bytes (8 bits)
.It Cm h
look at by half words (16 bits)
.It Cm l
look at by long words (32 bits)
.It Cm g
look at by quad words (64 bits)
.It Cm a
print the location being displayed
.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 are displayed as an octal escape code (e.g.,
.Ql \e000 ) .
.It Cm s
display the null-terminated string at the location.
Non-printing characters are displayed as octal escapes.
.It 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.
.It Cm i
display as an instruction
.It Cm I
display as an instruction with possible alternate formats depending on the
machine, but none of the supported architectures have an alternate format.
.It Cm S
display a symbol name for the pointer stored at the address
.El
.Pp
.It Ic xf
Examine forward:
execute an
.Ic examine
command with the last specified parameters to it
except that the next address displayed by it is used as the start address.
.Pp
.It Ic xb
Examine backward:
execute an
.Ic 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.
.Pp
.It Ic print Ns Op Li / Ns Cm acdoruxz
.It Ic p Ns Op Li / Ns Cm acdoruxz
Print
.Ar addr Ns s
according to the modifier character (as described above for
.Cm examine ) .
Valid formats are:
.Cm a , x , z , o , d , u , r ,
and
.Cm c .
If no modifier is specified, the last one specified to it is used.
The argument
.Ar addr
can be a string, in which case it is printed as it is.
For example:
.Bd -literal -offset indent
print/x "eax = " $eax "\enecx = " $ecx "\en"
.Ed
.Pp
will print like:
.Bd -literal -offset indent
eax = xxxxxx
ecx = yyyyyy
.Ed
.Pp
.It Xo
.Ic write Ns Op Li / Ns Cm bhl
.Ar addr expr1 Op Ar expr2 ...
.Xc
.It Xo
.Ic w Ns Op Li / Ns Cm bhl
.Ar addr expr1 Op Ar expr2 ...
.Xc
Write the expressions specified after
.Ar addr
on the command line at succeeding locations starting with
.Ar addr .
The write unit size can be specified in the modifier with a letter
.Cm b
(byte),
.Cm h
(half word) or
.Cm l
(long word) respectively.
If omitted,
long word is assumed.
.Pp
.Sy Warning :
since there is no delimiter between expressions, strange
things may happen.
It is best to enclose each expression in parentheses.
.Pp
.It Ic set Li $ Ns Ar variable Oo Li = Oc Ar expr
Set the named variable or register with the value of
.Ar expr .
Valid variable names are described below.
.Pp
.It Ic break Ns Op Li / Ns Cm u
.It Ic b Ns Op Li / Ns Cm u
Set a break point at
.Ar addr .
If
.Ar count
is supplied, continues
.Ar count
\- 1 times before stopping at the
break point.
If the break point is set, a break point number is
printed with
.Ql # .
This number can be used in deleting the break point
or adding conditions to it.
.Pp
If the
.Cm u
modifier is specified, this command sets a break point in user
address space.
Without the
.Cm u
option, the address is considered to be in the kernel
space, and a wrong space address is rejected with an error message.
This modifier can be used only if it is supported by machine dependent
routines.
.Pp
.Sy 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.
.Pp
.It Ic delete Ar addr
.It Ic d Ar addr
.It Ic delete Li # Ns Ar number
.It Ic d Li # Ns Ar number
Delete the break point.
The target break point can be specified by a
break point number with
.Ql # ,
or by using the same
.Ar addr
specified in the original
.Ic break
command.
.Pp
.It Ic watch Ar addr Ns Li , Ns Ar size
Set a watchpoint for a region.
Execution stops when an attempt to modify the region occurs.
The
.Ar size
argument defaults to 4.
If you specify a wrong space address, the request is rejected
with an error message.
.Pp
.Sy Warning :
Attempts to watch wired kernel memory
may cause unrecoverable error in some systems such as i386.
Watchpoints on user addresses work best.
.Pp
.It Ic hwatch Ar addr Ns Li , Ns Ar size
Set a hardware watchpoint for a region if supported by the
architecture.
Execution stops when an attempt to modify the region occurs.
The
.Ar size
argument defaults to 4.
.Pp
.Sy Warning :
The hardware debug facilities do not have a concept of separate
address spaces like the watch command does.
Use
.Ic hwatch
for setting watchpoints on kernel address locations only, and avoid
its use on user mode address spaces.
.Pp
.It Ic dhwatch Ar addr Ns Li , Ns Ar size
Delete specified hardware watchpoint.
.Pp
.It Ic step Ns Op Li / Ns Cm p
.It Ic s Ns Op Li / Ns Cm p
Single step
.Ar count
times (the comma is a mandatory part of the syntax).
If the
.Cm p
modifier is specified, print each instruction at each step.
Otherwise, only print the last instruction.
.Pp
.Sy 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.
.Pp
.It Ic continue Ns Op Li / Ns Cm c
.It Ic c Ns Op Li / Ns Cm c
Continue execution until a breakpoint or watchpoint.
If the
.Cm c
modifier is specified, count instructions while executing.
Some machines (e.g., pmax) also count loads and stores.
.Pp
.Sy Warning :
when counting, the debugger is really silently single-stepping.
This means that single-stepping on low-level code may cause strange
behavior.
.Pp
.It Ic until Ns Op Li / Ns Cm p
Stop at the next call or return instruction.
If the
.Cm 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.
.Pp
.It Ic next Ns Op Li / Ns Cm p
.It Ic match Ns Op Li / Ns Cm p
Stop at the matching return instruction.
If the
.Cm 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.
.Pp
.It Xo
.Ic trace Ns Op Li / Ns Cm u
.Op Ar pid | tid
.Op Li , Ns Ar count
.Xc
.It Xo
.Ic t Ns Op Li / Ns Cm u
.Op Ar pid | tid
.Op Li , Ns Ar count
.Xc
.It Xo
.Ic where Ns Op Li / Ns Cm u
.Op Ar pid | tid
.Op Li , Ns Ar count
.Xc
.It Xo
.Ic bt Ns Op Li / Ns Cm u
.Op Ar pid | tid
.Op Li , Ns Ar count
.Xc
Stack trace.
The
.Cm u
option traces user space; if omitted,
.Ic trace
only traces
kernel space.
The optional argument
.Ar count
is the number of frames to be traced.
If
.Ar count
is omitted, all frames are printed.
.Pp
.Sy Warning :
User space stack trace is valid
only if the machine dependent code supports it.
.Pp
.It Xo
.Ic search Ns Op Li / Ns Cm bhl
.Ar addr
.Ar value
.Op Ar mask
.Op Li , Ns Ar count
.Xc
Search memory for
.Ar value .
This command might fail in interesting
ways if it does not find the searched-for value.
This is because
.Nm
does not always recover from touching bad memory.
The optional
.Ar count
argument limits the search.
.\"
.Pp
.It Xo
.Ic findstack
.Ar addr
.Xc
Prints the thread address for a thread kernel-mode stack of which contains the
specified address.
If the thread is not found, search the thread stack cache and prints the
cached stack address.
Otherwise, prints nothing.
.Pp
.It Ic show Cm all procs Ns Op Li / Ns Cm m
.It Ic ps Ns Op Li / Ns Cm 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
.Cm m
modifier will alter the display to show VM map
addresses for the process and not show other information.
.\"
.Pp
.It Ic show Cm all ttys
Show all TTY's within the system.
Output is similar to
.Xr pstat 8 ,
but also includes the address of the TTY structure.
.\"
.Pp
.It Ic show Cm allchains
Show the same information like "show lockchain" does, but
for every thread in the system.
.\"
.Pp
.It Ic show Cm alllocks
Show all locks that are currently held.
This command is only available if
.Xr witness 4
is included in the kernel.
.\"
.Pp
.It Ic show Cm allpcpu
The same as "show pcpu", but for every CPU present in the system.
.\"
.Pp
.It Ic show Cm allrman
Show information related with resource management, including
interrupt request lines, DMA request lines, I/O ports, I/O memory
addresses, and Resource IDs.
.\"
.Pp
.It Ic show Cm apic
Dump data about APIC IDT vector mappings.
.\"
.Pp
.It Ic show Cm breaks
Show breakpoints set with the "break" command.
.\"
.Pp
.It Ic show Cm bio Ar addr
Show information about the bio structure
.Vt struct bio
present at
.Ar addr .
See the
.Pa sys/bio.h
header file and
.Xr g_bio 9
for more details on the exact meaning of the structure fields.
.\"
.Pp
.It Ic show Cm buffer Ar addr
Show information about the buf structure
.Vt struct buf
present at
.Ar addr .
See the
.Pa sys/buf.h
header file for more details on the exact meaning of the structure fields.
.\"
.Pp
.It Ic show Cm cbstat
Show brief information about the TTY subsystem.
.\"
.Pp
.It Ic show Cm cdev
Without argument, show the list of all created cdev's, consisting of devfs
node name and struct cdev address.
When address of cdev is supplied, show some internal devfs state of the cdev.
.\"
.Pp
.It Ic show Cm conifhk
Lists hooks currently waiting for completion in
run_interrupt_driven_config_hooks().
.\"
.Pp
.It Ic show Cm cpusets
Print numbered root and assigned CPU affinity sets.
See
.Xr cpuset 2
for more details.
.\"
.Pp
.It Ic show Cm cyrixreg
Show registers specific to the Cyrix processor.
.\"
.Pp
.It Ic show Cm devmap
Prints the contents of the static device mapping table.
Currently only available on the
ARM
architecture.
.\"
.Pp
.It Ic show Cm domain Ar addr
Print protocol domain structure
.Vt struct domain
at address
.Ar addr .
See the
.Pa sys/domain.h
header file for more details on the exact meaning of the structure fields.
.\"
.Pp
.It Ic show Cm ffs Op Ar addr
Show brief information about ffs mount at the address
.Ar addr ,
if argument is given.
Otherwise, provides the summary about each ffs mount.
.\"
.Pp
.It Ic show Cm file Ar addr
Show information about the file structure
.Vt struct file
present at address
.Ar addr .
.\"
.Pp
.It Ic show Cm files
Show information about every file structure in the system.
.\"
.Pp
.It Ic show Cm freepages
Show the number of physical pages in each of the free lists.
.\"
.Pp
.It Ic show Cm geom Op Ar addr
If the
.Ar addr
argument is not given, displays the entire GEOM topology.
If
.Ar addr
is given, displays details about the given GEOM object (class, geom,
provider or consumer).
.\"
.Pp
.It Ic show Cm idt
Show IDT layout.
The first column specifies the IDT vector.
The second one is the name of the interrupt/trap handler.
Those functions are machine dependent.
.\"
.Pp
.It Ic show Cm inodedeps Op Ar addr
Show brief information about each inodedep structure.
If
.Ar addr
is given, only inodedeps belonging to the fs located at the
supplied address are shown.
.\"
.Pp
.It Ic show Cm inpcb Ar addr
Show information on IP Control Block
.Vt struct in_pcb
present at
.Ar addr .
.\"
.Pp
.It Ic show Cm intr
Dump information about interrupt handlers.
.\"
.Pp
.It Ic show Cm intrcnt
Dump the interrupt statistics.
.\"
.Pp
.It Ic show Cm irqs
Show interrupt lines and their respective kernel threads.
.\"
.Pp
.It Ic show Cm jails
Show the list of
.Xr jail 8
instances.
In addition to what
.Xr jls 8
shows, also list kernel internal details.
.\"
.Pp
.It Ic show Cm lapic
Show information from the local APIC registers for this CPU.
.\"
.Pp
.It Ic show Cm lock Ar addr
Show lock structure.
The output format is as follows:
.Bl -tag -width "flags"
.It Ic class:
Class of the lock.
Possible types include
.Xr mutex 9 ,
.Xr rmlock 9 ,
.Xr rwlock 9 ,
.Xr sx 9 .
.It Ic name:
Name of the lock.
.It Ic flags:
Flags passed to the lock initialization function.
For exact possibilities see manual pages of possible lock types.
.It Ic state:
Current state of a lock.
As well as
.Ic flags
it's lock-specific.
.It Ic owner:
Lock owner.
.El
.\"
.Pp
.It Ic show Cm lockchain Ar addr
Show all threads a particular thread at address
.Ar addr
is waiting on based on non-sleepable and non-spin locks.
.\"
.Pp
.It Ic show Cm lockedbufs
Show the same information as "show buf", but for every locked
.Vt struct buf
object.
.\"
.Pp
.It Ic show Cm lockedvnods
List all locked vnodes in the system.
.\"
.Pp
.It Ic show Cm locks
Prints all locks that are currently acquired.
This command is only available if
.Xr witness 4
is included in the kernel.
.\"
.Pp
.It Ic show Cm locktree
.\"
.Pp
.It Ic show Cm malloc
Prints
.Xr malloc 9
memory allocator statistics.
The output format is as follows:
.Pp
.Bl -tag -compact -offset indent -width "Requests"
.It Ic Type
Specifies a type of memory.
It is the same as a description string used while defining the
given memory type with
.Xr MALLOC_DECLARE 9 .
.It Ic InUse
Number of memory allocations of the given type, for which
.Xr free 9
has not been called yet.
.It Ic MemUse
Total memory consumed by the given allocation type.
.It Ic Requests
Number of memory allocation requests for the given
memory type.
.El
.Pp
The same information can be gathered in userspace with
.Dq Nm vmstat Fl m .
.\"
.Pp
.It Ic show Cm map Ns Oo Li / Ns Cm f Oc Ar addr
Prints the VM map at
.Ar addr .
If the
.Cm f
modifier is specified the
complete map is printed.
.\"
.Pp
.It Ic show Cm msgbuf
Print the system's message buffer.
It is the same output as in the
.Dq Nm dmesg
case.
It is useful if you got a kernel panic, attached a serial cable
to the machine and want to get the boot messages from before the
system hang.
.\"
.It Ic show Cm mount
Displays short info about all currently mounted file systems.
.Pp
.It Ic show Cm mount Ar addr
Displays details about the given mount point.
.\"
.Pp
.It Ic show Cm object Ns Oo Li / Ns Cm f Oc Ar addr
Prints the VM object at
.Ar addr .
If the
.Cm f
option is specified the
complete object is printed.
.\"
.Pp
.It Ic show Cm page
Show statistics on VM pages.
.\"
.Pp
.It Ic show Cm pageq
Show statistics on VM page queues.
.\"
.Pp
.It Ic show Cm pciregs
Print PCI bus registers.
The same information can be gathered in userspace by running
.Dq Nm pciconf Fl lv .
.\"
.Pp
.It Ic show Cm pcpu
Print current processor state.
The output format is as follows:
.Pp
.Bl -tag -compact -offset indent -width "spin locks held:"
.It Ic cpuid
Processor identifier.
.It Ic curthread
Thread pointer, process identifier and the name of the process.
.It Ic curpcb
Control block pointer.
.It Ic fpcurthread
FPU thread pointer.
.It Ic idlethread
Idle thread pointer.
.It Ic APIC ID
CPU identifier coming from APIC.
.It Ic currentldt
LDT pointer.
.It Ic spin locks held
Names of spin locks held.
.El
.\"
.Pp
.It Ic show Cm pgrpdump
Dump process groups present within the system.
.\"
.Pp
.It Ic show Cm proc Op Ar addr
If no
.Op Ar addr
is specified, print information about the current process.
Otherwise, show information about the process at address
.Ar addr .
.\"
.Pp
.It Ic show Cm procvm
Show process virtual memory layout.
.\"
.Pp
.It Ic show Cm protosw Ar addr
Print protocol switch structure
.Vt struct protosw
at address
.Ar addr .
.\"
.Pp
.It Ic show Cm registers Ns Op Li / Ns Cm u
Display the register set.
If the
.Cm u
modifier is specified, it displays user registers instead of
kernel registers or the currently saved one.
.Pp
.Sy Warning :
The support of the
.Cm u
modifier depends on the machine.
If not supported, incorrect information will be displayed.
.\"
.Pp
.It Ic show Cm rman Ar addr
Show resource manager object
.Vt struct rman
at address
.Ar addr .
Addresses of particular pointers can be gathered with "show allrman"
command.
.\"
.Pp
.It Ic show Cm rtc
Show real time clock value.
Useful for long debugging sessions.
.\"
.Pp
.It Ic show Cm sleepchain
Show all the threads a particular thread is waiting on based on
sleepable locks.
.\"
.Pp
.It Ic show Cm sleepq
.It Ic show Cm sleepqueue
Both commands provide the same functionality.
They show sleepqueue
.Vt struct sleepqueue
structure.
Sleepqueues are used within the
.Fx
kernel to implement sleepable
synchronization primitives (thread holding a lock might sleep or
be context switched), which at the time of writing are:
.Xr condvar 9 ,
.Xr sx 9
and standard
.Xr msleep 9
interface.
.\"
.Pp
.It Ic show Cm sockbuf Ar addr
.It Ic show Cm socket Ar addr
Those commands print
.Vt struct sockbuf
and
.Vt struct socket
objects placed at
.Ar addr .
Output consists of all values present in structures mentioned.
For exact interpretation and more details, visit
.Pa sys/socket.h
header file.
.\"
.Pp
.It Ic show Cm sysregs
Show system registers (e.g.,
.Li cr0-4
on i386.)
Not present on some platforms.
.\"
.Pp
.It Ic show Cm tcpcb Ar addr
Print TCP control block
.Vt struct tcpcb
lying at address
.Ar addr .
For exact interpretation of output, visit
.Pa netinet/tcp.h
header file.
.\"
.Pp
.It Ic show Cm thread Op Ar addr
If no
.Ar addr
is specified, show detailed information about current thread.
Otherwise, information about thread at
.Ar addr
is printed.
.\"
.Pp
.It Ic show Cm threads
Show all threads within the system.
Output format is as follows:
.Pp
.Bl -tag -compact -offset indent -width "Second column"
.It Ic First column
Thread identifier (TID)
.It Ic Second column
Thread structure address
.It Ic Third column
Backtrace.
.El
.\"
.Pp
.It Ic show Cm tty Ar addr
Display the contents of a TTY structure in a readable form.
.\"
.Pp
.It Ic show Cm turnstile Ar addr
Show turnstile
.Vt struct turnstile
structure at address
.Ar addr .
Turnstiles are structures used within the
.Fx
kernel to implement
synchronization primitives which, while holding a specific type of lock, cannot
sleep or context switch to another thread.
Currently, those are:
.Xr mutex 9 ,
.Xr rwlock 9 ,
.Xr rmlock 9 .
.\"
.Pp
.It Ic show Cm uma
Show UMA allocator statistics.
Output consists five columns:
.Pp
.Bl -tag -compact -offset indent -width "Requests"
.It Cm "Zone"
Name of the UMA zone.
The same string that was passed to
.Xr uma_zcreate 9
as a first argument.
.It Cm "Size"
Size of a given memory object (slab).
.It Cm "Used"
Number of slabs being currently used.
.It Cm "Free"
Number of free slabs within the UMA zone.
.It Cm "Requests"
Number of allocations requests to the given zone.
.El
.Pp
The very same information might be gathered in the userspace
with the help of
.Dq Nm vmstat Fl z .
.\"
.Pp
.It Ic show Cm unpcb Ar addr
Shows UNIX domain socket private control block
.Vt struct unpcb
present at the address
.Ar addr .
.\"
.Pp
.It Ic show Cm vmochk
Prints, whether the internal VM objects are in a map somewhere
and none have zero ref counts.
.\"
.Pp
.It Ic show Cm vmopag
This is supposed to show physical addresses consumed by a
VM object.
Currently, it is not possible to use this command when
.Xr witness 4
is compiled in the kernel.
.\"
.Pp
.It Ic show Cm vnode Op Ar addr
Prints vnode
.Vt struct vnode
structure lying at
.Op Ar addr .
For the exact interpretation of the output, look at the
.Pa sys/vnode.h
header file.
.\"
.Pp
.It Ic show Cm vnodebufs Ar addr
Shows clean/dirty buffer lists of the vnode located at
.Ar addr .
.\"
.Pp
.It Ic show Cm watches
Displays all watchpoints.
Shows watchpoints set with "watch" command.
.\"
.Pp
.It Ic show Cm witness
Shows information about lock acquisition coming from the
.Xr witness 4
subsystem.
.\"
.Pp
.It Ic gdb
Toggles between remote GDB and DDB mode.
In remote GDB mode, another machine is required that runs
.Xr 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
architecture.
.Pp
.It Ic halt
Halt the system.
.Pp
.It Ic kill Ar sig pid
Send signal
.Ar sig
to process
.Ar pid .
The signal is acted on upon returning from the debugger.
This command can be used to kill a process causing resource contention
in the case of a hung system.
See
.Xr signal 3
for a list of signals.
Note that the arguments are reversed relative to
.Xr kill 2 .
.Pp
.It Ic reboot Op Ar seconds
.It Ic reset Op Ar seconds
Hard reset the system.
If the optional argument
.Ar seconds
is given, the debugger will wait for this long, at most a week,
before rebooting.
.Pp
.It Ic help
Print a short summary of the available commands and command
abbreviations.
.Pp
.It Ic capture on
.It Ic capture off
.It Ic capture reset
.It Ic capture status
.Nm
supports a basic output capture facility, which can be used to retrieve the
results of debugging commands from userspace using
.Xr sysctl 2 .
.Ic capture on
enables output capture;
.Ic capture off
disables capture.
.Ic capture reset
will clear the capture buffer and disable capture.
.Ic capture status
will report current buffer use, buffer size, and disposition of output
capture.
.Pp
Userspace processes may inspect and manage
.Nm
capture state using
.Xr sysctl 8 :
.Pp
.Dv debug.ddb.capture.bufsize
may be used to query or set the current capture buffer size.
.Pp
.Dv debug.ddb.capture.maxbufsize
may be used to query the compile-time limit on the capture buffer size.
.Pp
.Dv debug.ddb.capture.bytes
may be used to query the number of bytes of output currently in the capture
buffer.
.Pp
.Dv debug.ddb.capture.data
returns the contents of the buffer as a string to an appropriately privileged
process.
.Pp
This facility is particularly useful in concert with the scripting and
.Xr textdump 4
facilities, allowing scripted debugging output to be captured and
committed to disk as part of a textdump for later analysis.
The contents of the capture buffer may also be inspected in a kernel core dump
using
.Xr kgdb 1 .
.Pp
.It Ic run
.It Ic script
.It Ic scripts
.It Ic unscript
Run, define, list, and delete scripts.
See the
.Sx SCRIPTING
section for more information on the scripting facility.
.Pp
.It Ic textdump dump
.It Ic textdump set
.It Ic textdump status
.It Ic textdump unset
Use the
.Ic textdump dump
command to immediately perform a textdump.
More information may be found in
.Xr textdump 4 .
The
.Ic textdump set
command may be used to force the next kernel core dump to be a textdump
rather than a traditional memory dump or minidump.
.Ic textdump status
reports whether a textdump has been scheduled.
.Ic textdump unset
cancels a request to perform a textdump as the next kernel core dump.
.El
.Sh VARIABLES
The debugger accesses registers and variables as
.Li $ Ns Ar name .
Register names are as in the
.Dq Ic show Cm 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
.Cm u
modifier to indicate user register (e.g.,
.Dq Li $eax:u ) .
.Pp
Built-in variables currently supported are:
.Pp
.Bl -tag -width ".Va tabstops" -compact
.It Va radix
Input and output radix.
.It Va maxoff
Addresses are printed as
.Dq Ar symbol Ns Li + Ns Ar offset
unless
.Ar offset
is greater than
.Va maxoff .
.It Va maxwidth
The width of the displayed line.
.It Va lines
The number of lines.
It is used by the built-in pager.
.It Va tabstops
Tab stop width.
.It Va work Ns Ar xx
Work variable;
.Ar xx
can take values from 0 to 31.
.El
.Sh EXPRESSIONS
Most expression operators in C are supported except
.Ql ~ ,
.Ql ^ ,
and unary
.Ql & .
Special rules in
.Nm
are:
.Bl -tag -width ".No Identifiers"
.It Identifiers
The name of a symbol is translated to the value of the symbol, which
is the address of the corresponding object.
.Ql \&.
and
.Ql \&:
can be used in the identifier.
If supported by an object format dependent routine,
.Sm off
.Oo Ar filename : Oc Ar func : lineno ,
.Sm on
.Oo Ar filename : Oc Ns Ar variable ,
and
.Oo Ar filename : Oc Ns Ar lineno
can be accepted as a symbol.
.It Numbers
Radix is determined by the first two letters:
.Ql 0x :
hex,
.Ql 0o :
octal,
.Ql 0t :
decimal; otherwise, follow current radix.
.It Li \&.
.Va dot
.It Li +
.Va next
.It Li ..
address of the start of the last line examined.
Unlike
.Va dot
or
.Va next ,
this is only changed by
.Ic examine
or
.Ic write
command.
.It Li '
last address explicitly specified.
.It Li $ Ns Ar variable
Translated to the value of the specified variable.
It may be followed by a
.Ql \&:
and modifiers as described above.
.It Ar a Ns Li # Ns Ar b
A binary operator which rounds up the left hand side to the next
multiple of right hand side.
.It Li * Ns Ar expr
Indirection.
It may be followed by a
.Ql \&:
and modifiers as described above.
.El
.Sh SCRIPTING
.Nm
supports a basic scripting facility to allow automating tasks or responses to
specific events.
Each script consists of a list of DDB commands to be executed sequentially,
and is assigned a unique name.
Certain script names have special meaning, and will be automatically run on
various
.Nm
events if scripts by those names have been defined.
.Pp
The
.Ic script
command may be used to define a script by name.
Scripts consist of a series of
.Nm
commands separated with the
.Ql \&;
character.
For example:
.Bd -literal -offset indent
script kdb.enter.panic=bt; show pcpu
script lockinfo=show alllocks; show lockedvnods
.Ed
.Pp
The
.Ic scripts
command lists currently defined scripts.
.Pp
The
.Ic run
command execute a script by name.
For example:
.Bd -literal -offset indent
run lockinfo
.Ed
.Pp
The
.Ic unscript
command may be used to delete a script by name.
For example:
.Bd -literal -offset indent
unscript kdb.enter.panic
.Ed
.Pp
These functions may also be performed from userspace using the
.Xr ddb 8
command.
.Pp
Certain scripts are run automatically, if defined, for specific
.Nm
events.
The follow scripts are run when various events occur:
.Bl -tag -width kdb.enter.powerfail
.It Dv kdb.enter.acpi
The kernel debugger was entered as a result of an
.Xr acpi 4
event.
.It Dv kdb.enter.bootflags
The kernel debugger was entered at boot as a result of the debugger boot
flag being set.
.It Dv kdb.enter.break
The kernel debugger was entered as a result of a serial or console break.
.It Dv kdb.enter.cam
The kernel debugger was entered as a result of a
.Xr CAM 4
event.
.It Dv kdb.enter.mac
The kernel debugger was entered as a result of an assertion failure in the
.Xr mac_test 4
module of the
TrustedBSD MAC Framework.
.It Dv kdb.enter.ndis
The kernel debugger was entered as a result of an
.Xr ndis 4
breakpoint event.
.It Dv kdb.enter.netgraph
The kernel debugger was entered as a result of a
.Xr netgraph 4
event.
.It Dv kdb.enter.panic
.Xr panic 9
was called.
.It Dv kdb.enter.powerfail
The kernel debugger was entered as a result of a powerfail NMI on the sparc64
platform.
.It Dv kdb.enter.powerpc
The kernel debugger was entered as a result of an unimplemented interrupt
type on the powerpc platform.
.It Dv kdb.enter.sysctl
The kernel debugger was entered as a result of the
.Dv debug.kdb.enter
sysctl being set.
.It Dv kdb.enter.trapsig
The kernel debugger was entered as a result of a trapsig event on the sparc64
platform.
.It Dv kdb.enter.unionfs
The kernel debugger was entered as a result of an assertion failure in the
union file system.
.It Dv kdb.enter.unknown
The kernel debugger was entered, but no reason has been set.
.It Dv kdb.enter.vfslock
The kernel debugger was entered as a result of a VFS lock violation.
.It Dv kdb.enter.watchdog
The kernel debugger was entered as a result of a watchdog firing.
.It Dv kdb.enter.witness
The kernel debugger was entered as a result of a
.Xr witness 4
violation.
.El
.Pp
In the event that none of these scripts is found,
.Nm
will attempt to execute a default script:
.Bl -tag -width kdb.enter.powerfail
.It Dv kdb.enter.default
The kernel debugger was entered, but a script exactly matching the reason for
entering was not defined.
This can be used as a catch-all to handle cases not specifically of interest;
for example,
.Dv kdb.enter.witness
might be defined to have special handling, and
.Dv kdb.enter.default
might be defined to simply panic and reboot.
.El
.Sh HINTS
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
.Nm .
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.
There are many PCI and PCIe add-in cards which can generate NMI for
debugging.
Modern server systems typically use IPMI to generate signals to enter the
debugger.
The
.Dv devel/ipmitool
port can be used to send the
.Cd chassis power diag
command which delivers an NMI to the processor.
Embedded systems often use JTAG for debugging, but rarely use it in
combination with
.Nm .
.Pp
For serial consoles, you can enter the debugger by sending a BREAK
condition on the serial line if
.Cd options BREAK_TO_DEBUGGER
is specified in the kernel.
Most terminal emulation programs can send a break sequence with a
special key sequence or via a menu item.
However, in some setups, sending the break can be difficult to arrange
or happens spuriously, so if the kernel contains
.Cd options ALT_BREAK_TO_DEBUGGER
then the sequence of CR TILDE CTRL-B enters the debugger;
CR TILDE CTRL-P causes a panic instead of entering the
debugger; and
CR TILDE CTRL-R causes an immediate reboot.
In all the above sequences, CR is a Carriage Return and is usually
sent by hitting the Enter or Return key.
TILDE is the ASCII tilde character (~).
CTRL-x is Control x created by hitting the control key and then x
and then releasing both.
.Pp
The break to enter the debugger behavior may be enabled at run-time
by setting the
.Xr sysctl 8
.Dv debug.kdb.break_to_debugger
to 1.
The alternate sequence to enter the debugger behavior may be enabled
at run-time by setting the
.Xr sysctl 8
.Dv debug.kdb.alt_break_to_debugger
to 1.
The debugger may be entered by setting the
.Xr sysctl 8
.Dv debug.kdb.enter
to 1.
.Sh FILES
Header files mentioned in this manual page can be found below
.Pa /usr/include
directory.
.Pp
.Bl -dash -compact
.It
.Pa sys/buf.h
.It
.Pa sys/domain.h
.It
.Pa netinet/in_pcb.h
.It
.Pa sys/socket.h
.It
.Pa sys/vnode.h
.El
.Sh SEE ALSO
.Xr gdb 1 ,
.Xr kgdb 1 ,
.Xr acpi 4 ,
.Xr CAM 4 ,
.Xr mac_test 4 ,
.Xr ndis 4 ,
.Xr netgraph 4 ,
.Xr textdump 4 ,
.Xr witness 4 ,
.Xr ddb 8 ,
.Xr sysctl 8 ,
.Xr panic 9
.Sh HISTORY
The
.Nm
debugger was developed for Mach, and ported to
.Bx 386 0.1 .
This manual page translated from
.Xr man 7
macros by
.An Garrett Wollman .
.Pp
.An Robert N. M. Watson
added support for
.Nm
output capture,
.Xr textdump 4
and scripting in
.Fx 7.1 .