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DDB(4)			  OpenBSD Programmer's Manual			DDB(4)

     ddb - kernel debugger

     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:


     The general syntax of a ddb command is:

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

     To save typing, ddb makes use of a context inferred from previous
     commands.	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
     intuitive 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).
     Omitting 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.  Entering 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

     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.  Multiple modifier formats display multiple
		 locations.  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
		       characters 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 alternate format instruction depending on
		       the machine:

		       alpha	 Print affected register contents for every
		       i386	 Do not skip padding to the next long word
				 boundary for unconditional jumps.
		       m68k	 Use Motorola assembly syntax.
		       m88k	 Decode instructions for the opposite CPU
				 model (e.g. m88110 when running on an m88100
		       vax	 Don't assume that each external label is a
				 procedure entry mask.

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

     print [/axzodurc] [addr [addr ...]]
		 Print each 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.  The addr argument can be a string, and it is
		 printed as a literal.

		 For example,

		       print/x "eax = " $eax "\necx = " $ecx "\n"

		 will print something like this:

		       eax = xxxxxx
		       ecx = yyyyyy

     w[rite] [/bhl] [addr] expr [expr ...]
		 Write the value of each expr expression at succeeding
		 locations 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

		 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.
		 Valid variable names are described below.

     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.

		 If the break point is successfully set, a break point number
		 is displayed, in the form #number.  This can later be used in
		 deleting the break point or for adding conditions to it.

     d[elete] [addr | #number]
		 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),
		 stepping through code executed by interrupt handlers will
		 probably 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

		 Warning: when counting with /c, ddb is really silently
		 single-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
		 analysis, of which its depth is controlled by the optional
		 argument of the default value of five.	 It uses some
		 sophisticated 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 symbol name.	 This smart thing requires a little of
		 your attention, 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
		 modifier is specified, ddb prints the call nesting depth and
		 the cumulative instruction count at each call or return.
		 Otherwise, it remains mostly quiet.

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

     trace [/pu] [frameaddr] [,count]
		 Show the stack trace.	The /p modifier interprets the
		 frameaddr argument as the PID of a process and shows the
		 stack trace of that process.  The /p modifier 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

		 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
		 recover 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.

     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 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
		       specified 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

		 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 offset offset
		       Prints a list of the known kernel structure fields
		       which occur at the given offset from the beginning of
		       the struct, as well as their size.  The option
		       DDB_STRUCT is required for this command to be

		 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 [/clp] addr
		       Prints the pool at addr.	 Valid modifiers:
		       /c   Print the cachelist and its statistics for this
		       /l   Print the log entries for this pool.
		       /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
		       specified, 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 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.	 The
		       option DDB_STRUCT is required for this command to be

		 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)
		       currently attached to this vnode.

		 show watches
		       Displays all watchpoints set with the watch command.

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

		       /n   (Default) Show process information in a ps(1)-like
			    format.  Information printed includes process ID,
			    parent process ID, process group, UID, process
			    status, process flags, process command name, and
			    process wait channel message.
		       /a   Shows the kernel virtual addresses of each
			    process' proc structure, u-area, and vmspace
			    structure.	The vmspace address is also the
			    address of the process' vm_map structure and can
			    be used in the show map command.
		       /w   Shows each process' PID, command, system call
			    emulation, wait channel address, and wait channel

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

		       /f   For each filesystem, list all its struct vnode
			    addresses.	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
			    output.  See the show nfsreq command for more

		 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 [/anw]	 A synonym for show all procs.

     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
     modifier 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.

     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
			    address (or value) of the symbol.  `.' and `:' can
			    be used in the identifier.	The following can be
			    accepted as an identifier, if supported by an
			    object format dependent routine:
			    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
			    examined.  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
			    modifiers as described above.

     gdb(1), nm(1), sysctl.conf(5), hangman(6), kgdb(7), crash(8), sysctl(8),
     extent(9), pool(9), uvm(9)

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

OpenBSD 4.9		       November 27, 2010		   OpenBSD 4.9

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