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DDB(4)                   BSD Kernel Interfaces Manual                   DDB(4)

NAME
     ddb -- kernel debugger

DESCRIPTION
     The ddb debugger provides a means for debugging the kernel, and analysing
     the kernel after a system crash ("panic"), with a gdb(1)-like syntax.

     ddb is invoked upon a kernel panic when the sysctl(8) ddb.panic is set to
     1.  It may be invoked from the console when the sysctl ddb.console is set
     to 1, using any of the following methods:

        -   Using the key sequence Ctrl-Alt-Esc.

        -   Sending a BREAK when using a serial console.

        -   Writing to the sysctl ddb.trigger.

        -   For i386 and amd64 architectures, using the key sequence
            Ctrl-Alt-Delete when the sysctl machdep.kbdreset is set to 2.

     ddb prompts for commands on the console with:

           ddb>

     The general syntax of a ddb command is:

           command [/modifiers]   [address][,count]

     To save typing, ddb makes use of a context inferred from previous com-
     mands.  In this context, the current location is called dot.  The
     examine, search, show struct, and write commands update dot to be that of
     the last address examined or the last location modified, and have intu-
     itive effects on next and prev.  All the other commands do not change
     dot, and set next to be the same.  (See VARIABLES.)

     An expression can be used in place of address (see EXPRESSIONS).  Omit-
     ting address in a command uses the last value of dot.  A missing count is
     taken to be 1 for printing commands or infinity for stack traces.  Enter-
     ing a blank line causes the last command to be repeated using next in
     place of address, a count of 1, and no modifiers.

     ddb has a feature like more(1) for the output.  If the number of lines
     output in response to one command exceeds the number set in the $lines
     variable, it displays the message '--db_more--' and waits for a response.

     The valid responses are:

           <space>     One more page.
           <return>    One more line.
           q           Abort the current command, and return to the command
                       input mode.

     The following command line editing keys are provided:

           ^b          back one character
           ^f          forward one character
           ^a          beginning of line
           ^e          end of line
           ^w          erase word back
           ^h | <del>  erase previous character
           ^d          erase next character
           ^k          delete to end of line
           ^u          delete line
           ^p          previous in command history
           ^n          next in command history
           ^r          redraw line
           ^t          exchange the two characters to the left of the cursor

COMMANDS
     The following commands may be typed at the 'ddb>' prompt.  Some commands
     consist of more than one word, and if only the first word or words are
     entered, the possible alternatives to complete the command are displayed
     and no other action is performed.

     help        List the available commands.

     [e]x[amine] [/bhlqaAxzodurcsmiI] [addr][,count]
                 Display the contents at address addr according to the formats
                 in the modifier.  If no format is specified, the last formats
                 specified for this command are 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) (default)
                 /q    look at by long longs (64 bits) (only available on
                       64-bit platforms)
                 /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 char-
                       acters 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 alternate format instruction depending on
                       the machine:

                       alpha     Print affected register contents for every
                                 instruction.
                       amd64,
                       i386      Do not skip padding to the next long word
                                 boundary for unconditional jumps.
                       m88k      Decode instructions for the opposite CPU
                                 model (e.g. m88110 when running on an m88100
                                 processor).
                       vax       Don't assume that each external label is a
                                 procedure entry mask.

                 The value of next is set to the addr plus the size of the
                 data examined.

     print [/axzodurc] [addr [addr ...]]
                 Print each addr according to the modifier character.  The
                 valid modifiers are a subset of those from the examine com-
                 mand, and act as described there.  If no modifier is speci-
                 fied, the last one specified in a previous use of print is
                 used.  The addr argument can be a string, and it is printed
                 as a literal.

                 For example,

                       print/x "eax = " $eax "\necx = " $ecx "\n"

                 will print something like this:

                       eax = xxxxxx
                       ecx = yyyyyy

     w[rite] [/bhl] [addr] expr [expr ...]
                 Write the value of each expr expression at succeeding loca-
                 tions start at addr.  The write unit size can be specified
                 using one of the modifiers:

                       /b    byte (8 bits)
                       /h    half word (16 bits)
                       /l    long word (32 bits) (default)

                 The value of next is set to addr plus the size of values
                 written.

                 Warning: since there is no delimiter between expressions, the
                 command may not parse as you expect.  It is best to enclose
                 each expression in parentheses.

     set $name [=] expr
                 Set the named variable or register with the value of expr.
                 Valid variable names are described below.

     boot how    Reboot the machine depending on how:

                 boot sync      Sync disks and reboot.
                 boot crash     Dump core and reboot.
                 boot dump      Sync disks, dump core and reboot.
                 boot halt      Just halt.
                 boot reboot    Just reboot.
                 boot poweroff  Power down the machine whenever possible; if
                                it fails, just halt.

     break [addr][,count]
                 Set a break point at addr.  If count is supplied, ddb allows
                 the breakpoint to be silently hit (count - 1) times before
                 stopping at the break point.

                 If the break point is successfully set, a break point number
                 is displayed, in the form #number.  This can later be used in
                 deleting the break point or for adding conditions to it.

     d[elete] [addr | #number]
                 Delete the break point set with the break command.

     s[tep] [/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.  On machines
                 with software-emulated single-stepping (e.g., alpha), step-
                 ping through code executed by interrupt handlers will proba-
                 bly do the wrong thing.

     call name(expr [, expr ...] )
                 Call the function named by name with the argument(s) listed
                 in parentheses.  Parentheses may be omitted if the function
                 takes no arguments.  The number of arguments is currently
                 limited to 10.

     c[ontinue] [/c]
                 Continue execution until a breakpoint or watchpoint.  If the
                 /c modifier is given, instructions are counted while execut-
                 ing.

                 Warning: when counting with /c, ddb is really silently sin-
                 gle-stepping.  This means that single-stepping on low-level
                 code may cause strange behavior.

     watch addr [,size]
                 Set a watchpoint for the region starting at addr.  Execution
                 stops and control returns to ddb when an attempt is made to
                 modify a watched region.  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 an
                 unrecoverable error on some systems (e.g., i386).

     dwatch addr
                 Delete the watchpoint at address addr that was previously set
                 with a watch command.

     hangman [/s[0-9]]
                 This is a tiny and handy tool for random kernel hangs analy-
                 sis, of which its depth is controlled by the optional argu-
                 ment of the default value of five.  It uses some sophisti-
                 cated heuristics to spot the global symbol that caused the
                 hang.  Since the discovering algorithm is a probabilistic
                 one, you may spend substantial time to figure the exact sym-
                 bol name.  This smart thing requires a little of your atten-
                 tion, the input it accepts is mostly of the same format as
                 that of the famous hangman(6) game, to which it, apparently,
                 is obliged by the name.  Hint: the nm(1) utility might help.

     until [/p]  Stop at the next "call" or "return" instruction.  If the /p
                 modifier is specified, ddb prints the call nesting depth and
                 the cumulative instruction count at each call or return.
                 Otherwise, it stays silent until the matching return is hit.

     match [/p]  Stop at the next matching return instruction.  If the /p mod-
                 ifier is specified, ddb prints the call nesting depth and the
                 cumulative instruction count at each call or return.  Other-
                 wise, it remains mostly quiet.

     next [/p]   The next command is a synonym for match.

     trace [/pu] [frameaddr][,count]
                 Show the stack trace.  The /p modifier interprets the
                 frameaddr argument as the PID of a process and shows the
                 stack trace of that process.  The /p modifier is not sup-
                 ported on all platforms.  The /u modifier shows the stack
                 trace of user space; if omitted, the kernel stack is traced
                 instead.  The count argument is the limit on the number of
                 frames to be followed.  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 a value beginning at addr.  This command
                 might fail in interesting ways if it doesn't find the
                 searched-for value.  This is because ddb doesn't always
                 recover from touching bad memory.  The optional count argu-
                 ment limits the search.  The modifiers are the same as those
                 of the write command.

                 The next address is set to the address where value is found,
                 or just after where the search area finishes.

     show what   Displays various things, depending on what:

                 show bcstats
                       Prints the buffer cache statistics.

                 show breaks
                       Prints a list of all breakpoints that have been set
                       with the break command.

                 show buf [/f] addr
                       Prints the struct buf at addr.  If the /f modifier is
                       specified output will also include softdep printout, if
                       those are available.

                 show extents
                       Prints a detailed list of all extents.

                 show malloc [addr]
                       Prints malloc debugging information if available.  If
                       an optional address is specified, only information
                       about that address is printed.

                 show map [/f] addr
                       Prints the vm_map at addr.  If the /f modifier is spec-
                       ified the complete map is printed.

                 show mbuf addr
                       Prints the struct mbuf header at addr.  Depending on
                       the mbuf flags struct pkthdr and struct m_ext are
                       printed as well.

                 show mount [/f] addr
                       Prints the struct mount at addr.  If the /f modifier is
                       specified prints out all vnodes (see also show vnode)
                       and also all bufs (see also show buf) on all those
                       vnodes.

                 show nfsnode [/f] addr
                       Prints the struct nfsnode at addr.  If the /f modifier
                       is specified prints out additional information as well.

                 show nfsreq [/f] addr
                       Prints the struct nfsreq at addr.  If the /f modifier
                       is specified prints out additional information as well.

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

                 show offset offset
                       Prints a list of the known kernel structure fields
                       which occur at the given offset from the beginning of
                       the struct, as well as their size.  The option
                       DDB_STRUCT is required for this command to be avail-
                       able.

                 show page [/f] addr
                       Prints the vm_page at addr.  If the /f modifier is
                       specified the complete page is printed.

                 show panic
                       Prints the panic string.

                 show pool [/p] addr
                       Prints the pool at addr.  Valid modifiers:
                       /p   Print the pagelist for this pool.

                 show proc [addr]
                       Prints the struct proc at addr.  If an optional address
                       is not specified curproc is assumed.

                 show registers [/u]
                       Display the register set.  If the /u modifier is speci-
                       fied, it displays user registers (or the currently
                       saved registers) instead of the kernel's.  Note: The /u
                       modifier is not supported on every machine, in which
                       case incorrect information may be displayed.

                 show struct name [addr]
                       Prints the content of the memory at addr as a struct
                       name.  Nested structures and bit fields are not
                       printed.  Character arrays are printed as bytes.  The
                       option DDB_STRUCT is required for this command to be
                       available.

                 show uvmexp
                       Displays a selection of uvm counters and statistics.

                 show vnode [/f] addr
                       Prints the struct vnode at addr.  If the /f modifier is
                       specified prints out all bufs (see also show buf) cur-
                       rently attached to this vnode.

                 show watches
                       Displays all watchpoints set with the watch command.

                 show all procs [/anw]
                       Display information on all processes.

                       /n   (Default) Show process information in a ps(1)-like
                            format.  Information printed includes process ID,
                            parent process ID, process group, UID, process
                            status, process flags, process command name, and
                            process wait channel message.
                       /a   Shows the kernel virtual addresses of each
                            process' proc structure, u-area, and vmspace
                            structure.  The vmspace address is also the
                            address of the process' vm_map structure and can
                            be used in the show map command.
                       /w   Shows each process' PID, command, system call emu-
                            lation, wait channel address, and wait channel
                            message.

                 show all bufs [/f]
                       Display information about all buffers in the system.

                       /f   For each buffer, print a more detailed output.
                            See the show buf command for more information.

                 show all callout
                       Display the contents of the callout table.

                 show all pools [/a]
                       Display information about all system pools in a format
                       similar to vmstat(8).

                       /a   Displays ``interesting'' address information.

                 show all mounts [/f]
                       Display information on all mounted filesystems.

                       /f   For each filesystem, list all its struct vnode
                            addresses.  These addresses can be used in the
                            show vnode command.

                 show all nfsnodes [/f]
                       Display information about all nfsnodes in the system.

                       /f   For each nfsnode, print a more detailed output.
                            See the show nfsnode command for more information.

                 show all nfsreqs [/f]
                       Display information for all outstanding NFS requests.

                       /f   For each NFS requests, print a more detailed out-
                            put.  See the show nfsreq command for more infor-
                            mation.

                 show all vnodes [/f]
                       Display information about all vnodes in the system.

                       /f   For each vnode, print a more detailed output.  See
                            the show vnode command for more information.

     callout     A synonym for the show all callout command.

     ps [/anw]   A synonym for show all procs.

     mac[hine] subcommand [args ...]
                 Perform a platform-specific command.

                 The following commands are supported by multiprocessor ker-
                 nels on these platforms: amd64, i386, macppc, and mips64.

                 cpuinfo             Display the state of each CPU.
                 ddbcpu N            Stop the current CPU and start handling
                                     ddb on the selected CPU.
                 startcpu [N]        Resume normal processing on the selected
                                     CPU, or all CPUs if none is specified.
                 stopcpu [N]         Stop normal processing on the selected
                                     CPU, or all CPUs (except the one handling
                                     ddb) if none is specified.

                 Other platform-specific commands:

                 arm:
                 frame addr          Display the trapframe at addr.

                 i386:
                 sysregs             Display the contents of the privileged
                                     registers: IDTR, GDTR, LDTR, TR, CR0,
                                     CR2, CR3, and CR4.

                 m88k:
                 ddbcpu N            Stop the current CPU and start handling
                                     ddb on the selected CPU.
                 frame addr          Display the trapframe at addr.
                 regs                Display the registers from when ddb was
                                     entered.
                 searchframe [addr]  Search for and display stack exception
                                     frames, starting from addr if given, else
                                     the address in register r31, and stopping
                                     at the next 8k boundary.
                 where               Display where the current CPU was
                                     stopped.

                 mips64:
                 tlb [/p asid] [/c] [tlb]
                 trap ??

                 sh:
                 cache [addr]        Display the cache, starting from addr,
                                     defaulting to 0.
                 frame               Display the switch and trap frames.
                 tlb                 Display the TLB.

                 sparc:
                 proc ??

                 sparc64:
                 ctx                 Display the context addresses for all
                                     threads.

VARIABLES
     ddb denotes registers and variables by $name.  Register names can be
     found with the show registers command.

     Some variable names are suffixed with numbers, and some may have a modi-
     fier following a colon immediately after the variable name.  For example,
     register variables can have the ':u' modifier to indicate a user register
     (e.g., '$eax:u').

     Built-in debugger 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 lines.
           $lines      The number of lines to page.  This is used by the
                       ``more'' feature.
           $tabstops   Tab stop width.
           $log        Controls whether the output of ddb will also appear in
                       the system message buffer.

     These variables can also be controlled outside ddb via the 'ddb'
     sysctl(8) hierarchy.

EXPRESSIONS
     Almost all expression operators in C are supported except for '~', '^',
     and unary '&'.  Special rules for expressions in ddb are:
           identifier       The name of a symbol.  It is translated to the
                            address (or value) of the symbol.  '.' and ':' can
                            be used in the identifier.  The following can be
                            accepted as an identifier, if supported by an
                            object format dependent routine:
                                  [filename:]func[:linenumber]
                                  [filename:]variable
                                  filename[:linenumber]
                            The symbol may be prefixed with
                            'symboltablename::' (e.g.,
                            'emulator::mach_msg_trap') to specify other than
                            kernel symbols.
           number           The radix is determined by the first two letters:
                            '0x': hex, '0o': octal, '0t': decimal, otherwise,
                            the value of $radix is used.
           .                dot: the current address.
           +                next: the next address.
           ..               The address of the start of the last line exam-
                            ined.  Unlike dot or next, this is only changed by
                            the examine or write command.
           '                The last address explicitly specified.
           $variable        The value of a register or variable.  The name may
                            be followed by a ':' and modifiers as described
                            above with identifier.
           expr # expr      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 mod-
                            ifiers as described above.

SEE ALSO
     gdb(1), nm(1), sysctl.conf(5), hangman(6), kgdb(7), crash(8), sysctl(8),
     extent(9), pool(9), uvm(9)

HISTORY
     This kernel facility first appeared in the MACH 2 operating system devel-
     oped by CMU.  Hangman (which stands for "hangs maniacal analyzer") first
     appeared in OpenBSD 1.2.

BSD                             April 27, 2017                             BSD