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ddb (4)
  • >> ddb (4) ( FreeBSD man: Специальные файлы /dev/* )
  • ddb (8) ( FreeBSD man: Команды системного администрирования )

  • BSD mandoc


     - interactive kernel debugger


    options KDB options DDB

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


    The 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 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:

    one more page
    one more line
    abort the current command, and return to the command input mode

    Finally, 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 line.  


    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:

    look at by bytes (8 bits)
    look at by half words (16 bits)
    look at by long words (32 bits)
    print the location being displayed
    print the location with a line number if possible
    display in unsigned hex
    display in signed hex
    display in unsigned octal
    display in signed decimal
    display in unsigned decimal
    display in current radix, signed
    display low 8 bits as a character. Non-printing characters are displayed as an octal escape code (e.g., `\000' ) .
    display the null-terminated string at the location. Non-printing characters are displayed as octal escapes.
    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.
    display as an instruction
    display as an instruction with possible alternate formats depending on the machine:

    Show the registers of the instruction.
    No alternate format.
    No alternate format.
    No alternate format.
    No alternate format.
    No alternate format.

    display a symbol name for the pointer stored at the address

    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 [/ acdoruxz ]
    p [/ acdoruxz ]
    Print addr s 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 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.

    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 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 platforms.

    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.

    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 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).

    Hard reset the system.

    Print a short summary of the available commands and command abbreviations.

    capture on
    capture off
    capture reset
    capture status
    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 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 Sx 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:

    Input and output radix.
    Addresses are printed as ``symbol + offset '' unless offset is greater than maxoff
    The width of the displayed line.
    The number of lines. It is used by the built-in pager.
    Tab stop width.
    work xx
    Work variable; xx can take values from 0 to 31.



    Most expression operators in C are supported except `~' , `^' , and unary `&' Special rules in are:

    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.
    Radix is determined by the first two letters: `0x' : hex, `0o' : octal, `0t' : decimal; otherwise, follow current radix.
    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.



    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 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 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 events. The follow scripts are run when various events occur:

    The kernel debugger was entered as a result of an acpi(4) event.
    The kernel debugger was entered at boot as a result of the debugger boot flag being set.
    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.
    The kernel debugger was entered as a result of an assertion failure in the mac_test4 module of the TrustedBSD MAC Framework.
    The kernel debugger was entered as a result of an ndis(4) breakpoint event.
    The kernel debugger was entered as a result of a netgraph(4) event.
    panic(9) was called.
    The kernel debugger was entered as a result of a powerfail NMI on the sparc64 platform.
    The kernel debugger was entered as a result of an unimplemented interrupt type on the powerpc platform.
    The kernel debugger was entered as a result of the debug.kdb.enter sysctl being set.
    The kernel debugger was entered as a result of a trapsig event on the sparc64 or sun4v platform.
    The kernel debugger was entered as a result of an assertion failure in the union file system.
    The kernel debugger was entered, but no reason has been set.
    The kernel debugger was entered as a result of a VFS lock violation.
    The kernel debugger was entered as a result of a watchdog firing.
    The kernel debugger was entered as a result of a witness(4) violation.

    In the event that none of these scripts is found, will attempt to execute a default script:

    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 . 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_text4, ndis(4), netgraph(4), textdump(4), witness(4), ddb(8), sysctl(8), panic(9)  


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

    An Robert N. M. Watson added support for output capture, textdump(4) and scripting in Fx 7.1 .




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