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DDB(4)		       FreeBSD Kernel Interfaces Manual			DDB(4)

     ddb -- interactive	kernel debugger

     In	order to enable	kernel debugging facilities include:

	   options KDB
	   options DDB

     To	prevent	activation of the debugger on kernel panic(9):

	   options KDB_UNATTENDED

     In	order to print a stack trace of	the current thread on the console for
     a panic:

	   options KDB_TRACE

     To	print the numerical value of symbols in	addition to the	symbolic rep-
     resentation, define:

	   options DDB_NUMSYM

     To	enable the gdb(1) backend, so that remote debugging with kgdb(1) is
     possible, include:

	   options GDB

     The ddb kernel debugger is	an interactive debugger	with a 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 ker-
     nel 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	speci-

     The current location is called dot.  The dot is displayed with a hexadec-
     imal format at a prompt.  The commands examine and	write 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 pager feature (like	the more(1) command) for the
     output.  If an output line	exceeds	the number set in the lines variable,
     it	displays ``--More--'' and waits	for a response.	 The valid responses
     for it are:

     SPC  one more page
     RET  one more line
     q	  abort	the current command, and return	to the 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 may be used to
     browse through the	history	buffer,	and move the cursor within the current

     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.

	     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)
	     g	     look at by	quad words (64 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 charac-
		     ters are displayed	as an octal escape code	(e.g.,
	     s	     display the null-terminated string	at the location.  Non-
		     printing characters are displayed as octal	escapes.
	     m	     display in	unsigned hex with character dump at the	end of
		     each line.	 The location is also displayed	in hex at the
		     beginning of each line.
	     i	     display as	an instruction
	     I	     display as	an instruction with possible alternate formats
		     depending on the machine, but none	of the supported
		     architectures have	an alternate format.
	     S	     display a symbol name for the pointer stored at the

     xf	     Examine forward: execute an examine command with the last speci-
	     fied parameters to	it except that the next	address	displayed by
	     it	is used	as the start address.

     xb	     Examine backward: execute an examine command with the last	speci-
	     fied parameters to	it except that the last	start address sub-
	     tracted 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.  The	argument 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 ...]
     w[/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

	     Warning: since there is no	delimiter between expressions, strange
	     things may	happen.	 It is best to enclose each expression in

     set $variable [=] expr
	     Set the named variable or register	with the value of expr.	 Valid
	     variable names are	described below.

     b[/u]   Set a break point at addr.	 If count is supplied, continues count
	     - 1 times before stopping at the break point.  If the break point
	     is	set, a break point number is printed with `#'.	This number
	     can be used in deleting the break point or	adding conditions to

	     If	the u modifier is specified, this command sets a break point
	     in	user address space.  Without the u option, the address is con-
	     sidered 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.

	     Warning: If a user	text is	shadowed by a normal user space	debug-
	     ger, user space break points may not work correctly.  Setting a
	     break point at the	low-level code paths may also cause strange

     delete addr
     d addr
     delete #number
     d #number
	     Delete the	break point.  The target break point can be specified
	     by	a break	point number with `#', or by using the same addr spec-
	     ified in the original break command.

     watch addr,size
	     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 unrecov-
	     erable error in some systems such as i386.	 Watchpoints on	user
	     addresses work best.

     hwatch addr,size
	     Set a hardware watchpoint for a region if supported by the	archi-
	     tecture.  Execution stops when an attempt to modify the region
	     occurs.  The size argument	defaults to 4.

	     Warning: The hardware debug facilities do not have	a concept of
	     separate address spaces like the watch command does.  Use hwatch
	     for setting watchpoints on	kernel address locations only, and
	     avoid its use on user mode	address	spaces.

     dhwatch addr,size
	     Delete specified hardware watchpoint.

     s[/p]   Single step count times (the comma	is a mandatory part of the
	     syntax).  If the p	modifier is specified, print each instruction
	     at	each step.  Otherwise, only print the last instruction.

	     Warning: depending	on machine type, it may	not be possible	to
	     single-step through some low-level	code paths or user space code.
	     On	machines with software-emulated	single-stepping	(e.g., pmax),
	     stepping through code executed by interrupt handlers will proba-
	     bly do the	wrong thing.

     c[/c]   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 behavior.

	     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] [pid | tid] [,count]
     t[/u] [pid	| tid] [,count]
     where[/u] [pid | tid] [,count]
     bt[/u] [pid | tid]	[,count]
	     Stack trace.  The u option	traces user space; if omitted, trace
	     only traces kernel	space.	The optional argument 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.

     findstack addr
	     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.

     show all procs[/m]
     ps[/m]  Display all process information.  The process information may not
	     be	shown if it is not supported in	the machine, or	the bottom of
	     the stack of the target process is	not in the main	memory at that
	     time.  The	m modifier will	alter the display to show VM map
	     addresses for the process and not show other information.

     show all ttys
	     Show all TTY's within the system.	Output is similar to pstat(8),
	     but also includes the address of the TTY structure.

     show all vnets
	     Show the same output as "show vnet" does, but lists all virtual-
	     ized network stacks within	the system.

     show allchains
	     Show the same information like "show lockchain" does, but for
	     every thread in the system.

     show alllocks
	     Show all locks that are currently held.  This command is only
	     available if witness(4) is	included in the	kernel.

     show allpcpu
	     The same as "show pcpu", but for every CPU	present	in the system.

     show allrman
	     Show information related with resource management,	including
	     interrupt request lines, DMA request lines, I/O ports, I/O	memory
	     addresses,	and Resource IDs.

     show apic
	     Dump data about APIC IDT vector mappings.

     show breaks
	     Show breakpoints set with the "break" command.

     show bio addr
	     Show information about the	bio structure struct bio present at
	     addr.  See	the sys/bio.h header file and g_bio(9) for more
	     details on	the exact meaning of the structure fields.

     show buffer addr
	     Show information about the	buf structure struct buf present at
	     addr.  See	the sys/buf.h header file for more details on the
	     exact meaning of the structure fields.

     show callout addr
	     Show information about the	callout	structure struct callout
	     present at	addr.

     show cbstat
	     Show brief	information about the TTY subsystem.

     show 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.

     show conifhk
	     Lists hooks currently waiting for completion in run_inter-

     show cpusets
	     Print numbered root and assigned CPU affinity sets.  See
	     cpuset(2) for more	details.

     show cyrixreg
	     Show registers specific to	the Cyrix processor.

     show devmap
	     Prints the	contents of the	static device mapping table.  Cur-
	     rently only available on the ARM architecture.

     show domain addr
	     Print protocol domain structure struct domain at address addr.
	     See the sys/domain.h header file for more details on the exact
	     meaning of	the structure fields.

     show ffs [addr]
	     Show brief	information about ffs mount at the address addr, if
	     argument is given.	 Otherwise, provides the summary about each
	     ffs mount.

     show file addr
	     Show information about the	file structure struct file present at
	     address addr.

     show files
	     Show information about every file structure in the	system.

     show freepages
	     Show the number of	physical pages in each of the free lists.

     show geom [addr]
	     If	the addr argument is not given,	displays the entire GEOM
	     topology.	If addr	is given, displays details about the given
	     GEOM object (class, geom, provider	or consumer).

     show 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.

     show igi_list addr
	     Show information about the	IGMP structure struct igmp_ifsoftc
	     present at	addr.

     show inodedeps [addr]
	     Show brief	information about each inodedep	structure.  If addr is
	     given, only inodedeps belonging to	the fs located at the supplied
	     address are shown.

     show inpcb	addr
	     Show information on IP Control Block struct in_pcb	present	at

     show intr
	     Dump information about interrupt handlers.

     show intrcnt
	     Dump the interrupt	statistics.

     show irqs
	     Show interrupt lines and their respective kernel threads.

     show jails
	     Show the list of jail(8) instances.  In addition to what jls(8)
	     shows, also list kernel internal details.

     show lapic
	     Show information from the local APIC registers for	this CPU.

     show lock addr
	     Show lock structure.  The output format is	as follows:

		    Class of the lock.	Possible types include mutex(9),
		    rmlock(9), rwlock(9), sx(9).

	     name:  Name of the	lock.

		    Flags passed to the	lock initialization function.  For
		    exact possibilities	see manual pages of possible lock

		    Current state of a lock.  As well as flags it's lock-spe-

		    Lock owner.

     show lockchain addr
	     Show all threads a	particular thread at address addr is waiting
	     on	based on non-sleepable and non-spin locks.

     show lockedbufs
	     Show the same information as "show	buf", but for every locked
	     struct buf	object.

     show lockedvnods
	     List all locked vnodes in the system.

     show locks
	     Prints all	locks that are currently acquired.  This command is
	     only available if witness(4) is included in the kernel.

     show locktree

     show malloc
	     Prints malloc(9) memory allocator statistics.  The	output format
	     is	as follows:

		   Type	     Specifies a type of memory.  It is	the same as a
			     description string	used while defining the	given
			     memory type with MALLOC_DECLARE(9).
		   InUse     Number of memory allocations of the given type,
			     for which free(9) has not been called yet.
		   MemUse    Total memory consumed by the given	allocation
		   Requests  Number of memory allocation requests for the
			     given memory type.

	     The same information can be gathered in userspace with ``vmstat

     show map[/f] addr
	     Prints the	VM map at addr.	 If the	f modifier is specified	the
	     complete map is printed.

     show msgbuf
	     Print the system's	message	buffer.	 It is the same	output as in
	     the ``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.
     show mount
	     Displays short info about all currently mounted file systems.

     show mount	addr
	     Displays details about the	given mount point.

     show object[/f] addr
	     Prints the	VM object at addr.  If the f option is specified the
	     complete object is	printed.

     show panic
	     Print the panic message if	set.

     show page
	     Show statistics on	VM pages.

     show pageq
	     Show statistics on	VM page	queues.

     show pciregs
	     Print PCI bus registers.  The same	information can	be gathered in
	     userspace by running ``pciconf -lv''.

     show pcpu
	     Print current processor state.  The output	format is as follows:

		   cpuid	     Processor identifier.
		   curthread	     Thread pointer, process identifier	and
				     the name of the process.
		   curpcb	     Control block pointer.
		   fpcurthread	     FPU thread	pointer.
		   idlethread	     Idle thread pointer.
		   APIC	ID	     CPU identifier coming from	APIC.
		   currentldt	     LDT pointer.
		   spin	locks held   Names of spin locks held.

     show pgrpdump
	     Dump process groups present within	the system.

     show proc [addr]
	     If	no [addr] is specified,	print information about	the current
	     process.  Otherwise, show information about the process at
	     address addr.

     show procvm
	     Show process virtual memory layout.

     show protosw addr
	     Print protocol switch structure struct protosw at address addr.

     show registers[/u]
	     Display the register set.	If the u modifier is specified,	it
	     displays user registers instead of	kernel registers or the	cur-
	     rently saved one.

	     Warning: The support of the u modifier depends on the machine.
	     If	not supported, incorrect information will be displayed.

     show rman addr
	     Show resource manager object struct rman at address addr.
	     Addresses of particular pointers can be gathered with "show allr-
	     man" command.

     show rtc
	     Show real time clock value.  Useful for long debugging sessions.

     show sleepchain
	     Show all the threads a particular thread is waiting on based on
	     sleepable locks.

     show sleepq
     show sleepqueue
	     Both commands provide the same functionality.  They show sleep-
	     queue struct sleepqueue structure.	 Sleepqueues are used within
	     the FreeBSD kernel	to implement sleepable synchronization primi-
	     tives (thread holding a lock might	sleep or be context switched),
	     which at the time of writing are: condvar(9), sx(9) and standard
	     msleep(9) interface.

     show sockbuf addr
     show socket addr
	     Those commands print struct sockbuf and struct socket objects
	     placed at addr.  Output consists of all values present in struc-
	     tures mentioned.  For exact interpretation	and more details,
	     visit sys/socket.h	header file.

     show sysregs
	     Show system registers (e.g., cr0-4	on i386.)  Not present on some

     show tcpcb	addr
	     Print TCP control block struct tcpcb lying	at address addr.  For
	     exact interpretation of output, visit netinet/tcp.h header	file.

     show thread [addr]
	     If	no addr	is specified, show detailed information	about current
	     thread.  Otherwise, information about thread at addr is printed.

     show threads
	     Show all threads within the system.  Output format	is as follows:

		   First column	  Thread identifier (TID)
		   Second column  Thread structure address
		   Third column	  Backtrace.

     show tty addr
	     Display the contents of a TTY structure in	a readable form.

     show turnstile addr
	     Show turnstile struct turnstile structure at address addr.	 Turn-
	     stiles are	structures used	within the FreeBSD kernel to implement
	     synchronization primitives	which, while holding a specific	type
	     of	lock, cannot sleep or context switch to	another	thread.	 Cur-
	     rently, those are:	mutex(9), rwlock(9), rmlock(9).

     show uma
	     Show UMA allocator	statistics.  Output consists five columns:

		   Zone	     Name of the UMA zone.  The	same string that was
			     passed to uma_zcreate(9) as a first argument.
		   Size	     Size of a given memory object (slab).
		   Used	     Number of slabs being currently used.
		   Free	     Number of free slabs within the UMA zone.
		   Requests  Number of allocations requests to the given zone.

	     The very same information might be	gathered in the	userspace with
	     the help of ``vmstat -z''.

     show unpcb	addr
	     Shows UNIX	domain socket private control block struct unpcb
	     present at	the address addr.

     show vmochk
	     Prints, whether the internal VM objects are in a map somewhere
	     and none have zero	ref counts.

     show vmopag
	     This is supposed to show physical addresses consumed by a VM
	     object.  Currently, it is not possible to use this	command	when
	     witness(4)	is compiled in the kernel.

     show vnet addr
	     Prints virtualized	network	stack struct vnet structure present at
	     the address addr.

     show vnode	[addr]
	     Prints vnode struct vnode structure lying at [addr].  For the
	     exact interpretation of the output, look at the sys/vnode.h
	     header file.

     show vnodebufs addr
	     Shows clean/dirty buffer lists of the vnode located at addr.

     show watches
	     Displays all watchpoints.	Shows watchpoints set with "watch"

     show witness
	     Shows information about lock acquisition coming from the
	     witness(4)	subsystem.

     gdb     Toggles between remote GDB	and DDB	mode.  In remote GDB mode,
	     another machine is	required that runs gdb(1) using	the remote
	     debug feature, with a connection to the serial console port on
	     the target	machine.  Currently only available on the i386 archi-

     halt    Halt the system.

     kill sig pid
	     Send signal sig to	process	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 signal(3) for a list of signals.  Note	that the arguments are
	     reversed relative to kill(2).

     reboot [seconds]
     reset [seconds]
	     Hard reset	the system.  If	the optional argument seconds is
	     given, the	debugger will wait for this long, at most a week,
	     before rebooting.

     help    Print a short summary of the available commands and command

     capture on
     capture off
     capture reset
     capture status
	     ddb supports a basic output capture facility, which can be	used
	     to	retrieve the results of	debugging commands from	userspace
	     using sysctl(3).  capture on enables output capture; capture off
	     disables capture.	capture	reset will clear the capture buffer
	     and disable capture.  capture status will report current buffer
	     use, buffer size, and disposition of output capture.

	     Userspace processes may inspect and manage	ddb capture state
	     using sysctl(8):

	     debug.ddb.capture.bufsize may be used to query or set the current
	     capture buffer size.

	     debug.ddb.capture.maxbufsize may be used to query the compile-
	     time limit	on the capture buffer size.

	     debug.ddb.capture.bytes may be used to query the number of	bytes
	     of	output currently in the	capture	buffer. returns the	contents of the	buffer as a
	     string to an appropriately	privileged process.

	     This facility is particularly useful in concert with the script-
	     ing and textdump(4) facilities, allowing scripted debugging out-
	     put 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 kgdb(1).

	     Run, define, list,	and delete scripts.  See the SCRIPTING section
	     for more information on the scripting facility.

     textdump dump
     textdump set
     textdump status
     textdump unset
	     Use the textdump dump command to immediately perform a textdump.
	     More information may be found in textdump(4).  The	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.
	     textdump status reports whether a textdump	has been scheduled.
	     textdump unset cancels a request to perform a textdump as the
	     next kernel core dump.

     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 the built-in	pager.
     tabstops  Tab stop	width.
     workxx    Work variable; xx can take values from 0	to 31.

     Most 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 sym-
		  bol, 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.

     .		  dot

     +		  next

     ..		  address of the start of the last line	examined.  Unlike dot
		  or next, this	is only	changed	by examine or write command.

     '		  last address explicitly specified.

     $variable	  Translated to	the value of the specified variable.  It may
		  be followed by a `:' and modifiers as	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
		  described above.

     ddb supports a basic scripting facility to	allow automating tasks or
     responses to specific events.  Each script	consists of a list of DDB com-
     mands to be executed sequentially,	and is assigned	a unique name.	Cer-
     tain script names have special meaning, and will be automatically run on
     various ddb events	if scripts by those names have been defined.

     The script	command	may be used to define a	script by name.	 Scripts con-
     sist of a series of ddb commands separated	with the `;' character.	 For

	   script kdb.enter.panic=bt; show pcpu
	   script lockinfo=show	alllocks; show lockedvnods

     The scripts command lists currently defined scripts.

     The run command execute a script by name.	For example:

	   run lockinfo

     The unscript command may be used to delete	a script by name.  For exam-

	   unscript kdb.enter.panic

     These functions may also be performed from	userspace using	the ddb(8)

     Certain scripts are run automatically, if defined,	for specific ddb
     events.  The follow scripts are run when various events occur:

     kdb.enter.acpi	  The kernel debugger was entered as a result of an
			  acpi(4) event.

     kdb.enter.bootflags  The kernel debugger was entered at boot as a result
			  of the debugger boot flag being set.

     kdb.enter.break	  The kernel debugger was entered as a result of a
			  serial or console break.	  The kernel debugger was entered as a result of a
			  CAM(4) event.

     kdb.enter.mac	  The kernel debugger was entered as a result of an
			  assertion failure in the mac_test(4) module of the
			  TrustedBSD MAC Framework.

     kdb.enter.ndis	  The kernel debugger was entered as a result of an
			  ndis(4) breakpoint event.

     kdb.enter.netgraph	  The kernel debugger was entered as a result of a
			  netgraph(4) event.

     kdb.enter.panic	  panic(9) was called.

     kdb.enter.powerfail  The kernel debugger was entered as a result of a
			  powerfail NMI	on the sparc64 platform.

     kdb.enter.powerpc	  The kernel debugger was entered as a result of an
			  unimplemented	interrupt type on the powerpc plat-

     kdb.enter.sysctl	  The kernel debugger was entered as a result of the
			  debug.kdb.enter sysctl being set.

     kdb.enter.trapsig	  The kernel debugger was entered as a result of a
			  trapsig event	on the sparc64 platform.

     kdb.enter.unionfs	  The kernel debugger was entered as a result of an
			  assertion failure in the union file system.

     kdb.enter.unknown	  The kernel debugger was entered, but no reason has
			  been set.

     kdb.enter.vfslock	  The kernel debugger was entered as a result of a VFS
			  lock violation.

     kdb.enter.watchdog	  The kernel debugger was entered as a result of a
			  watchdog firing.

     kdb.enter.witness	  The kernel debugger was entered as a result of a
			  witness(4) violation.

     In	the event that none of these scripts is	found, ddb will	attempt	to
     execute a default script:

     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,
			  kdb.enter.witness might be defined to	have special
			  handling, and	kdb.enter.default might	be defined to
			  simply panic and reboot.

     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.
     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 devel/ipmitool	port can be used to send the
     chassis power diag	command	which delivers an NMI to the processor.
     Embedded systems often use	JTAG for debugging, but	rarely use it in com-
     bination with ddb.

     For serial	consoles, you can enter	the debugger by	sending	a BREAK	condi-
     tion on the serial	line if	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 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 usu-
     ally 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.

     The break to enter	the debugger behavior may be enabled at	run-time by
     setting the sysctl(8) debug.kdb.break_to_debugger to 1.  The alternate
     sequence to enter the debugger behavior may be enabled at run-time	by
     setting the sysctl(8) debug.kdb.alt_break_to_debugger to 1.  The debugger
     may be entered by setting the sysctl(8) debug.kdb.enter to	1.

     Header files mentioned in this manual page	can be found below
     /usr/include directory.

     -	 sys/buf.h
     -	 sys/domain.h
     -	 netinet/in_pcb.h
     -	 sys/socket.h
     -	 sys/vnode.h

     gdb(1), kgdb(1), acpi(4), CAM(4), mac_test(4), ndis(4), netgraph(4),
     textdump(4), witness(4), ddb(8), sysctl(8), panic(9)

     The ddb debugger was developed for	Mach, and ported to 386BSD 0.1.	 This
     manual page translated from man(7)	macros by Garrett Wollman.

     Robert N. M. Watson added support for ddb output capture, textdump(4) and
     scripting in FreeBSD 7.1.

FreeBSD	11.0			 June 6, 2016			  FreeBSD 11.0


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