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

NAME
     ddb -- interactive	kernel debugger

SYNOPSIS
     options DDB

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

DESCRIPTION
     The ddb kernel debugger has most of the features of the old kdb, but with
     a more rational 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 kernel	panic(9) if the
     debug.debugger_on_panic sysctl(8) MIB variable is set non-zero, which is
     the default unless	the DDB_UNATTENDED option is specified.

     The current location is called `dot'.  The	`dot' is displayed with	a
     hexadecimal format	at a prompt.  Examine and write	commands 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 don't 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	print-
     ing commands or infinity for stack	traces.

     The ddb debugger has a feature like the more(1) command for the output.
     If	an output line exceeds the number set in the $lines variable, it dis-
     plays ``--db_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 might	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 formats 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 characters 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 depend-
	     ing on the	machine:
	     alpha    Show the registers of the	instruction.
	     amd64    No alternate format
	     i386     No alternate format
	     ia64     No alternate format
	     powerpc  No alternate format
	     sparc64  No alternate format

     xf
     Examine forward: Execute an examine command with the last specified
     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	specified
     parameters	to it except that the last start address subtracted by the
     size displayed by it is used as the start address.

     print[/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.  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 ...]
     Write the expressions specified after addr	on the command line at suc-
     ceeding 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's best to enclose each expression in parentheses.

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

     break[/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
     space address.  Without the u option, the address is considered in	the
     kernel space, and 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	debugger, 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

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

     step[/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 probably do the wrong thing.

     continue[/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	speci-
     fied, 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] [frame] [,count]
     Stack trace.  The u option	traces user space; if omitted, trace only
     traces kernel space.  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	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.

     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 mod-
     ifier will	alter the display to show VM map addresses for the process and
     not show other info.

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

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

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

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

     show watches
     Displays all watchpoints.

     reset
     Hard reset	the system.

     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 unrecoverable
     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	architecture.
     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.

     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	and Alpha architectures.

     help
     Print a short summary of the available commands and command abbrevia-
     tions.

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 ``more'' feature.
     tabstops  Tab stop	width.
     workxx    Work variable.  xx can be 0 to 31.

EXPRESSIONS
     Almost all	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.

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.

SEE ALSO
     gdb(1)

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

FreeBSD	10.1		       January 16, 1996			  FreeBSD 10.1

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

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