Ubuntu Feisty 7.04 manual page repository

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      ddb - interactive kernel debugger


      options DDB
      options KDB_UNATTENDED


      The ddb kernel debugger has most of the features of the old kdb, but with
gdb(1).  If linked into the running
keymap(5) action.  The
panic(9) if the
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 hex‐
      adecimal format at a prompt.  Examine and write commands 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
      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 print‐
      ing commands or infinity for stack traces.
more(1) command for the output.
      If an output line exceeds the number set in the $lines variable, it dis‐
      plays “--db_more--” and waits for a response.  The valid responses for it
      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


      Display the addressed locations according to the formats in the modifier.
      Multiple modifier formats display multiple locations.  If no format is
      specified, the last formats 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 depend‐
              ing 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.
      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.
      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 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.  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 ...]
      Write the expressions specified after addr on the command line at suc‐
      ceeding 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 vari‐
      able names are described below.
      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
      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
      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
      delete #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.
      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 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
      Stop at the next call or return instruction.  If the p modifier is speci‐
      fied, 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] [frame] [,count]
      Stack trace.  The u option traces user space; if omitted, trace only
      traces kernel space.  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]
      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 mod‐
      ifier 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 option 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 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 watches
      Displays all watchpoints.
      Hard reset the system.
      watch addr,size
      Set a watchpoint for a region.  Execution stops when an attempt to modify
      the region occurs.  The size argument defaults to 4.  If you specify a
      wrong space address, the request is rejected with an error message.
      Warning: Attempts to watch wired kernel memory may cause unrecoverable
      error in some systems such as i386.  Watchpoints on user addresses work
      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.
      Toggles between remote GDB and DDB mode.  In remote GDB mode, another
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 and Alpha architectures.
      Print a short summary of the available commands and command abbrevia‐


      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 “more” feature.
      tabstops  Tab stop width.
      workxx    Work variable.  xx can be 0 to 31.


      Almost all 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 macros by Garrett Wollman.


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