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ddb - interactive kernel debugger



      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 hexadec‐
      imal 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 charac‐
                      ters 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.
      xf      Examine forward: execute an examine command with the last speci‐
              fied 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 speci‐
              fied parameters to it except that the last start address sub‐
              tracted by the size displayed by it is used as the start address.
              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 con‐
              sidered 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 debug‐
              ger, user space break points may not work correctly.  Setting a
              break point at the low-level code paths may also cause strange
      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 spec‐
              ified 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 unrecov‐
              erable 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 archi‐
              tecture.  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 proba‐
              bly 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 archi‐
      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


      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 sym‐
                   bol, 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.


      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.


      The ddb debugger was developed for Mach, and ported to 386BSD 0.1.  This
      manual page translated from man(7) macros by Garrett Wollman.


Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.