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DDB(4)			 BSD Kernel Interfaces Manual			DDB(4)

NAME
     ddb — interactive kernel debugger

SYNOPSIS
     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

DESCRIPTION
     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‐
     fied.

     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
     line.

COMMANDS
     examine
     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)
	     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.,
		     ‘\000’).
	     s	     display the null-terminated string at the location.  Non-
		     printing characters are displayed as octal escapes.
	     m	     display in unsigned hex with character dump at the end of
		     each line.	 The location is also displayed in hex at the
		     beginning of each line.
	     i	     display as an instruction
	     I	     display as an instruction with possible alternate formats
		     depending on the machine:
		     amd64    No alternate format.
		     i386     No alternate format.
		     ia64     No alternate format.
		     powerpc  No alternate format.
		     sparc64  No alternate format.
	     S	     display a symbol name for the pointer stored at the
		     address

     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[/acdoruxz]
     p[/acdoruxz]
	     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
	     assumed.

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

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

     break[/u]
     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
	     it.

	     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
	     behavior.

     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.

     step[/p]
     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.

     continue[/c]
     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.

     until[/p]
	     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.

     next[/p]
     match[/p]
	     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.

     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 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 and I/O
	     memory addresses.

     show apic
	     Dump data about APIC IDT vector mappings.

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

     show buffer
	     Show buffer structure of struct buf type.	Such a structure is
	     used within the FreeBSD kernel for the I/O subsystem implementa‐
	     tion.  For an exact interpretation of the output, please see the
	     sys/buf.h header file.

     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‐
	     rupt_driven_config_hooks().

     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 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 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
	     addr.

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

	     name:  Name of the lock.

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

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

	     owner:
		    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
			     type.
		   Requests  Number of memory allocation requests for the
			     given memory type.

	     The same information can be gathered in userspace with “vmstat
	     -m”.

     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 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
	     platforms.

     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 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"
	     command.

     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‐
	     tecture.

     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
	     abbreviations.

     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 userpsace
	     using sysctl(2).  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.

	     debug.ddb.capture.data 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
     script
     scripts
     unscript
	     Run, define, list, and delete scripts.  See the SCRIPTING section
	     for more information on the scripting facility.

     textdump set
     textdump status
     textdump unset
	     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.  More information may be
	     found in textdump(4).

VARIABLES
     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.

EXPRESSIONS
     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.

SCRIPTING
     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
     example:

	   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‐
     ple:

	   unscript kdb.enter.panic

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

     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.

     kdb.enter.cam	  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‐
			  form.

     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 or sun4v 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.

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 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.

FILES
     Header files mention 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

SEE ALSO
     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)

HISTORY
     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.

BSD			       September 5, 2010			   BSD
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