Manual Pages  — DDB

ddb – interactive kernel debugger

SYNOPSIS DESCRIPTION COMMANDS VARIABLES EXPRESSIONS SCRIPTING HINTS FILES SEE ALSO HISTORY

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 an instruction
I	display as an 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 = " $ecx "\n" will print like: eax = xxxxxx ecx = yyyyyy
write[/ bhl] addr expr1 [expr2 ...]
w[/ bhl] addr expr1 [expr2 ...] Write the expressions specified after addr on the command line at succeeding locations starting with addr. The write unit size can be specified in the modifier with a letter b (byte), h (half word) or l (long word) respectively. If omitted, long word is assumed. Warning: since there is no delimiter between expressions, strange things may happen. It is best to enclose each expression in parentheses.
set$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.
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.
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.
findstack addr Prints the thread address for a thread kernel-mode stack of which contains the specified address. If the thread is not found, search the thread stack cache and prints the cached stack address. Otherwise, prints nothing.
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 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 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 cbstat	Show brief information about the TTY subsystem.
show cdev	Without argument, show the list of all created cdev's, consisting of devfs node name and struct cdev address. When address of cdev is supplied, show some internal devfs state of the cdev.
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 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 jails	Show the list of jail(8) instances. In addition to what jls(8) shows, also list kernel internal details.
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
	Prints malloc(9) memory allocator statistics. The output format is 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".

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	Displays short info about all currently mounted file systems.
show mount addr	Displays details about the given mount point.
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".
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 proc [addr]	If no [addr] is specified, print information about the current process. Otherwise, show information about the process at address addr.
show procvm	Show process virtual memory layout.
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 rtc	Show real time clock value. Useful for long debugging sessions.
show sleepchain	Deprecated. Now an alias for show lockchain.
show sleepq show sleepqueue
	Both commands provide the same functionality. They show sleepqueue struct sleepqueue structure. Sleepqueues are used within the FreeBSD kernel to implement sleepable synchronization primitives (thread holding a lock might sleep or be context switched), which at the time of writing are: condvar(9), sx(9) and standard msleep(9) interface.
show sockbuf addr show socket addr
	Those commands print struct sockbuf and struct socket objects placed at addr. Output consists of all values present in structures mentioned. For exact interpretation and more details, visit sys/socket.h header file.
show sysregs	Show system registers (e.g., cr0-4 on i386.) Not present on some platforms.
show tcpcb addr	Print TCP control block struct tcpcb lying at address addr. For exact interpretation of output, visit netinet/tcp.h header file.
show thread [addr]	If no addr is specified, show detailed information about current thread. Otherwise, information about thread at addr is printed.
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	Show UMA allocator statistics. Output consists five 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.

The very same information might be gathered in the userspace with the help of "vmstat".

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	This is supposed to show physical addresses consumed by a VM object. Currently, it is not possible to use this command when witness(4) is compiled in the kernel.
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.
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 architecture.
halt	Halt the system.
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).
reboot [seconds] reset [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.
help	Print a short summary of the available commands and command abbreviations.
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).
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.

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.

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. ‘amp;.’ and ‘amp;:’ 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.
amp;.	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 ‘amp;:’ 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 ‘amp;:’ and modifiers as described above.

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