---
name: "ddb"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "openbsd"]
url: "https://new.carta.tech/ddb/4"
---

# ddb(4)

interactive kernel debugger

## 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)](/panic/9.md):

**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(4)](/gdb/4.md) backend, so that remote debugging with kgdb(1) (*ports/devel/gdb*) is possible, include:

**options** **GDB**

## DESCRIPTION

The **ddb** kernel debugger is an interactive debugger with a syntax inspired by [gdb(1)](/gdb/1.md) (*ports/devel/gdb*). If linked into the running kernel, it can be invoked locally with the ‘debug’ [keymap(5)](/keymap/5.md) action, usually mapped to Ctrl+Alt+Esc, or by setting the *debug.kdb.enter* sysctl to 1. The debugger is also invoked on kernel [panic(9)](/panic/9.md) if the *debug.debugger\_on\_panic* [sysctl(8)](/sysctl/8.md) MIB variable is set non-zero, which is the default unless the KDB\_UNATTENDED option is specified. Similarly, if the *debug.debugger\_on\_recursive\_panic* variable is set to 1, then the debugger will be invoked on a recursive kernel panic. This variable has a default value of 0, and has no effect if *debug.debugger\_on\_panic* is already set non-zero.

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*\[/*modifier*\] \[*addr*\]\[,*count*\]

A blank line repeats the previous command from the address *next* with count 1 and no modifiers. Specifying *addr* sets *dot* to the address. Omitting *addr* uses *dot*. A missing *count* is taken to be 1 for printing commands or infinity for stack traces. A *count* of -1 is equivalent to a missing *count*. Options that are supplied but not supported by the given *command* are usually ignored.

The **ddb** debugger has a pager feature (like the [more(1)](/more/1.md) 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

### COMMON DEBUGGER COMMANDS

- **help**

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

-

  **examine**\[/**AISabcdghilmorsuxz** **...**\] \[*addr*\]\[,*count*\]

-

  **x**\[/**AISabcdghilmorsuxz** **...**\] \[*addr*\]\[,*count*\] 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)

  - **g**

    look at by quad words (64 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 a disassembled instruction

  - **I**

    display as a disassembled instruction with possible alternate formats depending on the machine. On i386, this selects the alternate format for the instruction decoding (16 bits in a 32-bit code segment and vice versa).

  - **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 *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
  ```

- **pprint**\[/**d** **depth**\] \[*name*\]

  Pretty-print symbol specified by *name* using CTF debugging data. Works for all symbols exported by the kernel and loaded kernel modules.

  If the **d** modifier has been specified, contents of structs nested up to *depth* levels deep will also be included in the output.

- **pprint** **struct**\[/**d** **depth**\] \[*name*\]\[*addr*\]

  Print memory at *addr* as struct *name*. Works for all structs defined by the kernel and loaded kernel modules.

  If the **d** modifier has been specified, contents of structs nested up to *depth* levels deep will also be included in the output.

-

  **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**\[/**u**\] \[*addr*\]\[,*count*\]

  - **b**\[/**u**\] \[*addr*\]\[,*count*\]

  Set a break point at *addr*. If *count* is supplied, the **continue** command will not stop at this break point on the first *count* - 1 times that it is hit. 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 specified break point. The break point can be specified by a break point number with ‘#’, or by using the same *addr* specified in the original **break** command, or by omitting *addr* to get the default address of *dot*.

- **halt**

  Halt 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 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.

- **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)](/signal/3.md) for a list of signals. Note that the arguments are reversed relative to [kill(2)](/kill/2.md).

- **step**\[/**p**\]\[,*count*\]

  - **s**\[/**p**\]\[,*count*\]

  Single step *count* times. 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*. The optional *count* argument limits the search.

-

  **reboot**\[/**s**\] \[*seconds*\]

-

  **reset**\[/**s**\] \[*seconds*\] Hard reset the system. If the optional argument *seconds* is given, the debugger will wait for this long, at most a week, before rebooting. When the **s** modifier is given, the command will skip running any registered shutdown handlers and attempt the most basic reset.

- **thread** *addr* \| *tid*

  Switch the debugger to the thread with ID *tid*, if the argument is a decimal number, or address *addr*, otherwise.

- **watchdog** \[*exp*\]

  Program the [watchdog(4)](/watchdog/4.md) timer to fire in 2^*exp* seconds. If no argument is provided, the watchdog timer is disabled.

### SPECIALIZED HELPER COMMANDS

-

  **findstack** *addr* Prints the address of the thread whose kernel-mode stack contains *addr*, if any.

- **show** **active** **trace**

  - **acttrace**

  Show a stack trace for every thread running on a CPU.

- **show** **all** **procs**\[/**a**\]

  - **ps**\[/**a**\]

  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 **a** modifier will print command line arguments for each process.

- **show** **all** **tcpcbs**\[/**b****i****l**\]

  Show the same output as "show tcpcb" does, but for all TCP control blocks within the system. The **b** modifier will request BBLog entries to be printed. If the **i** modifier is provided, the corresponding IP control block is also shown. Using the **l** modifier will limit the output to TCP control blocks, which are locked.

- **show** **all** **trace**

  - **alltrace**

  Show a stack trace for every thread in the system.

- **show** **all** **ttys**

  Show all TTY's within the system. Output is similar to [pstat(8)](/pstat/8.md), but also includes the address of the TTY structure.

- **show** **all** **vnets**

  Show the same output as "show vnet" does, but lists all virtualized network stacks within the system.

- **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)](/witness/4.md) 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, I/O memory addresses, and Resource IDs.

- **show** **apic**

  Dump data about APIC IDT vector mappings.

- **show** **badstacks**

  Walk the [witness(4)](/witness/4.md) graph and print any lock-order violations. This command is only available if [witness(4)](/witness/4.md) is included in the kernel.

- **show** **breaks**

  Show breakpoints set with the "break" command.

- **show** **bio** *addr*

  Show information about the bio structure *struct* *bio* present at *addr*. See the *sys/bio.h* header file and [g\_bio(9)](/g_bio/9.md) for more details on the exact meaning of the structure fields.

- **show** **buffer** *addr*

  Show information about the buf structure *struct* *buf* present at *addr*. See the *sys/buf.h* header file for more details on the exact meaning of the structure fields.

- **show** **callout** *addr*

  Show information about the callout structure *struct* *callout* present at *addr*.

- **show** **cdev** \[*addr*\]

  Show the internal devfs state of the cdev structure located at *addr*. If no argument is provided, show the list of all created cdevs, consisting of the devfs node name and the *struct* *cdev* address.

- **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)](/cpuset/2.md) for more details.

- **show** **cyrixreg**

  Show registers specific to the Cyrix processor.

- **show** **devmap**

  Prints the contents of the static device mapping table. Currently only available on the ARM architecture.

- **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** **igi\_list** *addr*

  Show information about the IGMP structure *struct* *igmp\_ifsoftc* present at *addr*.

- **show** **iosched** *addr*

  Show information about the I/O scheduler *struct* *cam\_iosched\_softc* located at *addr*.

- **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** **ktr**\[/**a****v****V**\]

  Print the contents of the [ktr(4)](/ktr/4.md) trace buffer. The **v** modifier will request fully verbose output, causing the file, line number, and timestamp to be printed for each trace entry. The **V** modifier will request only the timestamps to be printed. The **a** modifier will request that the output be unpaginated.

- **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)](/mutex/9.md), [rmlock(9)](/rmlock/9.md), [rwlock(9)](/rwlock/9.md), [sx(9)](/sx/9.md).

  - **name**:

    Name of the lock.

  - **flags**:

    Flags passed to the lock initialization function. *flags* values are lock class specific.

  - **state**:

    Current state of a lock. *state* values are lock class specific.

  - **owner**:

    Lock owner.

- **show** **lockchain** *addr*

  Show all threads a particular thread at address *addr* is waiting on based on 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)](/witness/4.md) is included in the kernel.

- **show** **locktree**

  - **show** **malloc**\[/**i**\]

  Prints [malloc(9)](/malloc/9.md) memory allocator statistics. If the **i** modifier is specified, format output as machine-parseable comma-separated values ("CSV"). The output columns are 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)](/MALLOC_DECLARE/9.md).

  - **InUse**

    Number of memory allocations of the given type, for which [free(9)](/free/9.md) 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** \[*addr*\]

  Displays details about the mount point located at *addr*. If no *addr* is specified, displays short info about all currently mounted file systems.

- **show** **object**\[/**f**\] *addr*

  Prints the VM object at *addr*. If the **f** option is specified the complete object is printed.

- **show** **panic**

  Print the panic message if set.

- **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** **preload**

  Pretty-print the metadata passed to the kernel by [loader(8)](/loader/8.md).

- **show** **prison** \[*addr*\]

  Show the prison structure located at *addr*. If no *addr* argument is specified, show information about all prisons in the system.

- **show** **proc** \[*addr*\]

  Show information about the process structure located at address *addr*, or the current process if no argument is specified.

- **show** **procvm** \[*addr*\]

  Show process virtual memory layout for the process located at *addr*, or the current process if no argument is specified.

- **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, the register contents of the thread's previous trapframe are displayed instead. Usually, this corresponds to the saved state from userspace.

- **show** **rman** *addr*

  Show resource manager object *struct* *rman* at address *addr*. Addresses of particular pointers can be gathered with "show allrman" command.

- **show** **route** *addr*

  Show route table result for destination *addr*. At this time, INET and INET6 formatted addresses are supported.

- **show** **routetable** \[*af*\]

  Show full route table or tables. If *af* is specified, show only routes for the given numeric address family. If no argument is specified, dump the route table for all address families.

- **show** **rtc**

  Show real time clock value. Useful for long debugging sessions.

- **show** **sleepchain**

  Deprecated. Now an alias for **show** **lockchain**.

- **show** **sleepq** *addr*

  - **show** **sleepqueue** *addr*

  Show the [sleepqueue(9)](/sleepqueue/9.md) structure located at *addr*.

- **show** **sockbuf** *addr*

  Show the socket buffer *struct* *sockbuf* located at *addr*.

- **show** **socket** *addr*

  Show the socket object *struct* *socket* located at *addr*.

- **show** **sysregs**

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

- **show** **tcpcb**\[/**b****i**\] *addr*

  Print TCP control block *struct* *tcpcb* lying at address *addr*. For exact interpretation of output, visit *netinet/tcp.h* header file. The **b** modifier will request BBLog entries to be printed. If the **i** modifier is provided, the corresponding IP control block is also shown.

- **show** **thread** \[*addr* \| *tid*\]

  If no *addr* or *tid* is specified, show detailed information about current thread. Otherwise, print information about the thread with ID *tid* or kernel address *addr*. (If the argument is a decimal number, it is assumed to be a tid.)

- **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)](/mutex/9.md), [rwlock(9)](/rwlock/9.md), [rmlock(9)](/rmlock/9.md).

- **show** **uma**\[/**i**\]

  Show UMA allocator statistics. If the **i** modifier is specified, format output as machine-parseable comma-separated values ("CSV"). The output contains the following columns:

  - **Zone**

    Name of the UMA zone. The same string that was passed to [uma\_zcreate(9)](/uma_zcreate/9.md) 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.

  - **Total Mem**

    Total memory in use (either allocated or free) by a zone, in bytes.

  - **XFree**

    Number of free slabs within the UMA zone that were freed on a different NUMA domain than allocated. (The count in the **Free** column is inclusive of **XFree**.)

    The same information might be gathered in the userspace with the help of “**vmstat** **-z**”.

- **show** **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**

  Walk the list of VM objects in the system, printing the indices and physical addresses of the VM pages belonging to each object.

- **show** **vnet** *addr*

  Prints virtualized network stack *struct* *vnet* structure present at the address *addr*.

- **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** **vpath** *addr*

  Walk the namecache to lookup the pathname 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)](/witness/4.md) subsystem.

### OFFLINE DEBUGGING COMMANDS

- **dump**

  Initiate a kernel core dump to the device(s) configured by [dumpon(8)](/dumpon/8.md).

- **gdb**

  Switches to remote GDB mode. In remote GDB mode, another machine is required that runs [gdb(1)](/gdb/1.md) (*ports/devel/gdb*) using the remote debug feature, with a connection to the serial console port on the target machine.

- **netdump** **-s** *server* \[**-g** *gateway* **-c** *client* **-i** *iface*\]

  Configure [netdump(4)](/netdump/4.md) with the provided parameters, and immediately perform a netdump.

  There are some known limitations. Principally, [netdump(4)](/netdump/4.md) only supports IPv4 at this time. The address arguments to the **netdump** command must be dotted decimal IPv4 addresses. (Hostnames are not supported.) At present, the command only works if the machine is in a panic state. Finally, the **ddb** **netdump** command does not provide any way to configure compression or encryption.

- **netgdb** **-s** *server* \[**-g** *gateway* **-c** *client* **-i** *iface*\]

  Initiate a [netgdb(4)](/netgdb/4.md) session with the provided parameters.

  **netgdb** has identical limitations to **netdump**.

- **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 userspace using [sysctl(3)](/sysctl/3.md). **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)](/sysctl/8.md):

  *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)](/textdump/4.md) 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) (*ports/devel/gdb*).

- **run**

  - **script**

  - **scripts**

  - **unscript**

  Run, define, list, and delete scripts. See the SCRIPTING section for more information on the scripting facility.

- **textdump** **dump**

  - **textdump** **set**

  - **textdump** **status**

  - **textdump** **unset**

  Use the **textdump** **dump** command to immediately perform a textdump. More information may be found in [textdump(4)](/textdump/4.md). 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.

## 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. Setting it to 0 disables paging.

- *tabstops*

  Tab stop width.

- *work**xx*

  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)](/ddb/8.md) 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)](/acpi/4.md) 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)](/CAM/4.md) event.

- *kdb.enter.mac*

  The kernel debugger was entered as a result of an assertion failure in the [mac\_test(4)](/mac_test/4.md) module of the TrustedBSD MAC Framework.

- *kdb.enter.netgraph*

  The kernel debugger was entered as a result of a [netgraph(4)](/netgraph/4.md) event.

- *kdb.enter.panic*

  [panic(9)](/panic/9.md) was called.

- *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.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)](/witness/4.md) 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. There are many PCI and PCIe add-in cards which can generate NMI for debugging. Modern server systems typically use IPMI to generate signals to enter the debugger. The *devel/ipmitool* port can be used to send the **chassis** **power** **diag** command which delivers an NMI to the processor. Embedded systems often use JTAG for debugging, but rarely use it in combination with **ddb**.

Serial consoles can break to the debugger by sending a BREAK condition on the serial line. This requires a kernel built with **options** **BREAK\_TO\_DEBUGGER** is specified in the kernel. Most terminal emulation programs can send a break sequence with a special key sequence or menu selection. Sending the break can be difficult or even happen spuriously in some setups. An alternative method is to build a kernel with **options** **ALT\_BREAK\_TO\_DEBUGGER** then the sequence of CR TILDE CTRL-B enters the debugger; CR TILDE CTRL-P causes a panic; and CR TILDE CTRL-R causes an immediate reboot. In all these sequences, CR represents Carriage Return and is usually sent by pressing the Enter or Return key. TILDE is the ASCII tilde character (~). CTRL-x is Control x, sent by pressing the Control key, then x, then releasing both.

The break-to-debugger behavior can be enabled by setting [sysctl(8)](/sysctl/8.md) *debug.kdb.break\_to\_debugger* to 1. The alt-break-to-debugger behavior can be enabled by setting [sysctl(8)](/sysctl/8.md) *debug.kdb.alt\_break\_to\_debugger* to 1. The debugger can be entered by setting [sysctl(8)](/sysctl/8.md) *debug.kdb.enter* to 1.

Output can be interrupted, paused, and resumed with the control characters CTRL-C, CTRL-S, and CTRL-Q. Because these control characters are received as in-band data from the console, there is an input buffer, and once that buffer fills **ddb** must either stop responding to control characters or drop additional input while continuing to search for control characters. This behavior is controlled by the tunable [sysctl(8)](/sysctl/8.md) *debug.ddb.prioritize\_control\_input*, which defaults to 1. The input buffer size is 512 bytes.

## FILES

Header files mentioned 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)](/gdb/1.md) (*ports/devel/gdb*), kgdb(1) (*ports/devel/gdb*), [acpi(4)](/acpi/4.md), [CAM(4)](/CAM/4.md), [gdb(4)](/gdb/4.md), [mac\_ddb(4)](/mac_ddb/4.md), [mac\_test(4)](/mac_test/4.md), [netgraph(4)](/netgraph/4.md), [textdump(4)](/textdump/4.md), [witness(4)](/witness/4.md), [ddb(8)](/ddb/8.md), [sysctl(8)](/sysctl/8.md), [panic(9)](/panic/9.md)

## HISTORY

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

Robert N. M. Watson added support for **ddb** output capture, [textdump(4)](/textdump/4.md) and scripting in FreeBSD 7.1.