Db is a general purpose debugging program. It may be used to examine
files and to provide a controlled environment for the execution
of Plan 9 programs.
A textfile is a file containing the text and initialized data
of an executable program. A memfile is the memory image of an
executing process. It is usually accessed via the process id (pid)
of the process in /proc/pid/mem. A memfile contains the text,
data, and saved registers and process state. A map associated
with each textfile or memfile supports accesses to instructions
and data in the file; see `Addresses'.
An argument consisting entirely of digits is assumed to be a process
id; otherwise, it is the name of a textfile. When a textfile is
given, the textfile map is associated with it. If only a pid is
given, the textfile map is associated with /proc/pid/text. When
a pid is given, the memfile map is associated with
/proc/pid/mem; otherwise it is undefined and accesses to the memfile
are not permitted.
Commands to db are read from the standard input and responses
are to the standard output. The options are|
–k Use the kernel stack of process pid to debug the executing kernel
process. If textfile is not specified, file /$cputype/9type is
used, where type is the second word in $terminal.
–w Create textfile and memfile if they don't exist; open them for
writing as well as reading.
Directory in which to look for relative path names in $< and $<<
Most db commands have the following form:
Assume instructions are for the given CPU type (any standard architecture
name, such as alpha or 386, plus mipsco and sunsparc, which cause
disassembly to the manufacturer's syntax) instead of using the
magic number to select the CPU type.
If address is present then the current position, called `dot',
is set to address. Initially dot is set to 0. Most commands are
repeated count times with dot advancing between repetitions. The
default count is 1. Address and count are expressions. Multiple
commands on one line must be separated by ;.
[address] [, count] [command]
Expressions are evaluated as long ints.
. The value of dot.
+ The value of dot incremented by the current increment.
^ The value of dot decremented by the current increment.
" The last address typed.
A number, in decimal radix by default. The prefixes 0 and 0o and
0O (zero oh) force interpretation in octal radix; the prefixes
0t and 0T force interpretation in decimal radix; the prefixes
0x, 0X, and # force interpretation in hexadecimal radix. Thus
020, 0o20, 0t16, and #10 all represent sixteen.
'c' The 16–bit value of a character. \ may be used to escape a '.
A single–precision floating point number.|
The value of name, which is a register name. The register names
are those printed by the $r command.|
A symbol is a sequence of upper or lower case letters, underscores
or digits, not starting with a digit. \ may be used to escape
other characters. The location of the symbol is calculated from
the symbol table in textfile.|
The address of the variable name in the specified C routine. Both
routine and name are symbols. If name is omitted the value is
the address of the most recently activated stack frame corresponding
to routine; if routine is omitted, the active procedure is assumed.|
The address of the instruction corresponding to the source statement
at the indicated line number of the file. If the source line contains
no executable statement, the address of the instruction associated
with the nearest executable source line is returned. Files begin
at line 1. If multiple files of the same name
are loaded, an expression of this form resolves to the first file
encountered in the symbol table.|
The value of the expression exp.
Dyadic operators are left–associative and are less binding than
*exp The contents of the location addressed by exp in memfile.|
@exp The contents of the location addressed by exp in textfile.
–exp Integer negation.
~exp Bitwise complement.
%exp When used as an address, exp is an offset into the segment
named ublock; see `Addresses'.
e1#e2E1 rounded up to the next multiple of e2.
A format consists of one or more characters that specify a style
of printing. Each format character may be preceded by a decimal
integer that is a repeat count for the format character. If no
format is given then the last format is used.
Most format letters fetch some data, print it, and advance (a
local copy of) dot by the number of bytes fetched. The total number
of bytes in a format becomes the current increment.
Most commands have the following syntax:
?f Locations starting at address in textfile are printed according
to the format f.
/f Locations starting at address in memfile are printed according
to the format f.
=f The value of address itself is printed according to the format
Other commands include:
o Print two–byte integer in octal.|
O Print four–byte integer in octal.
q Print two–byte integer in signed octal.
Q Print four–byte integer in signed octal.
d Print two–byte integer in decimal.
D Print four–byte integer in decimal.
V Print eight–byte integer in decimal.
Z Print eight–byte integer in unsigned decimal.
x Print two–byte integer in hexadecimal.
X Print four–byte integer in hexadecimal.
Y Print eight–byte integer in hexadecimal.
u Print two–byte integer in unsigned decimal.
U Print four–byte integer in unsigned decimal.
f Print as a single–precision floating point number.
F Print double–precision floating point.
b Print the addressed byte in hexadecimal.
c Print the addressed byte as an ASCII character.
C Print the addressed byte as a character. Printable ASCII characters
are represented normally; others are printed in the form \xnn.
s Print the addressed characters, as a UTF string, until a zero
byte is reached. Advance dot by the length of the string, including
the zero terminator.
S Print a string using the escape convention (see C above).
r Print as UTF the addressed four–byte integer (rune).
R Print as UTF the addressed four–byte integers as runes until a
zero rune is reached. Advance dot by the length of the string,
including the zero terminator.
i Print as machine instructions. Dot is incremented by the size
of the instruction.
I As i above, but print the machine instructions in an alternate
form if possible: sunsparc and mipsco reproduce the manufacturers'
M Print the addressed machine instruction in a machine–dependent
a Print the value of dot in symbolic form. Dot is unaffected.
A Print the value of dot in hexadecimal. Dot is unaffected.
z Print the function name, source file, and line number corresponding
to dot (textfile only). Dot is unaffected.
p Print the addressed value in symbolic form. Dot is advanced by
the size of a machine address.
t When preceded by an integer, tabs to the next appropriate tab
stop. For example, 8t moves to the next 8–space tab stop. Dot is
n Print a newline. Dot is unaffected.
"..." Print the enclosed string. Dot is unaffected.
^ Dot is decremented by the current increment. Nothing is printed.
+ Dot is incremented by 1. Nothing is printed.
– Dot is decremented by 1. Nothing is printed.
[?/]l value mask
Update dot by the current increment. Repeat the previous command
with a count of 1.|
[?/]w value ...
Words starting at dot are masked with mask and compared with value
until a match is found. If l is used, the match is for a two–byte
integer; L matches four bytes. If no match is found then dot is
unchanged; otherwise dot is set to the matched location. If mask
is omitted then ~0 is used.
[?/]m s b e f [?]
Write the two–byte value into the addressed location. If the command
is W, write four bytes.|
New values for (b, e, f) in the segment named s are recorded.
Valid segment names are text, data, or ublock. If less than three
address expressions are given, the remaining parameters are left
unchanged. If the list is terminated by ? or / then the file (textfile
or memfile respectively) is used for subsequent
requests. For example, /m? causes / to refer to textfile.|
! The rest of the line is passed to rc(1) for execution.
Dot is assigned to the variable or register named.|
Miscellaneous commands. The available modifiers are:|
<f Read commands from the file f. If this command is executed in
a file, further commands in the file are not seen. If f is omitted,
the current input stream is terminated. If a count is given, and
is zero, the command is ignored.
<<f Similar to < except it can be used in a file of commands without
causing the file to be closed. There is a (small) limit to the
number of << files that can be open at once.
>f Append output to the file f, which is created if it does not
exist. If f is omitted, output is returned to the terminal.
? Print process id, the condition which caused stopping or termination,
the registers and the instruction addressed by pc. This is the
default if modifier is omitted.
r Print the general registers and the instruction addressed by
pc. Dot is set to pc.
R Like $r, but include miscellaneous processor control registers
and floating point registers.
f Print floating–point register values as single–precision floating
F Print floating–point register values as double–precision floating
b Print all breakpoints and their associated counts and commands.
`B' produces the same results.
c Stack backtrace. If address is given, it specifies the address
of a pair of 32–bit values containing the sp and pc of an active
process. This allows selecting among various contexts of a multi–threaded
process. If C is used, the names and (long) values of all parameters,
automatic and static variables are
a Attach to the running process whose pid is contained in address.
printed for each active function. If count is given, only the
first count frames are printed.|
e The names and values of all external variables are printed.
w Set the page width for output to address (default 80).
q Exit from db.
m Print the address maps.
k Simulate kernel memory management.
Set the machine type used for disassembling instructions.|
Manage a subprocess. Available modifiers are:|
h Halt an asynchronously running process to allow breakpointing.
Unnecessary for processes created under db, e.g. by :r.
bc Set breakpoint at address. The breakpoint is executed count–1
times before causing a stop. Also, if a command c is given it
is executed at each breakpoint and if it sets dot to zero the
breakpoint causes a stop.
d Delete breakpoint at address.
r Run textfile as a subprocess. If address is given the program
is entered at that point; otherwise the standard entry point is
used. Count specifies how many breakpoints are to be ignored before
stopping. Arguments to the subprocess may be supplied on the same
line as the command. An argument
cs The subprocess is continued. If s is omitted or nonzero, the
subprocess is sent the note that caused it to stop. If 0 is specified,
no note is sent. (If the stop was due to a breakpoint or single–step,
the corresponding note is elided before continuing.) Breakpoint
skipping is the same as for r.
ss As for c except that the subprocess is single stepped for count
machine instructions. If a note is pending, it is received before
the first instruction is executed. If there is no current subprocess
then textfile is run as a subprocess as for r. In this case no
note can be sent; the remainder of the line is
starting with < or > causes the standard input or output to be established
for the command.|
Ss Identical to s except the subprocess is single stepped for count
lines of C source. In optimized code, the correspondence between
C source and the machine instructions is approximate at best.
treated as arguments to the subprocess.|
x The current subprocess, if any, is released by db and allowed
to continue executing normally.
k The current subprocess, if any, is terminated.
nc Display the pending notes for the process. If c is specified,
first delete c'th pending note.
Usually, the text and initialized data of a program are mapped
by segments called text and data. Since a program file does not
contain bss, stack or ublock data, these data are not mapped by
the data segment. The text segment is mapped similarly in a normal
(i.e., non–kernel) memfile. However, the segment called
data maps memory from the beginning of the program's data space
to the base of the ublock. This region contains the program's
static data, the bss, the heap and the stack. A segment called
ublock maps the page containing its registers and process state.
Sometimes it is useful to define a map with a single segment mapping
the region from 0 to 0xFFFFFFFF; a map of this type allows the
entire file to be examined without address translation.
Registers are saved at a machine–dependent offset in the ublock.
It is usually not necessary to know this offset; the $r, $R, $f,
and $F commands calculate it and display the register contents.
The $m command dumps the currently active maps. The ?m and /m
commands modify the segment parameters in the textfile and memfile
The location in a file or memory image associated with an address
is calculated from a map associated with the file. Each map contains
one or more quadruples (t, b, e, f), defining a segment named
t (usually, text, data, or ublock) mapping addresses in the range
b through e to the part of the file beginning at offset f.
The memory model of a Plan 9 process assumes that segments are
disjoint. There can be more than one segment of a given type (e.g.,
a process may have more than one text segment) but segments may
not overlap. An address a is translated to a file address by finding
a segment for which b≤a<e; the location in the
file is then address+f–b.