ddb(4)

NAME
ddb - kernel debugger

DESCRIPTION

The kernel debugger has most of the features of the old kdb, but with a more rational (gdb-like) syntax.

The current location is called 'dot'. The 'dot' is displayed with a hexadecimal format at a prompt. Examine and write commands update 'dot' to the address of the last line examined or the last location modified, and set 'next' to the address of the next location to be examined or changed. Other commands don't change 'dot', and set 'next' to be the same as 'dot'.

The general command syntax is:

command[/modifier] address [,count]

A blank line repeats 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.

"ddb" has a feature like a command "more" for the output. If an output line exceeds the number set in the $lines variable, it displays "--db_more--" and waits for a response. The valid responses for it are:

<space>
    one more page

<return>
    one more line

q   abort the current command, and return to the command input mode.

COMMANDS

Display the addressed locations according to the formats in the modifier. Multiple modifier formats display multiple locations. If no format is specified, the last formats specified for this command is used.
The format characters are

b   look at by bytes(8 bits)

h   look at by half words(16 bits)

l   look at by long words(32 bits)

a   print the location being displayed

A   print the location with a line number if possible

x   display in unsigned hex

z   display in signed hex

o   display in unsigned octal

d   display in signed decimal

u   display in unsigned decimal

r   display in current radix, signed

c   display low 8 bits as a character. Non-printing characters are displayed as an octal escape code (e.g. '\000').

s   display the null-terminated string at the location. Non-printing characters are displayed as octal escapes.

m   display in unsigned hex with character dump at the end of each line. The location is also displayed in hex at the beginning of each line.

i   display as an instruction

I   display as an instruction with possible alternate formats depending on the machine:

vax don't assume that each external label is a procedure entry mask

i386
    don't round to the next long word boundary

mips
    print register contents

Examine forward. It executes an examine command with the last specified parameters to it except that the next address displayed by it is used as the start address. Examine backward. It executes 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 'addr's according to the modifier character. Valid formats are: a x z o d u r c. If no modifier is specified, the last one specified to it is used. 'addr' can be a string, and it is printed as it is. For example,

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

will print like

eax = xxxxxx
ecx = yyyyyy

Write the expressions at succeeding locations. 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's best to enclose each expression in parentheses. Set the named variable or register with the value of 'expr'. Valid variable names are described below. Set a break point at 'addr'. If count is supplied, continues (count-1) times before stopping at the break point. If the break point is set, a break point number is printed with '#'. This number can be used in deleting the break point or adding conditions to it.

u   Set a break point in user space address. Without 'u' option, the address is considered in the kernel space, and wrong space address is rejected with an error message. This option 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 the break point. The target break point can be specified by a break point number with '#', or by 'addr' like specified in 'break' command. Single step 'count' times. If 'p' option is specified, print each instruction at each step. Otherwise, only print the last instruction.

Warning: depending on machine type, it may not be possible to single-step through some low-level code paths or user space code. On machines with software-emulated single-stepping (e.g., pmax), stepping through code executed by interrupt handlers will probably do the wrong thing. Continue execution until a breakpoint or watchpoint. If /c, 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. Stop at the next call or return instruction. If 'p' option 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. Stop at the matching return instruction. If 'p' option 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. A synonym for 'next'. Stack trace. 'u' option traces user space; if omitted, only traces kernel space. 'count' is the number of frames to be traced. If the 'count' is omitted, all frames are printed.

Warning: User space stack trace is valid only if the machine dependent code supports it. Search memory for a value. 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 argument limits the search. Display all process information. This version of "ddb" prints more information than previous one. It shows process information like "ps". 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' options will alter the 'ps' display to show vm_map addresses for the process and not show other info.
A synonym for 'show all procs'. Display the register set. If 'u' option is specified, it displays user registers instead of kernel or currently saved one.

Warning: The support of 'u' option depends on the machine. If not supported, incorrect information will be displayed. Prints the vm_map at 'addr'. If the 'f' option is specified the complete map is printed. Prints the vm_object at 'addr'. If the 'f' option is specified the complete object is printed. Displays all watchpoints. 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.

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 'u' modifier to indicate user register (e.g. $eax:u).

Built-in variables currently supported are:

radix
    Input and output radix

maxoff
    Addresses are printed as 'symbol'+offset unless offset is greater than maxoff.

maxwidth
    The width of the displayed line.

lines
    The number of lines. It is used by "more" feature.

tabstops
    Tab stop width.

workxx
    Work variable. 'xx' can be 0 to 31.

EXPRESSIONS

Almost all expression operators in C are supported except '~', '^', and unary '&'. Special rules in "ddb" are:

<identifier>
    name of a symbol. It is translated to the address(or value) of it. '.' and ':' can be used in the identifier. If supported by an object format dependent routine, [<file_name>:]<func>[:<line_number>] [<file_name>:]<variable>, and <file_name>[:<line_number>] can be accepted as a symbol. The symbol may be prefixed with '<symbol_table_name>::' like 'emulator::mach_msg_trap' to specify other than kernel symbols.

<number>
    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>
    register name or variable. It is translated to the value of it. It may be followed by a ':' and modifiers as described above.

#   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.