Provided by: freebsd-manpages_8.0-1_all bug

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):

           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(1) backend, so that remote debugging with kgdb(1) is
     possible, include:

           options GDB

DESCRIPTION

     The ddb kernel debugger is an interactive debugger with a 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 may be used to
     browse through the history buffer, and move the cursor within the current
     line.

COMMANDS

     examine
     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.,
                     ‘\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
                     depending on the machine:
                     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
                     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 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
             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 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 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
             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 information.

     show all ttys
             Show all TTY’s within the system.  Output is similar to pstat(8),
             but also includes the address of the TTY structure.

     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) 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 and I/O
             memory addresses.

     show apic
             Dump data about APIC IDT vector mappings.

     show breaks
             Show breakpoints set with the "break" command.

     show buffer
             Show buffer structure of struct buf type.  Such a structure is
             used within the FreeBSD kernel for the I/O subsystem
             implementation.  For an exact interpretation of the output,
             please see the sys/buf.h header file.

     show cbstat
             Show brief information about the TTY subsystem.

     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) for more details.

     show cyrixreg
             Show registers specific to the Cyrix processor.

     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 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 jails
             Show the list of jail(8) instances.  In addition to what jls(8)
             shows, also list kernel internal details.

     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),
                    rmlock(9), rwlock(9), sx(9).

             name:  Name of the lock.

             flags:
                    Flags passed to the lock initialization function.  For
                    exact possibilities see manual pages of possible lock
                    types.

             state:
                    Current state of a lock.  As well as flags it’s lock-
                    specific.

             owner:
                    Lock owner.

     show lockchain addr
             Show all threads a particular thread at address addr is waiting
             on based on non-sleepable and 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) is included in the kernel.

     show locktree

     show malloc
             Prints malloc(9) memory allocator statistics.  The output format
             is 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).
                   InUse     Number of memory allocations of the given type,
                             for which free(9) 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
             Displays short info about all currently mounted file systems.

     show mount addr
             Displays details about the given mount point.

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

     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 proc [addr]
             If no [addr] is specified, print information about the current
             process.  Otherwise, show information about the process at
             address addr.

     show procvm
             Show process virtual memory layout.

     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, it
             displays user registers instead of kernel registers or the
             currently saved one.

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

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

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

     show sleepchain
             Show all the threads a particular thread is waiting on based on
             sleepable locks.

     show sleepq
     show sleepqueue
             Both commands provide the same functionality.  They show
             sleepqueue struct sleepqueue structure.  Sleepqueues are used
             within the FreeBSD kernel to implement sleepable synchronization
             primitives (thread holding a lock might sleep or be context
             switched), which at the time of writing are: condvar(9), sx(9)
             and standard msleep(9) interface.

     show sockbuf addr
     show socket addr
             Those commands print struct sockbuf and struct socket objects
             placed at addr.  Output consists of all values present in
             structures mentioned.  For exact interpretation and more details,
             visit sys/socket.h header file.

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

     show tcpcb addr
             Print TCP control block struct tcpcb lying at address addr.  For
             exact interpretation of output, visit netinet/tcp.h header file.

     show thread [addr]
             If no addr is specified, show detailed information about current
             thread.  Otherwise, information about thread at addr is printed.

     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), rwlock(9), rmlock(9).

     show uma
             Show UMA allocator statistics.  Output consists five columns:

                   Zone      Name of the UMA zone.  The same string that was
                             passed to uma_zcreate(9) 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.

             The very same information might be gathered in the userspace with
             the help of “vmstat -zshow 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
             This is supposed to show physical addresses consumed by a VM
             object.  Currently, it is not possible to use this command when
             witness(4) is compiled in the kernel.

     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 watches
             Displays all watchpoints.  Shows watchpoints set with "watch"
             command.

     show witness
             Shows information about lock acquisition coming from the
             witness(4) subsystem.

     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
             architecture.

     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).

     reboot
     reset   Hard reset the system.

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

     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.

             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) 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
     script
     scripts
     unscript
             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).

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.
     tabstops  Tab stop width.
     workxx    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)
     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) 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) 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
                          platform.

     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.

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.

FILES

     Header files mention 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), kgdb(1), acpi(4), CAM(4), mac_test(4), ndis(4), netgraph(4),
     textdump(4), witness(4), ddb(8), sysctl(8), panic(9)

HISTORY

     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.