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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(1) backend, so that remote debugging with kgdb(1) is possible, include:

           options GDB

DESCRIPTION

     The ddb kernel debugger is an interactive debugger with a 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 KDB_UNATTENDED option is
     specified.

     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] 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 printing commands or infinity for stack traces.

     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

     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
             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)
             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:
                     amd64    No alternate format.
                     i386     No alternate format.
                     ia64     No alternate format.
                     powerpc  No alternate format.
                     sparc64  No alternate format.
             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]
     b[/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 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 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.

     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]
     s[/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]
     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.  This command might fail in interesting ways if it does not
             find the searched-for value.  This is because ddb does not 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 modifier will alter the display to show
             VM map addresses for the process and not show other information.

     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 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 and I/O memory addresses.

     show apic
             Dump data about APIC IDT vector mappings.

     show breaks
             Show breakpoints set with the "break" command.

     show buffer
             Show buffer structure of struct buf type.  Such a structure is used within the
             FreeBSD kernel for the I/O subsystem implementation.  For an exact interpretation of
             the output, please see the sys/buf.h header file.

     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 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 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.  For exact possibilities
                    see manual pages of possible lock types.

             state:
                    Current state of a lock.  As well as flags it's lock-specific.

             owner:
                    Lock owner.

     show lockchain addr
             Show all threads a particular thread at address addr is waiting on based on non-
             sleepable and 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 -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
             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 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 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, it displays user
             registers instead of kernel registers or the currently saved one.

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

     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
             Show all the threads a particular thread is waiting on based on sleepable locks.

     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 -zshow 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 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 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 userpsace using sysctl(2).  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 set
     textdump status
     textdump unset
             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.  More information may be found in
             textdump(4).

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.
     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.ndis       The kernel debugger was entered as a result of an ndis(4) breakpoint
                          event.

     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.powerfail  The kernel debugger was entered as a result of a powerfail NMI on the
                          sparc64 platform.

     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.trapsig    The kernel debugger was entered as a result of a trapsig event on the
                          sparc64 or sun4v platform.

     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.

FILES

     Header files mention 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), kgdb(1), acpi(4), CAM(4), mac_test(4), ndis(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.