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.\"
.\" Mach Operating System
.\" Copyright (c) 1991,1990 Carnegie Mellon University
.\" All Rights Reserved.
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.\" Permission to use, copy, modify and distribute this software and its
.\" documentation is hereby granted, provided that both the copyright
.\" notice and this permission notice appear in all copies of the
.\" software, derivative works or modified versions, and any portions
.\" thereof, and that both notices appear in supporting documentation.
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.\" CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
.\" CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
.\" ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
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.\" Carnegie Mellon requests users of this software to return to
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.\"  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
.\"  School of Computer Science
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.\" any improvements or extensions that they make and grant Carnegie Mellon
.\" the rights to redistribute these changes.
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.\" changed a \# to #, since groff choked on it.
.\"
.\" HISTORY
.\" ddb.4,v
.\" Revision 1.1  1993/07/15  18:41:02  brezak
.\" Man page for DDB
.\"
.\" Revision 2.6  92/04/08  08:52:57  rpd
.\"     Changes from OSF.
.\"     [92/01/17  14:19:22  jsb]
.\"     Changes for OSF debugger modifications.
.\"     [91/12/12            tak]
.\"
.\" Revision 2.5  91/06/25  13:50:22  rpd
.\"     Added some watchpoint explanation.
.\"     [91/06/25            rpd]
.\"
.\" Revision 2.4  91/06/17  15:47:31  jsb
.\"     Added documentation for continue/c, match, search, and watchpoints.
.\"     I've not actually explained what a watchpoint is; maybe Rich can
.\"     do that (hint, hint).
.\"     [91/06/17  10:58:08  jsb]
.\"
.\" Revision 2.3  91/05/14  17:04:23  mrt
.\"     Correcting copyright
.\"
.\" Revision 2.2  91/02/14  14:10:06  mrt
.\"     Changed to new Mach copyright
.\"     [91/02/12  18:10:12  mrt]
.\"
.\" Revision 2.2  90/08/30  14:23:15  dbg
.\"     Created.
.\"     [90/08/30            dbg]
.\"
.\" $FreeBSD: src/share/man/man4/ddb.4,v 1.7.2.6 2001/08/17 13:08:37 ru Exp $
.\" $DragonFly: src/share/man/man4/ddb.4,v 1.3 2006/05/11 08:23:19 swildner Exp $
.Dd May 11, 2006
.Dt DDB 4
.Os
.Sh NAME
.Nm ddb
.Nd interactive kernel debugger
.Sh SYNOPSIS
.Cd options DDB
.Pp
.Cd options DDB_TRACE
.Cd options DDB_UNATTENDED
.Sh DESCRIPTION
The
.Nm
kernel debugger has most of the features of the old kdb,
but with a more rational syntax
inspired by
.Xr gdb 1 .
If linked into the running kernel,
it can be invoked locally with the
.Ql debug
.Xr keymap 5
action.
The debugger is also invoked on kernel
.Xr panic 9
if the
.Va debug.debugger_on_panic
.Xr sysctl 8
MIB variable is set non-zero,
which is the default
unless the
.Dv DDB_UNATTENDED
option is specified.
If set, the
.Va debug.trace_on_panic
.Xr sysctl 8
MIB variable will cause
.Nm
to print a stack trace on
.Xr panic 9 .
It is zero by default unless the
.Dv DDB_TRACE
option is specified.
.Pp
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'.
.Pp
The general command syntax is:
.Cm command Ns Op Li \&/ Ns Ar modifier
.Ar address Ns Op Li , Ns Ar count
.Pp
A blank line repeats the previous command from the address `next' with
count 1 and no modifiers.  Specifying
.Ar address
sets `dot' to the
address.  Omitting
.Ar address
uses `dot'.  A missing
.Ar count
is taken
to be 1 for printing commands or infinity for stack traces.
.Pp
The
.Nm
debugger has a feature like the
.Xr more 1
command
for the output.  If an output line exceeds the number set in the
.Li \&$lines
variable, it displays
.Dq Em --db_more--
and waits for a response.
The valid responses for it are:
.Pp
.Bl -tag -compact -width SPC
.It Li SPC
one more page
.It Li RET
one more line
.It Li q
abort the current command, and return to the command input mode
.El
.Pp
Finally,
.Nm
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.
.Sh COMMANDS
.Bl -ohang
.It Cm examine
.It Cm 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.
.Pp
The format characters are:
.Bl -tag -compact -width indent
.It Li b
look at by bytes (8 bits)
.It Li h
look at by half words (16 bits)
.It Li l
look at by long words (32 bits)
.It Li a
print the location being displayed
.It Li A
print the location with a line number if possible
.It Li x
display in unsigned hex
.It Li z
display in signed hex
.It Li o
display in unsigned octal
.It Li d
display in signed decimal
.It Li u
display in unsigned decimal
.It Li r
display in current radix, signed
.It Li c
display low 8 bits as a character.
Non-printing characters are displayed as an octal escape code (e.g., `\e000').
.It Li s
display the null-terminated string at the location.
Non-printing characters are displayed as octal escapes.
.It Li 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.
.It Li i
display as an instruction
.It Li I
display as an instruction with possible alternate formats depending on the
machine:
.Bl -tag -width MIPS -compact
.It Tn VAX
don't assume that each external label is a procedure entry mask
.It Tn i386
don't round to the next long word boundary
.It Tn MIPS
print register contents
.El
.El
.It Cm 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.
.It Cm 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.
.It Cm print Ns Op Cm /acdoruxz
Print
.Ar addr Ns s
according to the modifier character (as described above for
.Li examine ) .
Valid formats are:
.Li a ,
.Li x ,
.Li z ,
.Li o ,
.Li d ,
.Li u ,
.Li r ,
and
.Li c .
If no modifier is specified, the last one specified to it is used.
.Ar addr
can be a string, in which case it is printed as it is.  For example:
.Bd -literal -offset indent
print/x \&"eax = \&" $eax \&"\enecx = \&" $ecx \&"\en\&"
.Ed
.Pp
will print like:
.Bd -literal -offset indent
eax = xxxxxx
ecx = yyyyyy
.Ed
.It Xo
.Cm write Ns Op Cm /bhl
.Ar addr Ar expr1 Op Ar "expr2 ..."
.Xc
Write the expressions specified after
.Ar addr
on the command line at succeeding locations starting with
.Ar addr
The write unit size can be specified in the modifier with a letter
.Li b
(byte),
.Li h
(half word) or
.Li l
(long word) respectively.  If omitted,
long word is assumed.
.Pp
.Sy Warning :
since there is no delimiter between expressions, strange
things may happen.
It's best to enclose each expression in parentheses.
.It Xo
.Cm set
.Li \&$ Ns Ar variable
.Op Li =
.Ar expr
.Xc
Set the named variable or register with the value of
.Ar expr .
Valid variable names are described below.
.It Cm break Ns Op Cm /u
Set a break point at
.Ar addr .
If
.Ar count
is supplied, continues
.Ar count
- 1 times before stopping at the
break point.  If the break point is set, a break point number is
printed with
.Sq Li \&# .
This number can be used in deleting the break point
or adding conditions to it.
.Pp
If the
.Li u
modifier is specified, this command sets a break point in user space
address.  Without the
.Li 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.
.Pp
.Sy 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.
.It Cm delete Ar addr
.It Cm delete Li \&# Ns Ar number
Delete the break point.  The target break point can be specified by a
break point number with
.Li # ,
or by using the same
.Ar addr
specified in the original
.Cm break
command.
.It Cm step Ns Op Cm /p
Single step
.Ar count
times (the comma is a mandatory part of the syntax).
If the
.Li p
modifier is specified, print each instruction at each step.
Otherwise, only print the last instruction.
.Pp
.Sy 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.
.It Cm continue Ns Op Cm /c
Continue execution until a breakpoint or watchpoint.
If the
.Li c
modifier is specified, count instructions while executing.
Some machines (e.g., pmax) also count loads and stores.
.Pp
.Sy Warning :
when counting, the debugger is really silently single-stepping.
This means that single-stepping on low-level code may cause strange
behavior.
.It Cm until Ns Op Cm /p
Stop at the next call or return instruction.
If the
.Li 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.
.It Cm next Ns Op Cm /p
.It Cm match Ns Op Cm /p
Stop at the matching return instruction.
If the
.Li 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.
.It Xo
.Cm trace Ns Op Cm /u
.Op Ar frame
.Op , Ns Ar count
.Xc
Stack trace.  The
.Li u
option traces user space; if omitted,
.Cm trace
only traces
kernel space.
.Ar count
is the number of frames to be traced.
If
.Ar count
is omitted, all frames are printed.
.Pp
.Sy Warning :
User space stack trace is valid
only if the machine dependent code supports it.
.It Xo
.Cm search Ns Op Cm /bhl
.Ar addr
.Ar value
.Op Ar mask
.Op , Ns Ar count
.Xc
Search memory for
.Ar 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
.Ar count
argument limits the search.
.It Cm show all procs Ns Op Cm /m
.It Cm ps Ns Op Cm /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
.Li m
modifier will alter the display to show VM map
addresses for the process and not show other info.
.It Cm show registers Ns Op Cm /u
Display the register set.
If the
.Li u
option is specified, it displays user registers instead of
kernel or currently saved one.
.Pp
.Sy Warning :
The support of the
.Li u
modifier depends on the machine.  If
not supported, incorrect information will be displayed.
.It Xo
.Cm show map Ns Op Cm /f
.Ar addr
.Xc
Prints the VM map at
.Ar addr .
If the
.Li f
modifier is specified the
complete map is printed.
.It Xo
.Cm show object Ns Op Cm /f
.Ar addr
.Xc
Prints the VM object at
.Ar addr .
If the
.Li f
option is specified the
complete object is printed.
.It Cm "show watches"
Displays all watchpoints.
.It Xo
.Cm watch
.Ar addr Ns Li \&, Ns Ar size
.Xc
Set a watchpoint for a region.  Execution stops
when an attempt to modify the region occurs.
The
.Ar size
argument defaults to 4.
If you specify a wrong space address, the request is rejected
with an error message.
.Pp
.Sy Warning :
Attempts to watch wired kernel memory
may cause unrecoverable error in some systems such as i386.
Watchpoints on user addresses work best.
.It Cm gdb
Toggles between remote GDB and DDB mode.  In remote GDB mode, another
machine is required that runs
.Xr gdb 1
using the remote debug feature, with a connection to the serial
console port on the target machine.  Currently only available on the
.Em i386
architecture.
.It Cm help
Print a short summary of the available commands and command
abbreviations.
.El
.Sh VARIABLES
The debugger accesses registers and variables as
.Li \&$ Ns Em name .
Register names are as in the
.Dq Cm 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
.Li u
modifier to indicate user register (e.g.,
.Li $eax:u ) .
.Pp
Built-in variables currently supported are:
.Bl -tag -width tabstops -compact
.It Li radix
Input and output radix
.It Li maxoff
Addresses are printed as 'symbol'+offset unless offset is greater than maxoff.
.It Li maxwidth
The width of the displayed line.
.It Li lines
The number of lines.  It is used by "more" feature.
.It Li tabstops
Tab stop width.
.It Li work Ns Ar xx
Work variable.
.Ar xx
can be 0 to 31.
.El
.Sh EXPRESSIONS
Almost all expression operators in C are supported except
.Sq Li \&~ ,
.Sq Li \&^ ,
and unary
.Sq Li \&& .
Special rules in
.Nm
are:
.Bl -tag -width Identifiers
.It Em Identifiers
The name of a symbol is translated to the value of the symbol, which
is the address of the corresponding object.
.Sq Li \&.
and
.Sq Li \&:
can be used in the identifier.
If supported by an object format dependent routine,
.Sm off
.Oo Em filename : Oc Em func : lineno ,
.Sm on
.Oo Em filename : Oc Ns Em variable ,
and
.Oo Em filename : Oc Ns Em lineno
can be accepted as a symbol.
.It Em Numbers
Radix is determined by the first two letters:
.Li 0x :
hex,
.Li 0o :
octal,
.Li 0t :
decimal; otherwise, follow current radix.
.It Li \&.
`dot'
.It Li \&+
`next'
.It Li \&..
address of the start of the last line examined.
Unlike `dot' or `next', this is only changed by
.Dq Li examine
or
.Dq Li write
command.
.It Li \&'
last address explicitly specified.
.It Li \&$ Ns Em variable
Translated to the value of the specified variable.
It may be followed by a
.Li :
and modifiers as described above.
.It Em a Ns Li \&# Ns Em b
a binary operator which rounds up the left hand side to the next
multiple of right hand side.
.It Li \&* Ns Em expr
indirection.  It may be followed by a
.Sq Li :
and modifiers as described above.
.El
.Sh SEE ALSO
.Xr gdb 1
.Sh HISTORY
The
.Nm
debugger was developed for Mach, and ported to
.Bx 386 0.1 .
This manual page translated from
.Fl man
macros by Garrett Wollman.