DBX(1) BSD General Commands Manual DBX(1)NAMEdbx — debugger
Dbx [-r] [-i] [-k] [-I dir] [-c file] [objfile [coredump]]
Dbx is a tool for source level debugging and execution of programs under
UNIX. The objfile is an object file produced by a compiler with the
appropriate flag (usually -g) specified to produce symbol information in
the object file. Currently, cc(1), f77(1), pc(1), and the DEC Western
Research Laboratory Modula-2 compiler, mod(l), produce the appropriate
source information. The machine level facilities of dbx can be used on
The object file contains a symbol table that includes the names of all
the source files translated by the compiler to create it. These files
are available for perusal while using the debugger.
If a file named core exists in the current directory or a coredump file
is specified, dbx can be used to examine the state of the program when it
If the file .dbxinit exists in the current directory then the debugger
commands in it are executed. Dbx also checks for a .dbxinit in the
user's home directory if there isn't one in the current directory.
The command line options and their meanings are:
-r Execute objfile immediately. If it terminates successfully dbx
exits. Otherwise the reason for termination will be reported and
the user offered the option of entering the debugger or letting
the program fault. Dbx will read from /dev/tty when -r is speci‐
fied and standard input is not a terminal.
-i Force dbx to act as though standard input is a terminal.
-k Map memory addresses, useful for kernel debugging.
-I dir Add dir to the list of directories that are searched when looking
for a source file. Normally dbx looks for source files in the
current directory and in the directory where objfile is located.
The directory search path can also be set with the use command.
Execute the dbx commands in the file before reading from standard
Unless -r is specified, dbx just prompts and waits for a command.
Execution and Tracing Commands
run [args] [< filename] [> filename]
rerun [args] [< filename] [> filename]
Start executing objfile, passing args as command line arguments;
< or > can be used to redirect input or output in the usual man‐
ner. When rerun is used without any arguments the previous argu‐
ment list is passed to the program; otherwise it is identical to
run. If objfile has been written since the last time the sym‐
bolic information was read in, dbx will read in the new informa‐
trace [in procedure/function] [if condition]
trace source-line-number [if condition]
trace procedure/function [in procedure/function] [if condition]
trace expression at source-line-number [if condition]
trace variable [in procedure/function] [if condition]
Have tracing information printed when the program is executed. A
number is associated with the command that is used to turn the
tracing off (see the delete command).
The first argument describes what is to be traced. If it is a
source-line-number, then the line is printed immediately prior to
being executed. Source line numbers in a file other than the
current one must be preceded by the name of the file in quotes
and a colon, e.g. "mumble.p":17.
If the argument is a procedure or function name then every time
it is called, information is printed telling what routine called
it, from what source line it was called, and what parameters were
passed to it. In addition, its return is noted, and if it's a
function then the value it is returning is also printed.
If the argument is an expression with an at clause then the value
of the expression is printed whenever the identified source line
If the argument is a variable then the name and value of the
variable is printed whenever it changes. Execution is substan‐
tially slower during this form of tracing.
If no argument is specified then all source lines are printed
before they are executed. Execution is substantially slower dur‐
ing this form of tracing.
The clause in procedure/function restricts tracing information to
be printed only while executing inside the given procedure or
Condition is a boolean expression and is evaluated prior to
printing the tracing information; if it is false then the infor‐
mation is not printed.
stop if condition
stop at source-line-number [if condition]
stop in source-line-number [if condition]
stop variable [if condition]
Stop execution when the given line is reached, procedure or func‐
tion called, variable changed, or condition true.
status [> filename]
Print out the currently active trace and stop commands.
delete command-number ...
The traces or stops corresponding to the given numbers are
removed. The numbers associated with traces and stops are
printed by the status command.
Start or stop trapping a signal before it is sent to the program.
This is useful when a program being debugged handles signals such
as interrupts. A signal may be specified by number or by a name
(e.g., SIGINT). Signal names are case insensitive and the “SIG”
prefix is optional. By default all signals are trapped except
SIGCONT, SIGCHILD, SIGALRM and SIGKILL.
Continue execution from where it stopped. If a signal is speci‐
fied, the process continues as though it received the signal.
Otherwise, the process is continued as though it had not been
Execution cannot be continued if the process has ``finished'',
that is, called the standard procedure ``exit''. Dbx does not
allow the process to exit, thereby letting the user to examine
the program state.
step Execute one source line.
next Execute up to the next source line. The difference between this
and step is that if the line contains a call to a procedure or
function the step command will stop at the beginning of that
block, while the next command will not.
Continue until a return to procedure is executed, or until the
current procedure returns if none is specified.
Execute the object code associated with the named procedure or
Printing Variables and Expressions
Names are resolved first using the static scope of the current function,
then using the dynamic scope if the name is not defined in the static
scope. If static and dynamic searches do not yield a result, an arbi‐
trary symbol is chosen and the message “[using qualified name]” is
printed. The name resolution procedure may be overridden by qualifying
an identifier with a block name, e.g., “module.variable” For C, source
files are treated as modules named by the file name without ``.c''.
Expressions are specified with an approximately common subset of C and
Pascal (or equivalently Modula-2) syntax. Indirection can be denoted
using either a prefix ``*'' or a postfix ``^'' and array expressions are
subscripted by brackets (“”) The field reference operator (``.'') can
be used with pointers as well as records, making the C operator ``->''
unnecessary (although it is supported).
Types of expressions are checked; the type of an expression may be over‐
ridden by using “type-name(expression)”. When there is no corresponding
named type the special constructs “&type-name” and “$$tag-name” can be
used to represent a pointer to a named type or C structure tag.
assign variable = expression
Assign the value of the expression to the variable.
dump [procedure] [> filename]
Print the names and values of variables in the given procedure,
or the current one if none is specified. If the procedure given
is ``.'', then the all active variables are dumped.
print expression [, expression ...]
Print out the values of the expressions.
Print the declaration of the given name, which may be qualified
with block names as above.
Print the full qualification of the given identifer, i.e. the
outer blocks that the identifier is associated with.
Move the current function, which is used for resolving names, up
or down the stack count levels. The default count is 1.
where Print out a list of the active procedures and function.
Print the full qualification of all the symbols whose name
matches the given identifier. The order in which the symbols are
printed is not meaningful.
Accessing Source Files
/ regular expression[/]
? regular expression?
Search forward or backward in the current source file for the
Invoke an editor on filename or the current source file if none
is specified. If a procedure or function name is specified, the
editor is invoked on the file that contains it. Which editor is
invoked by default depends on the installation. The default can
be overridden by setting the environment variable EDITOR to the
name of the desired editor.
Change the current source file name to filename. If none is
specified then the current source file name is printed.
Change the current function. If none is specified then print the
current function. Changing the current function implicitly
changes the current source file to the one that contains the
function; it also changes the current scope used for name resolu‐
list [source-line-number [, source-line-number]]
List the lines in the current source file from the first line
number to the second inclusive. If no lines are specified, the
next 10 lines are listed. If the name of a procedure or function
is given lines n-k to n+k are listed where n is the first state‐
ment in the procedure or function and k is small.
Set the list of directories to be searched when looking for
Command Aliases and Variables
alias name name
alias name string
alias name (parameters) “string”
When commands are processed, dbx first checks to see if the word
is an alias for either a command or a string. If it is an alias,
then dbx treats the input as though the corresponding string
(with values substituted for any parameters) had been entered.
For example, to define an alias ``rr'' for the command ``rerun'',
one can say
alias rr rerun
To define an alias called ``b'' that sets a stop at a particular
line one can say
alias b(x) ``stop at x''
Subsequently, the command ``b(12)'' will expand to ``stop at
set name [= expression]
The set command defines values for debugger variables. The names
of these variables cannot conflict with names in the program
being debugged, and are expanded to the corresponding expression
within other commands. The following variables have a special
Setting this variable to an address causes dbx to use the
stack frame pointed to by the address for doing stack
traces and accessing local variables. This facility is of
particular use for kernel debugging.
When set, dbx prints out out characters, integers, offsets
from registers, or character pointers respectively in hexa‐
The value of this variable specifies the number of lines to
list around a function or when the list command is given
without any parameters. Its default value is 10.
Setting (unsetting) this variable causes dbx to start
(stop) mapping addresses. As with ``$frame'', this is use‐
ful for kernel debugging.
When ``$unsafecall'' is set, strict type checking is turned
off for arguments to subroutine or function calls ( e .g.
in the call statement). When ``$unsafeassign'' is set,
strict type checking between the two sides of an assign
statement is turned off. These variables should be used
only with great care, because they severely limit dbx's
usefulness for detecting errors.
Remove the alias with the given name.
Delete the debugger variable associated with name.
Machine Level Commands
tracei [address] [if cond]
tracei [variable] [at address] [if cond]
stopi [address] [if cond]
stopi [at] [address] [if cond]
Turn on tracing or set a stop using a machine instruction
nexti Single step as in step or next, but do a single instruction
rather than source line.
Print the contents of memory starting at the first address
and continuing up to the second address or until count items
are printed. If the address is ``.'', the address following
the one printed most recently is used. The mode specifies
how memory is to be printed; if it is omitted the previous
mode specified is used. The initial mode is ``X''. The fol‐
lowing modes are supported:
i Print the machine instruction.
d Print a short word in decimal.
D Print a long word in decimal.
o Print a short word in octal.
O Print a long word in octal.
x Print a short word in hexadecimal.
X Print a long word in hexadecimal.
b Print a byte in octal.
c Print a byte as a character.
s Print a string of characters terminated by a null byte.
f Print a single precision real number.
g Print a double precision real number.
Symbolic addresses are specified by preceding the name with an ``&''.
Registers are denoted by ``$rN'' where N is the number of the register.
Addresses may be expressions made up of other addresses and the operators
``+'', ``-'', and indirection (unary ``*'').
gripe Invoke a mail program to send a message to the person in charge
help Print out a synopsis of dbx commands.
quit Exit dbx.
Pass the command line to the shell for execution. The SHELL
environment variable determines which shell is used.
Read dbx commands from the given filename.
Dbx examines the following environment variables:
a.out Object file.
.dbxinit Initial commands.
SEE ALSOcc(1), mod(l), f77(1), pc(1)HISTORY
The dbx command appeared in 4.2BSD.
Dbx suffers from the same ``multiple include'' malady as did sdb. If you
have a program consisting of a number of object files and each is built
from source files that include header files, the symbolic information for
the header files is replicated in each object file. Since about one
debugger start-up is done for each link, having the linker ld(1) re-orga‐
nize the symbol information would not save much time, though it would
reduce some of the disk space used.
This problem is an artifact of the unrestricted semantics of #include's
in C; for example an include file can contain static declarations that
are separate entities for each file in which they are included. However,
even with Modula-2 there is a substantial amount of duplication of symbol
information necessary for inter-module type checking.
Some problems remain with the support for individual languages. Fortran
problems include: inability to assign to logical, logical*2, complex and
double complex variables; inability to represent parameter constants
which are not type integer or real; peculiar representation for the val‐
ues of dummy procedures (the value shown for a dummy procedure is actu‐
ally the first few bytes of the procedure text; to find the location of
the procedure, use ``&'' to take the address of the variable).
4.2 Berkeley Distribution August 9, 1991 4.2 Berkeley Distribution