DDB(4) FreeBSD Kernel Interfaces Manual DDB(4)
NAME
ddb - interactive kernel debugger
SYNOPSIS
In order to enable kernel debugging facilities include:
options KDB
options DDB
To prevent activation of the debugger on kernel panic(9):
options KDB_UNATTENDED
In order to print a stack trace of the current thread on the console for
a panic:
options KDB_TRACE
To print the numerical value of symbols in addition to the symbolic
representation, define:
options DDB_NUMSYM
To enable the gdb(4) backend, so that remote debugging with kgdb(1)
(ports/devel/gdb) is possible, include:
options GDB
DESCRIPTION
The ddb kernel debugger is an interactive debugger with a syntax inspired
by gdb(1) (ports/devel/gdb). If linked into the running kernel, it can
be invoked locally with the `debug' keymap(5) action, usually mapped to
Ctrl+Alt+Esc, or by setting the debug.kdb.enter sysctl to 1. 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 KDB_UNATTENDED option is specified. Similarly, if
the debug.debugger_on_recursive_panic variable is set to 1, then the
debugger will be invoked on a recursive kernel panic. This variable has
a default value of 0, and has no effect if debug.debugger_on_panic is
already set non-zero.
The current location is called dot. The dot is displayed with a
hexadecimal format at a prompt. The commands examine and write 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 do not change dot, and set next to be the
same as dot.
The general command syntax is: command[/modifier] [addr][,count]
A blank line repeats the previous command from the address next with
count 1 and no modifiers. Specifying addr sets dot to the address.
Omitting addr uses dot. A missing count is taken to be 1 for printing
commands or infinity for stack traces. A count of -1 is equivalent to a
missing count. Options that are supplied but not supported by the given
command are usually ignored.
The ddb debugger has a pager feature (like the more(1) command) for the
output. If an output line exceeds the number set in the lines variable,
it displays "--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 may be used to
browse through the history buffer, and move the cursor within the current
line.
COMMANDS
COMMON DEBUGGER COMMANDS
help Print a short summary of the available commands and command
abbreviations.
examine[/AISabcdghilmorsuxz ...] [addr][,count]
x[/AISabcdghilmorsuxz ...] [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 format 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)
g look at by quad words (64 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 a disassembled instruction
I display as a disassembled instruction with possible
alternate formats depending on the machine. On i386,
this selects the alternate format for the instruction
decoding (16 bits in a 32-bit code segment and vice
versa).
S display a symbol name for the pointer stored at the
address
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]
p[/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. The argument 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
pprint[/d depth] [name]
Pretty-print symbol specified by name using CTF debugging data.
Works for all symbols exported by the kernel and loaded kernel
modules.
If the d modifier has been specified, contents of structs nested
up to depth levels deep will also be included in the output.
pprint struct[/d depth] [name][addr]
Print memory at addr as struct name. Works for all structs
defined by the kernel and loaded kernel modules.
If the d modifier has been specified, contents of structs nested
up to depth levels deep will also be included in the output.
write[/bhl] addr expr1 [expr2 ...]
w[/bhl] addr expr1 [expr2 ...]
Write the expressions specified after addr on the command line at
succeeding 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 is best to enclose each expression in
parentheses.
set $variable [=] expr
Set the named variable or register with the value of expr. Valid
variable names are described below.
break[/u] [addr][,count]
b[/u] [addr][,count]
Set a break point at addr. If count is supplied, the continue
command will not stop at this break point on the first count - 1
times that it is hit. 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 address space. Without the u option, the address is
considered to be in the kernel space, and a 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]
d [addr]
delete #number
d #number
Delete the specified break point. The break point can be
specified by a break point number with `#', or by using the same
addr specified in the original break command, or by omitting addr
to get the default address of dot.
halt Halt 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.
kill sig pid
Send signal sig to process pid. The signal is acted on upon
returning from the debugger. This command can be used to kill a
process causing resource contention in the case of a hung system.
See signal(3) for a list of signals. Note that the arguments are
reversed relative to kill(2).
step[/p][,count]
s[/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 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]
c[/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 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.
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] [pid | tid][,count]
t[/u] [pid | tid][,count]
where[/u] [pid | tid][,count]
bt[/u] [pid | tid][,count]
Stack trace. The u option traces user space; if omitted, trace
only traces kernel space. The optional argument 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. The optional count argument limits the
search.
reboot[/s] [seconds]
reset[/s] [seconds]
Hard reset the system. If the optional argument seconds is
given, the debugger will wait for this long, at most a week,
before rebooting. When the s modifier is given, the command will
skip running any registered shutdown handlers and attempt the
most basic reset.
thread addr | tid
Switch the debugger to the thread with ID tid, if the argument is
a decimal number, or address addr, otherwise.
watchdog [exp]
Program the watchdog(4) timer to fire in 2^exp seconds. If no
argument is provided, the watchdog timer is disabled.
SPECIALIZED HELPER COMMANDS
findstack addr
Prints the address of the thread whose kernel-mode stack contains
addr, if any.
show active trace
acttrace
Show a stack trace for every thread running on a CPU.
show all procs[/a]
ps[/a] 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 a modifier will print command line arguments for each
process.
show all tcpcbs[/bil]
Show the same output as "show tcpcb" does, but for all TCP
control blocks within the system. The b modifier will request
BBLog entries to be printed. If the i modifier is provided, the
corresponding IP control block is also shown. Using the l
modifier will limit the output to TCP control blocks, which are
locked.
show all trace
alltrace
Show a stack trace for every thread in the system.
show all ttys
Show all TTY's within the system. Output is similar to pstat(8),
but also includes the address of the TTY structure.
show all vnets
Show the same output as "show vnet" does, but lists all
virtualized network stacks within the system.
show allchains
Show the same information like "show lockchain" does, but for
every thread in the system.
show alllocks
Show all locks that are currently held. This command is only
available if witness(4) is included in the kernel.
show allpcpu
The same as "show pcpu", but for every CPU present in the system.
show allrman
Show information related with resource management, including
interrupt request lines, DMA request lines, I/O ports, I/O memory
addresses, and Resource IDs.
show apic
Dump data about APIC IDT vector mappings.
show badstacks
Walk the witness(4) graph and print any lock-order violations.
This command is only available if witness(4) is included in the
kernel.
show breaks
Show breakpoints set with the "break" command.
show bio addr
Show information about the bio structure struct bio present at
addr. See the sys/bio.h header file and g_bio(9) for more
details on the exact meaning of the structure fields.
show buffer addr
Show information about the buf structure struct buf present at
addr. See the sys/buf.h header file for more details on the
exact meaning of the structure fields.
show callout addr
Show information about the callout structure struct callout
present at addr.
show cdev [addr]
Show the internal devfs state of the cdev structure located at
addr. If no argument is provided, show the list of all created
cdevs, consisting of the devfs node name and the struct cdev
address.
show conifhk
Lists hooks currently waiting for completion in
run_interrupt_driven_config_hooks().
show cpusets
Print numbered root and assigned CPU affinity sets. See
cpuset(2) for more details.
show cyrixreg
Show registers specific to the Cyrix processor.
show devmap
Prints the contents of the static device mapping table.
Currently only available on the ARM architecture.
show domain addr
Print protocol domain structure struct domain at address addr.
See the sys/domain.h header file for more details on the exact
meaning of the structure fields.
show ffs [addr]
Show brief information about ffs mount at the address addr, if
argument is given. Otherwise, provides the summary about each
ffs mount.
show file addr
Show information about the file structure struct file present at
address addr.
show files
Show information about every file structure in the system.
show freepages
Show the number of physical pages in each of the free lists.
show geom [addr]
If the addr argument is not given, displays the entire GEOM
topology. If addr is given, displays details about the given
GEOM object (class, geom, provider or consumer).
show idt
Show IDT layout. The first column specifies the IDT vector. The
second one is the name of the interrupt/trap handler. Those
functions are machine dependent.
show igi_list addr
Show information about the IGMP structure struct igmp_ifsoftc
present at addr.
show iosched addr
Show information about the I/O scheduler struct cam_iosched_softc
located at addr.
show inodedeps [addr]
Show brief information about each inodedep structure. If addr is
given, only inodedeps belonging to the fs located at the supplied
address are shown.
show inpcb addr
Show information on IP Control Block struct in_pcb present at
addr.
show intr
Dump information about interrupt handlers.
show intrcnt
Dump the interrupt statistics.
show irqs
Show interrupt lines and their respective kernel threads.
show ktr[/avV]
Print the contents of the ktr(4) trace buffer. The v modifier
will request fully verbose output, causing the file, line number,
and timestamp to be printed for each trace entry. The V modifier
will request only the timestamps to be printed. The a modifier
will request that the output be unpaginated.
show lapic
Show information from the local APIC registers for this CPU.
show lock addr
Show lock structure. The output format is as follows:
class:
Class of the lock. Possible types include mutex(9),
rmlock(9), rwlock(9), sx(9).
name: Name of the lock.
flags:
Flags passed to the lock initialization function. flags
values are lock class specific.
state:
Current state of a lock. state values are lock class
specific.
owner:
Lock owner.
show lockchain addr
Show all threads a particular thread at address addr is waiting
on based on non-spin locks.
show lockedbufs
Show the same information as "show buf", but for every locked
struct buf object.
show lockedvnods
List all locked vnodes in the system.
show locks
Prints all locks that are currently acquired. This command is
only available if witness(4) is included in the kernel.
show locktree
show malloc[/i]
Prints malloc(9) memory allocator statistics. If the i modifier
is specified, format output as machine-parseable comma-separated
values ("CSV"). The output columns are as follows:
Type Specifies a type of memory. It is the same as a
description string used while defining the given
memory type with MALLOC_DECLARE(9).
InUse Number of memory allocations of the given type,
for which free(9) has not been called yet.
MemUse Total memory consumed by the given allocation
type.
Requests Number of memory allocation requests for the
given memory type.
The same information can be gathered in userspace with "vmstat
-m".
show map[/f] addr
Prints the VM map at addr. If the f modifier is specified the
complete map is printed.
show msgbuf
Print the system's message buffer. It is the same output as in
the "dmesg" case. It is useful if you got a kernel panic,
attached a serial cable to the machine and want to get the boot
messages from before the system hang.
show mount [addr]
Displays details about the mount point located at addr. If no
addr is specified, displays short info about all currently
mounted file systems.
show object[/f] addr
Prints the VM object at addr. If the f option is specified the
complete object is printed.
show panic
Print the panic message if set.
show page
Show statistics on VM pages.
show pageq
Show statistics on VM page queues.
show pciregs
Print PCI bus registers. The same information can be gathered in
userspace by running "pciconf -lv".
show pcpu
Print current processor state. The output format is as follows:
cpuid Processor identifier.
curthread Thread pointer, process identifier and
the name of the process.
curpcb Control block pointer.
fpcurthread FPU thread pointer.
idlethread Idle thread pointer.
APIC ID CPU identifier coming from APIC.
currentldt LDT pointer.
spin locks held Names of spin locks held.
show pgrpdump
Dump process groups present within the system.
show prison [addr]
Show the prison structure located at addr. If no addr argument
is specified, show information about all prisons in the system.
show proc [addr]
Show information about the process structure located at address
addr, or the current process if no argument is specified.
show procvm [addr]
Show process virtual memory layout for the process located at
addr, or the current process if no argument is specified.
show protosw addr
Print protocol switch structure struct protosw at address addr.
show registers[/u]
Display the register set. If the u modifier is specified, the
register contents of the thread's previous trapframe are
displayed instead. Usually, this corresponds to the saved state
from userspace.
show rman addr
Show resource manager object struct rman at address addr.
Addresses of particular pointers can be gathered with "show
allrman" command.
show route addr
Show route table result for destination addr. At this time, INET
and INET6 formatted addresses are supported.
show routetable [af]
Show full route table or tables. If af is specified, show only
routes for the given numeric address family. If no argument is
specified, dump the route table for all address families.
show rtc
Show real time clock value. Useful for long debugging sessions.
show sleepchain
Deprecated. Now an alias for show lockchain.
show sleepq addr
show sleepqueue addr
Show the sleepqueue(9) structure located at addr.
show sockbuf addr
Show the socket buffer struct sockbuf located at addr.
show socket addr
Show the socket object struct socket located at addr.
show sysregs
Show system registers (e.g., cr0-4 on i386.) Not present on some
platforms.
show tcpcb[/bi] addr
Print TCP control block struct tcpcb lying at address addr. For
exact interpretation of output, visit netinet/tcp.h header file.
The b modifier will request BBLog entries to be printed. If the
i modifier is provided, the corresponding IP control block is
also shown.
show thread [addr | tid]
If no addr or tid is specified, show detailed information about
current thread. Otherwise, print information about the thread
with ID tid or kernel address addr. (If the argument is a
decimal number, it is assumed to be a tid.)
show threads
Show all threads within the system. Output format is as follows:
First column Thread identifier (TID)
Second column Thread structure address
Third column Backtrace.
show tty addr
Display the contents of a TTY structure in a readable form.
show turnstile addr
Show turnstile struct turnstile structure at address addr.
Turnstiles are structures used within the FreeBSD kernel to
implement synchronization primitives which, while holding a
specific type of lock, cannot sleep or context switch to another
thread. Currently, those are: mutex(9), rwlock(9), rmlock(9).
show uma[/i]
Show UMA allocator statistics. If the i modifier is specified,
format output as machine-parseable comma-separated values
("CSV"). The output contains the following columns:
Zone Name of the UMA zone. The same string that was
passed to uma_zcreate(9) as a first argument.
Size Size of a given memory object (slab).
Used Number of slabs being currently used.
Free Number of free slabs within the UMA zone.
Requests Number of allocations requests to the given
zone.
Total Mem Total memory in use (either allocated or free)
by a zone, in bytes.
XFree Number of free slabs within the UMA zone that
were freed on a different NUMA domain than
allocated. (The count in the Free column is
inclusive of XFree.)
The same information might be gathered in the userspace with the
help of "vmstat -z".
show unpcb addr
Shows UNIX domain socket private control block struct unpcb
present at the address addr.
show vmochk
Prints, whether the internal VM objects are in a map somewhere
and none have zero ref counts.
show vmopag
Walk the list of VM objects in the system, printing the indices
and physical addresses of the VM pages belonging to each object.
show vnet addr
Prints virtualized network stack struct vnet structure present at
the address addr.
show vnode addr
Prints vnode struct vnode structure lying at addr. For the exact
interpretation of the output, look at the sys/vnode.h header
file.
show vnodebufs addr
Shows clean/dirty buffer lists of the vnode located at addr.
show vpath addr
Walk the namecache to lookup the pathname of the vnode located at
addr.
show watches
Displays all watchpoints. Shows watchpoints set with "watch"
command.
show witness
Shows information about lock acquisition coming from the
witness(4) subsystem.
OFFLINE DEBUGGING COMMANDS
dump Initiate a kernel core dump to the device(s) configured by
dumpon(8).
gdb Switches to remote GDB mode. In remote GDB mode, another machine
is required that runs gdb(1) (ports/devel/gdb) using the remote
debug feature, with a connection to the serial console port on
the target machine.
netdump -s server [-g gateway -c client -i iface]
Configure netdump(4) with the provided parameters, and
immediately perform a netdump.
There are some known limitations. Principally, netdump(4) only
supports IPv4 at this time. The address arguments to the netdump
command must be dotted decimal IPv4 addresses. (Hostnames are
not supported.) At present, the command only works if the
machine is in a panic state. Finally, the ddb netdump command
does not provide any way to configure compression or encryption.
netgdb -s server [-g gateway -c client -i iface]
Initiate a netgdb(4) session with the provided parameters.
netgdb has identical limitations to netdump.
capture on
capture off
capture reset
capture status
ddb supports a basic output capture facility, which can be used
to retrieve the results of debugging commands from userspace
using sysctl(3). capture on enables output capture; capture off
disables capture. capture reset will clear the capture buffer
and disable capture. capture status will report current buffer
use, buffer size, and disposition of output capture.
Userspace processes may inspect and manage ddb capture state
using sysctl(8):
debug.ddb.capture.bufsize may be used to query or set the current
capture buffer size.
debug.ddb.capture.maxbufsize may be used to query the compile-
time limit on the capture buffer size.
debug.ddb.capture.bytes may be used to query the number of bytes
of output currently in the capture buffer.
debug.ddb.capture.data returns the contents of the buffer as a
string to an appropriately privileged process.
This facility is particularly useful in concert with the
scripting and textdump(4) facilities, allowing scripted debugging
output to be captured and committed to disk as part of a textdump
for later analysis. The contents of the capture buffer may also
be inspected in a kernel core dump using kgdb(1)
(ports/devel/gdb).
run
script
scripts
unscript
Run, define, list, and delete scripts. See the SCRIPTING section
for more information on the scripting facility.
textdump dump
textdump set
textdump status
textdump unset
Use the textdump dump command to immediately perform a textdump.
More information may be found in textdump(4). The textdump set
command may be used to force the next kernel core dump to be a
textdump rather than a traditional memory dump or minidump.
textdump status reports whether a textdump has been scheduled.
textdump unset cancels a request to perform a textdump as the
next kernel core dump.
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 the built-in pager.
Setting it to 0 disables paging.
tabstops Tab stop width.
workxx Work variable; xx can take values from 0 to 31.
EXPRESSIONS
Most 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
symbol, 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.
SCRIPTING
ddb supports a basic scripting facility to allow automating tasks or
responses to specific events. Each script consists of a list of DDB
commands to be executed sequentially, and is assigned a unique name.
Certain script names have special meaning, and will be automatically run
on various ddb events if scripts by those names have been defined.
The script command may be used to define a script by name. Scripts
consist of a series of ddb commands separated with the `;' character.
For example:
script kdb.enter.panic=bt; show pcpu
script lockinfo=show alllocks; show lockedvnods
The scripts command lists currently defined scripts.
The run command execute a script by name. For example:
run lockinfo
The unscript command may be used to delete a script by name. For
example:
unscript kdb.enter.panic
These functions may also be performed from userspace using the ddb(8)
command.
Certain scripts are run automatically, if defined, for specific ddb
events. The follow scripts are run when various events occur:
kdb.enter.acpi The kernel debugger was entered as a result of an
acpi(4) event.
kdb.enter.bootflags The kernel debugger was entered at boot as a result
of the debugger boot flag being set.
kdb.enter.break The kernel debugger was entered as a result of a
serial or console break.
kdb.enter.cam The kernel debugger was entered as a result of a
CAM(4) event.
kdb.enter.mac The kernel debugger was entered as a result of an
assertion failure in the mac_test(4) module of the
TrustedBSD MAC Framework.
kdb.enter.netgraph The kernel debugger was entered as a result of a
netgraph(4) event.
kdb.enter.panic panic(9) was called.
kdb.enter.powerpc The kernel debugger was entered as a result of an
unimplemented interrupt type on the powerpc
platform.
kdb.enter.sysctl The kernel debugger was entered as a result of the
debug.kdb.enter sysctl being set.
kdb.enter.unionfs The kernel debugger was entered as a result of an
assertion failure in the union file system.
kdb.enter.unknown The kernel debugger was entered, but no reason has
been set.
kdb.enter.vfslock The kernel debugger was entered as a result of a VFS
lock violation.
kdb.enter.watchdog The kernel debugger was entered as a result of a
watchdog firing.
kdb.enter.witness The kernel debugger was entered as a result of a
witness(4) violation.
In the event that none of these scripts is found, ddb will attempt to
execute a default script:
kdb.enter.default The kernel debugger was entered, but a script
exactly matching the reason for entering was not
defined. This can be used as a catch-all to handle
cases not specifically of interest; for example,
kdb.enter.witness might be defined to have special
handling, and kdb.enter.default might be defined to
simply panic and reboot.
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.
There are many PCI and PCIe add-in cards which can generate NMI for
debugging. Modern server systems typically use IPMI to generate signals
to enter the debugger. The devel/ipmitool port can be used to send the
chassis power diag command which delivers an NMI to the processor.
Embedded systems often use JTAG for debugging, but rarely use it in
combination with ddb.
Serial consoles can break to the debugger by sending a BREAK condition on
the serial line. This requires a kernel built with options
BREAK_TO_DEBUGGER is specified in the kernel. Most terminal emulation
programs can send a break sequence with a special key sequence or menu
selection. Sending the break can be difficult or even happen spuriously
in some setups. An alternative method is to build a kernel with options
ALT_BREAK_TO_DEBUGGER then the sequence of CR TILDE CTRL-B enters the
debugger; CR TILDE CTRL-P causes a panic; and CR TILDE CTRL-R causes an
immediate reboot. In all these sequences, CR represents Carriage Return
and is usually sent by pressing the Enter or Return key. TILDE is the
ASCII tilde character (~). CTRL-x is Control x, sent by pressing the
Control key, then x, then releasing both.
The break-to-debugger behavior can be enabled by setting sysctl(8)
debug.kdb.break_to_debugger to 1. The alt-break-to-debugger behavior can
be enabled by setting sysctl(8) debug.kdb.alt_break_to_debugger to 1.
The debugger can be entered by setting sysctl(8) debug.kdb.enter to 1.
Output can be interrupted, paused, and resumed with the control
characters CTRL-C, CTRL-S, and CTRL-Q. Because these control characters
are received as in-band data from the console, there is an input buffer,
and once that buffer fills ddb must either stop responding to control
characters or drop additional input while continuing to search for
control characters. This behavior is controlled by the tunable sysctl(8)
debug.ddb.prioritize_control_input, which defaults to 1. The input
buffer size is 512 bytes.
FILES
Header files mentioned in this manual page can be found below
/usr/include directory.
- sys/buf.h
- sys/domain.h
- netinet/in_pcb.h
- sys/socket.h
- sys/vnode.h
SEE ALSO
gdb(1) (ports/devel/gdb), kgdb(1) (ports/devel/gdb), acpi(4), CAM(4),
gdb(4), mac_ddb(4), mac_test(4), netgraph(4), textdump(4), witness(4),
ddb(8), sysctl(8), panic(9)
HISTORY
The ddb debugger was developed for Mach, and ported to 386BSD-0.1. This
manual page translated from man(7) macros by Garrett Wollman.
Robert N. M. Watson added support for ddb output capture, textdump(4) and
scripting in FreeBSD 7.1.
FreeBSD 15.1-STABLE-HBSD October 31, 2025 DDB(4)