path: root/share/man/man4/ddb.4

.\"	$OpenBSD: ddb.4,v 1.10 1997/12/10 06:15:54 gene Exp $
.\"	$NetBSD: ddb.4,v 1.5 1994/11/30 16:22:09 jtc Exp $
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.Dd November 30, 1993
.Dt DDB 4
.Os
.Sh NAME
.Nm ddb
.Nd kernel debugger
.Sh DESCRIPTION
The kernel debugger has most of the features of the old kdb,
but with a more rational (gdb-like) syntax.
.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:
.Pp
command[/modifier] address [,count]
.Pp
A blank line repeats 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.
.Pp
.Nm ddb
has a feature like a command
.Nm more
for the output.  If an output line exceeds the number set in the
.Nm $lines
variable, it displays
.Nm "--db_more--"
and waits for a response.
The valid responses for it are:
.Bl -tag -width 10n
.It <space>
one more page
.It <return>
one more line
.It q
abort the current command, and return to the command input mode.
.El
.Sh COMMANDS
.Bl -tag -width 10n
.It examine(x) [/<modifier>] <addr>[,<count>]
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 -width 4n
.It b
look at by bytes(8 bits)
.It h
look at by half words(16 bits)
.It l
look at by long words(32 bits)
.It a
print the location being displayed
.It A
print the location with a line number if possible
.It x
display in unsigned hex
.It z
display in signed hex
.It o
display in unsigned octal
.It d
display in signed decimal
.It u
display in unsigned decimal
.It r
display in current radix, signed
.It c
display low 8 bits as a character.
Non-printing characters are displayed as an octal escape code (e.g. '\\000').
.It s
display the null-terminated string at the location.
Non-printing characters are displayed as octal escapes.
.It 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 i
display as an instruction
.It I
display as an instruction with possible alternate formats depending on the
machine:
.Bl -tag -width powerpc_
.It vax
don't assume that each external label is a procedure entry mask
.It i386
don't round to the next long word boundary
.It mips
print register contents
.El
.El
.It xf
Examine forward.
It executes 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 xb
Examine backward.
It executes 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 print[/axzodurc] <addr1> [ <addr2> ... ]
Print 'addr's according to the modifier character.
Valid formats are: a x z o d u r c.
If no modifier is specified, the last one specified to it is used.  'addr'
can be a string, and it is printed as it is.  For example,
.ti +4n
print/x "eax = " $eax "\\necx = " $ecx "\\n"
.br
will print like
.br
.in +4n
eax = xxxxxx
.br
ecx = yyyyyy
.in -4n
.Pp
.It write[/bhl] <addr> <expr1> [ <expr2> ... ]
Write the expressions at succeeding locations.
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.
.Pp
Warning: since there is no delimiter between expressions, strange
things may happen.
It's best to enclose each expression in parentheses.
.It set $<variable> [=] <expr>
Set the named variable or register with the value of 'expr'.
Valid variable names are described below.
.It boot {sync,crash,dump}
Reboot the machine with syncing disks, not syncing or dumping core, 
syncing disk and dumping core, respectively.
.It break[/u] <addr>[,<count>]
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.
.XS 2n
.IP u 5n
Set a break point in user space address. Without 'u' option,
the address is considered in the kernel space, and wrong space address
is rejected with an error message.
This option can be used only if it is supported by machine dependent
routines.
.XE 2n
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 delete <addr>|#<number>
Delete the break point.  The target break point can be specified by a
break point number with '#', or by 'addr' like specified in 'break'
command.
.It step[/p] [,<count>]
Single step 'count' times.
If 'p' option is specified, print each instruction at each step.
Otherwise, only print the last instruction.
.Pp
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 call <name>[(<args>)]
Call the named function with argument(s) listed in parentheses,
wich maybe omitted if there is no one.
Current upper limit for number of arguments is 10.
.It callout
See
.Nm show all callout
command.
.It continue[/c]
Continue execution until a breakpoint or watchpoint.
If /c, count instructions while executing.
Some machines (e.g., pmax) also count loads and stores.
.Pp
Warning: when counting, the debugger is really silently single-stepping.
This means that single-stepping on low-level code may cause strange
behavior.
.It dwatch <addr>
Delete watchpoint at 
.Nm <addr>
address, that was previously set with
.Nm watch
command.
.It hangman [/s[0-9]]
This is tiny and handy tool for random kernel hangs analysis, which
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 algorythm 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
.Xr hangman 6
game, to which it, apparently, is obliged by the name.
Hint: the
.Xr nm 1
might help.
.It until[/p]
Stop at the next call or return instruction.
If 'p' option 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 next[/p]
Stop at the matching return instruction.
If 'p' option 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 match[/p]
A synonym for 'next'.
.It trace[/u] [ <frame_addr> ][,<count>]
Stack trace.  'u' option traces user space; if omitted, only traces
kernel space. 'count' is the number of frames to be traced. 
If the 'count' is omitted, all frames are printed.
.Pp
Warning: User space stack trace is valid
only if the machine dependent code supports it.
.It search[/bhl] <addr> <value> [<mask>] [,<count>]
Search memory for a 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.
.It show all callout
Display callout table contents.
.It ps[/a][/n][/w]
A synonym for 'show all procs'.
.It show all procs[/a][/n][/w]
Display all process information.
The 'n' option (the default) shows process information in a 
UNIX-like 'ps' format.  Information printed includes process ID, parent 
process ID, process group, UID, process status, process flags, process
command name, and process wait channel message.
The 'a' option shows the kernel virtual addesses 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.
The 'w' option shows each process' PID, command, system call emulation,
wait channel addresss, and wait channel message.
.It show breaks
Prints the list of all breakpoints had been set.
.It show map[/f] <addr>
Prints the vm_map at 'addr'. If the 'f' option is specified the
complete map is printed.
.It show object[/f] <addr>
Prints the vm_object at 'addr'. If the 'f' option is specified the
complete object is printed.
.It show registers
Display the register set.
If 'u' option is specified, it displays user registers instead of
kernel or currently saved one.
.Pp
Warning: The support of 'u' option depends on the machine.  If
not supported, incorrect information will be displayed.
.It show watches
Displays all watchpoints.
.It 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.
.Pp
If you specify a wrong space address, the request is rejected
with an error message.
.Pp
Warning: Attempts to watch wired kernel memory
may cause unrecoverable error in some systems such as i386.
Watchpoints on user addresses work best.
.Sh VARIABLES
The debugger accesses registers and variables as
.I $<name>.
Register names are as in the
.Nm 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 'u' modifier to indicate
user register (e.g. $eax:u).
.Pp
Built-in variables currently supported are:
.Bl -tag -width 10n
.It radix
Input and output radix
.It maxoff
Addresses are printed as 'symbol'+offset unless offset is greater than maxoff.
.It maxwidth
The width of the displayed line.
.It lines
The number of lines.  It is used by "more" feature.
.It tabstops
Tab stop width.
.It work\fIxx\fR
Work variable.
.I 'xx'
can be 0 to 31.
.El
.Sh EXPRESSIONS
Almost all expression operators in C are supported except '~', '^',
and unary '&'.
Special rules in
.Nm ddb
are:
.Bl -tag -width 15n
.It "<identifier>"
name of a symbol.  It is translated to the address(or value) of it.  '.'
and ':' can be used in the identifier.  If supported by an object format
dependent routine,
[\fI<filename>\fR:]\fI<func>\fR[:\fI<linenumber>\fR]
[\fI<filename>\fR:]\fI<variable>\fR, and
\fI<filename>\fR[:\fI<linenumber>\fR]
can be accepted as a symbol.
The symbol may be prefixed with '\fI<symbol_table_name>\fR::'
like 'emulator::mach_msg_trap' to specify other than kernel symbols.
.It "<number>"
radix is determined by the first two letters:
0x: hex, 0o: octal, 0t: decimal, otherwise, follow current radix.
.It \.
\'dot'
.It \+
\'next'
.It \..
address of the start of the last line examined.
Unlike 'dot' or 'next', this is only changed by "examine" or
"write" command.
.It \'
last address explicitly specified.
.It "$<variable>"
register name or variable.  It is translated to the value of it.
It may be followed by a ':' and modifiers as described above.
.It \#
a binary operator which rounds up the left hand side to the next
multiple of right hand side.
.It "*<expr>"
indirection.  It may be followed by a ':' and  modifiers as described above.
.Sh SEE ALSO
.Xr gdb 1 ,
.Xr hangman 6 ,
.Xr nm 1 .
.Sh HISTORY
This kernel facility first appeared in MACH 2 operating system
developed by CMU.
Hangman (wich stands for "hangs maniacal analyzer") first appeared in
.Ox 1.2 .