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DDB(4)                 FreeBSD Kernel Interfaces Manual                 DDB(4)

     ddb - interactive kernel debugger

     options KDB
     options DDB

     To prevent activation of the debugger on kernel panic(9):
     options KDB_UNATTENDED

     The ddb kernel debugger has most of the features of the old kdb, but with
     a more rational 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 might be used to
     browse through the history buffer, and move the cursor within the current

     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.,
             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:
                     alpha        Show the registers of the instruction.
                     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

     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

             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

             Warning: since there is no delimiter between expressions, strange
             things may happen.  It is best to enclose each expression in

     set $variable [=] expr
             Set the named variable or register with the value of expr.  Valid
             variable names are described below.

     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

             If the u modifier is specified, this command sets a break point
             in user space address.  Without the u option, the address is
             considered in the kernel space, and wrong space address is
             rejected with an error message.  This modifier can be used only
             if it is supported by machine dependent routines.

             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.

     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.

     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.

             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.

             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 info.

     show registers[/u]
             Display the register set.  If the u modifier is specified, it
             displays user registers instead of kernel or currently saved one.

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

     show sysregs
             Show system registers (e.g., cr0-4 on i386.)  Not present on some

     show geom [addr]
             If the addr argument is not given, displays the entire GEOM
             topology.  If the addr is given, displays details about the given
             GEOM object (class, geom, provider or consumer).

     show map[/f] addr
             Prints the VM map at addr.  If the f modifier is specified the
             complete map is printed.

     show object[/f] addr
             Prints the VM object at addr.  If the f option is specified the
             complete object is printed.

     show vnode addr
             Displays details about the given vnode.

     show watches
             Displays all watchpoints.

     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

     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).

     reset   Hard reset the system.

     help    Print a short summary of the available commands and command

     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.

    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, 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).

     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.

     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

     .                dot

     +                next

     ..               address of the start of the last line examined.  Unlike
                      dot or next, this is only changed by examine or write

     '                last address explicitly specified.

     $variable        Translated to the value of the specified variable.  It
                      may be followed by a `:' and modifiers as described

     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.

     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

           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

           unscript kdb.enter.panic

     These functions may also be performed from userspace using the ddb(8)

     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.        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

     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.

     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.

     gdb(1), kgdb(1), acpi(4), CAM(4), mac_text(4), ndis(4), netgraph(4),
     textdump(4), witness(4), ddb(8), sysctl(8), panic(9)

     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 11.0-PRERELEASE        December 26, 2007       FreeBSD 11.0-PRERELEASE


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