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

NAME
     ddb -- kernel debugger

DESCRIPTION
     The ddb debugger provides a means for debugging the kernel, and analysing
     the kernel	after a	system crash ("panic"),	with a gdb(1)-like syntax.

     ddb is invoked upon a kernel panic	when the sysctl(8) ddb.panic is	set to
     1.	 It may	be invoked from	the console when the sysctl ddb.console	is set
     to	1, using any of	the following methods:

	-   Using the key sequence Ctrl-Alt-Esc.

	-   Sending a BREAK when using a serial	console.

	-   Writing to the sysctl ddb.trigger.

	-   For	i386 and amd64 architectures, using the	key sequence
	    Ctrl-Alt-Delete when the sysctl machdep.kbdreset is	set to 2.

     ddb prompts for commands on the console with:

	   ddb>

     The general syntax	of a ddb command is:

	   command [/modifiers]	  [address][,count]

     To	save typing, ddb makes use of a	context	inferred from previous com-
     mands.  In	this context, the current location is called dot.  The
     examine, search, show struct, and write commands update dot to be that of
     the last address examined or the last location modified, and have intu-
     itive effects on next and prev.  All the other commands do	not change
     dot, and set next to be the same.	(See VARIABLES.)

     An	expression can be used in place	of address (see	EXPRESSIONS).  Omit-
     ting address in a command uses the	last value of dot.  A missing count is
     taken to be 1 for printing	commands or infinity for stack traces.	Enter-
     ing a blank line causes the last command to be repeated using next	in
     place of address, a count of 1, and no modifiers.

     ddb has a feature like more(1) for	the output.  If	the number of lines
     output in response	to one command exceeds the number set in the $lines
     variable, it displays the message `--db_more--' and waits for a response.

     The valid responses are:

	   <space>     One more	page.
	   <return>    One more	line.
	   q	       Abort the current command, and return to	the command
		       input mode.

     The following command line	editing	keys are provided:

	   ^b	       back one	character
	   ^f	       forward one character
	   ^a	       beginning of line
	   ^e	       end of line
	   ^w	       erase word back
	   ^h |	<del>  erase previous character
	   ^d	       erase next character
	   ^k	       delete to end of	line
	   ^u	       delete line
	   ^p	       previous	in command history
	   ^n	       next in command history
	   ^r	       redraw line
	   ^t	       exchange	the two	characters to the left of the cursor

COMMANDS
     The following commands may	be typed at the	`ddb>' prompt.	Some commands
     consist of	more than one word, and	if only	the first word or words	are
     entered, the possible alternatives	to complete the	command	are displayed
     and no other action is performed.

     help	 List the available commands.

     [e]x[amine] [/bhlqaAxzodurcsmiI] [addr][,count]
		 Display the contents at address addr according	to the formats
		 in the	modifier.  If no format	is specified, the last formats
		 specified for this command are	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) (default)
		 /q    look at by long longs (64 bits) (only available on
		       64-bit platforms)
		 /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	char-
		       acters 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 alternate format instruction depending on
		       the machine:

		       alpha	 Print affected	register contents for every
				 instruction.
		       amd64,
		       i386	 Do not	skip padding to	the next long word
				 boundary for unconditional jumps.
		       m88k	 Decode	instructions for the opposite CPU
				 model (e.g. m88110 when running on an m88100
				 processor).

		 The value of next is set to the addr plus the size of the
		 data examined.

     p[rint] [/axzodurc] [addr]
		 Print addr according to the modifier character.  The valid
		 modifiers are a subset	of those from the examine command, and
		 act as	described there.  If no	modifier is specified, the
		 last one specified in a previous use of print is used.

		 For example,

		       print/x $eax

		 will print something like this:

		       xxxxxx

     pp[rint] [addr]
		 Pretty-print addr using CTF debug symbols included in the
		 kernel	binary image.  The CTF section is normally added by
		 running ctfstrip(1) as	part of	building a new kernel.

     w[rite] [/bhl] [addr] expr	[expr ...]
		 Write the value of each expr expression at succeeding loca-
		 tions start at	addr.  The write unit size can be specified
		 using one of the modifiers:

		       /b    byte (8 bits)
		       /h    half word (16 bits)
		       /l    long word (32 bits) (default)

		 The value of next is set to addr plus the size	of values
		 written.

		 Warning: since	there is no delimiter between expressions, the
		 command may not parse as you expect.  It is best to enclose
		 each expression in parentheses.

     set $name [=] expr
		 Set the named variable	or register with the value of expr.
		 For valid variable names, see VARIABLES.

     boot how	 Reboot	the machine depending on how:

		 boot sync	Sync disks and reboot.
		 boot crash	Dump core and reboot.
		 boot dump	Sync disks, dump core and reboot.
		 boot halt	Just halt.
		 boot reboot	Just reboot.
		 boot poweroff	Power down the machine whenever	possible; if
				it fails, just halt.

     break [addr][,count]
		 Set a break point at addr.  If	count is supplied, ddb allows
		 the breakpoint	to be silently hit (count - 1) times before
		 stopping at the break point.

     d[elete] [addr]
		 Delete	the break point	set with the break command.

     s[tep] [/p] [,count]
		 Single	step count times.  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.	 On machines
		 with software-emulated	single-stepping	(e.g., alpha), step-
		 ping through code executed by interrupt handlers will proba-
		 bly do	the wrong thing.

     call name(expr [, expr ...] )
		 Call the function named by name with the argument(s) listed
		 in parentheses.  Parentheses may be omitted if	the function
		 takes no arguments.  The number of arguments is currently
		 limited to 10.

     c[ontinue]	[/c]
		 Continue execution until a breakpoint or watchpoint.  If the
		 /c modifier is	given, instructions are	counted	while execut-
		 ing.

		 Warning: when counting	with /c, ddb is	really silently	sin-
		 gle-stepping.	This means that	single-stepping	on low-level
		 code may cause	strange	behavior.

     watch addr	[,size]
		 Set a watchpoint for the region starting at addr.  Execution
		 stops and control returns to ddb when an attempt is made to
		 modify	a watched region.  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 an
		 unrecoverable error on	some systems (e.g., i386).

     dwatch addr
		 Delete	the watchpoint at address addr that was	previously set
		 with a	watch command.

     hangman [/s[0-9]]
		 This is a tiny	and handy tool for random kernel hangs analy-
		 sis, of which its depth is controlled by the optional argu-
		 ment of the default value of five.  It	uses some sophisti-
		 cated heuristics to spot the global symbol that caused	the
		 hang.	Since the discovering algorithm	is a probabilistic
		 one, you may spend substantial	time to	figure the exact sym-
		 bol name.  This smart thing requires a	little of your atten-
		 tion, the input it accepts is mostly of the same format as
		 that of the famous hangman(6) game, to	which it, apparently,
		 is obliged by the name.  Hint:	the nm(1) utility might	help.

     until [/p]	 Stop at the next "call" or "return" instruction.  If the /p
		 modifier is specified,	ddb prints the call nesting depth and
		 the cumulative	instruction count at each call or return.
		 Otherwise, it stays silent until the matching return is hit.

     match [/p]	 Stop at the next matching return instruction.	If the /p mod-
		 ifier is specified, ddb prints	the call nesting depth and the
		 cumulative instruction	count at each call or return.  Other-
		 wise, it remains mostly quiet.

     next [/p]	 The next command is a synonym for match.

     kill pid	 Send an uncatchable SIGABRT signal to the process specified
		 by the	pid argument.

     trace [/tu] [frameaddr][,count]
		 Show the stack	trace.	The /t modifier	interprets the
		 frameaddr argument as the TID of a process and	shows the
		 stack trace of	that process.  frameaddr is subject to the
		 radix;	use the	0t prefix to enter a decimal TID.  The /t mod-
		 ifier is not supported	on all platforms.  The /u modifier
		 shows the stack trace of user space; if omitted, the kernel
		 stack is traced instead.  The count argument is the limit on
		 the number of frames to be followed.  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 a value beginning at	addr.  This command
		 might fail in interesting ways	if it doesn't find the
		 searched-for value.  This is because ddb doesn't always re-
		 cover from touching bad memory.  The optional count argument
		 limits	the search.  The modifiers are the same	as those of
		 the write command.

		 The next address is set to the	address	where value is found,
		 or just after where the search	area finishes.

     reboot	 Shortcut for boot reboot

     show what	 Displays various things, depending on what:

		 show bcstats
		       Prints the buffer cache statistics.

		 show breaks
		       Prints a	list of	all breakpoints	that have been set
		       with the	break command.

		 show buf [/f] addr
		       Prints the struct buf at	addr.  If the /f modifier is
		       specified output	will also include softdep printout, if
		       those are available.

		 show extents
		       Prints a	detailed list of all extents.

		 show locks [addr]
		       Prints the list of locks	held by	a thread.  If an op-
		       tional address is not specified,	curproc	is assumed.
		       The option WITNESS is required for this command to be
		       available.

		 show malloc [addr]
		       Prints malloc debugging information if available.  If
		       an optional address is specified, only information
		       about that address is printed.

		 show map [/f] addr
		       Prints the vm_map at addr.  If the /f modifier is spec-
		       ified the complete map is printed.

		 show mbuf addr
		       Prints the struct mbuf header at	addr.  Depending on
		       the mbuf	flags struct pkthdr and	struct m_ext are
		       printed as well.

		 show mount [/f] addr
		       Prints the struct mount at addr.	 If the	/f modifier is
		       specified prints	out all	vnodes (see also show vnode)
		       and also	all bufs (see also show	buf) on	all those vn-
		       odes.

		 show nfsnode [/f] addr
		       Prints the struct nfsnode at addr.  If the /f modifier
		       is specified prints out additional information as well.

		 show nfsreq [/f] addr
		       Prints the struct nfsreq	at addr.  If the /f modifier
		       is specified prints out additional information as well.

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

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

		 show panic
		       Prints the panic	string.

		 show pool [/p]	addr
		       Prints the pool at addr.	 Valid modifiers:
		       /p   Print the pagelist for this	pool.

		 show proc [addr]
		       Prints the struct proc at addr.	If an optional address
		       is not specified	curproc	is assumed.

		 show registers	[/u]
		       Display the register set.  If the /u modifier is	speci-
		       fied, it	displays user registers	(or the	currently
		       saved registers)	instead	of the kernel's.  Note:	The /u
		       modifier	is not supported on every machine, in which
		       case incorrect information may be displayed.

		 show socket addr
		       Prints the struct socket	at addr.  If the socket	is
		       spliced,	the struct sosplice associated with the	socket
		       is printed as well.

		 show struct name [addr]
		       Prints the content of the memory	at addr	as a struct
		       name.  Nested structures	and bit	fields are not
		       printed.	 Character arrays are printed as bytes.

		 show uvmexp
		       Displays	a selection of uvm counters and	statistics.

		 show vnode [/f] addr
		       Prints the struct vnode at addr.	 If the	/f modifier is
		       specified prints	out all	bufs (see also show buf) cur-
		       rently attached to this vnode.

		 show watches
		       Displays	all watchpoints	set with the watch command.

		 show witness [/b]
		       Prints the current order	list.  If the /b modifier is
		       specified, the list of found lock order violations is
		       printed instead.	 The option WITNESS is required	for
		       this command to be available.

		 show all procs	[/anow]
		       Display information on all processes.

		       /n   (Default) Show process information in a ps(1)-like
			    format.  Information printed includes process ID,
			    thread ID, parent process ID, UID, process status,
			    process flags, process wait	channel	message	and
			    process command name.
		       /a   Shows the kernel virtual addresses of each
			    process' proc structure, u-area, and vmspace
			    structure.	The vmspace address is also the	ad-
			    dress of the process' vm_map structure and can be
			    used in the	show map command.
		       /o   Shows non-idle threads that	were on	CPU when ddb
			    was	entered.  Information printed includes thread
			    ID,	process	ID, UID, process flags,	thread flags,
			    current CPU, and command name.
		       /w   Shows each thread's	ID, command, process group,
			    wait channel address, and wait channel message.

		 show all bufs [/f]
		       Display information about all buffers in	the system.

		       /f   For	each buffer, print a more detailed output.
			    See	the show buf command for more information.

		 show all callout
		       Display the contents of the callout table.

		 show all pools	[/a]
		       Display information about all system pools in a format
		       similar to vmstat(8).

		       /a   Displays "interesting" address information.

		 show all locks
		       Prints the list of locks	held by	all threads in the
		       system.	The option WITNESS is required for this	com-
		       mand to be available.

		 show all mounts [/f]
		       Display information on all mounted filesystems.

		       /f   For	each filesystem, list all its struct vnode ad-
			    dresses.  These addresses can be used in the show
			    vnode command.

		 show all nfsnodes [/f]
		       Display information about all nfsnodes in the system.

		       /f   For	each nfsnode, print a more detailed output.
			    See	the show nfsnode command for more information.

		 show all nfsreqs [/f]
		       Display information for all outstanding NFS requests.

		       /f   For	each NFS requests, print a more	detailed out-
			    put.  See the show nfsreq command for more infor-
			    mation.

		 show all vnodes [/f]
		       Display information about all vnodes in the system.

		       /f   For	each vnode, print a more detailed output.  See
			    the	show vnode command for more information.

     callout	 A synonym for the show	all callout command.

     ps	[/anow]	 A synonym for show all	procs.

     mac[hine] subcommand [args	...]
		 Perform a platform-specific command.

		 The following commands	are supported by multiprocessor	ker-
		 nels on these platforms: amd64, i386, macppc, mips64, and
		 sparc64.

		 cpuinfo	     Display the state of each CPU.
		 ddbcpu	N	     Stop the current CPU and start handling
				     ddb on the	selected CPU.
		 startcpu [N]	     Resume normal processing on the selected
				     CPU, or all CPUs if none is specified.
		 stopcpu [N]	     Stop normal processing on the selected
				     CPU, or all CPUs (except the one handling
				     ddb) if none is specified.

		 Other platform-specific commands:

		 arm:
		 frame addr	     Display the trapframe at addr.

		 i386:
		 sysregs	     Display the contents of the privileged
				     registers:	IDTR, GDTR, LDTR, TR, CR0,
				     CR2, CR3, and CR4.

		 m88k:
		 ddbcpu	N	     Stop the current CPU and start handling
				     ddb on the	selected CPU.
		 frame addr	     Display the trapframe at addr.
		 regs		     Display the registers from	when ddb was
				     entered.
		 searchframe [addr]  Search for	and display stack exception
				     frames, starting from addr	if given, else
				     the address in register r31, and stopping
				     at	the next 8k boundary.
		 where		     Display where the current CPU was
				     stopped.

		 mips64:
		 tlb [/p asid] [/c] [tlb]
		 trap ??

		 sh:
		 cache [addr]	     Display the cache,	starting from addr,
				     defaulting	to 0.
		 frame		     Display the switch	and trap frames.
		 tlb		     Display the TLB.

		 sparc64:
		 ctx		     Display the context addresses for all
				     threads.

VARIABLES
     ddb denotes registers and variables by $name.  Register names can be
     found with	the show registers command.

     Some variable names are suffixed with numbers, and	some may have a	modi-
     fier following a colon immediately	after the variable name.  For example,
     register variables	can have the `:u' modifier to indicate a user register
     (e.g., `$eax:u').

     Built-in debugger 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 lines.
	   $lines      The number of lines to page.  This is used by the
		       "more" feature.
	   $tabstops   Tab stop	width.
	   $log	       Controls	whether	the output of ddb will also appear in
		       the system message buffer.

     These variables can also be controlled outside ddb	via the	`ddb'
     sysctl(8) hierarchy.

EXPRESSIONS
     Almost all	expression operators in	C are supported	except for `~',	`^',
     and unary `&'.  Special rules for expressions in ddb are:
	   identifier	    The	name of	a symbol.  It is translated to the ad-
			    dress (or value) of	the symbol.  `.' and `:' can
			    be used in the identifier.	The following can be
			    accepted as	an identifier, if supported by an ob-
			    ject format	dependent routine:
				  [filename:]func[:linenumber]
				  [filename:]variable
				  filename[:linenumber]
			    The	symbol may be prefixed with
			    `symboltablename::'	(e.g.,
			    `emulator::mach_msg_trap') to specify other	than
			    kernel symbols.
	   number	    The	radix is determined by the first two letters:
			    `0x': hex, `0o': octal, `0t': decimal, otherwise,
			    the	value of $radix	is used.
	   .		    dot: the current address.
	   +		    next: the next address.
	   ..		    The	address	of the start of	the last line exam-
			    ined.  Unlike dot or next, this is only changed by
			    the	examine	or write command.
	   '		    The	last address explicitly	specified.
	   $variable	    The	value of a register or variable.  The name may
			    be followed	by a `:' and modifiers as described
			    above with identifier.
	   expr	# expr	    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 mod-
			    ifiers as described	above.

SEE ALSO
     ctfstrip(1), gdb(1), nm(1), witness(4), sysctl.conf(5), hangman(6),
     crash(8), sysctl(8), extent(9), pool(9), uvm_init(9)

HISTORY
     This kernel facility first	appeared in the	MACH 2 operating system	devel-
     oped by CMU.  Hangman (which stands for "hangs maniacal analyzer")	first
     appeared in OpenBSD 1.2.

FreeBSD	13.0		       October 26, 2020			  FreeBSD 13.0

NAME | DESCRIPTION | COMMANDS | VARIABLES | EXPRESSIONS | SEE ALSO | HISTORY

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