dbx.1

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of lines 
.PN dbx
lists around a procedure or function or displays when you issue the \fBlist\fP command
without arguments.
The default value for this variable is 10. 
.IP "$mapaddrs"
Setting (unsetting) this variable causes 
.PN dbx
to start (stop)
mapping addresses.
This variable is useful for kernel debugging.
.IP "$unsafecall"
.ns
.IP "$unsafeassign"
If you set one of these variables,
.PN dbx
does not perform strict type checking. 
When the 
.B $unsafecall 
variable is set,
strict type checking is turned off for arguments to
procedure or function calls (for example, in 
the \fBcall\fP command). When the 
.B $unsafeassign 
variable is set, 
strict type checking between the two sides
of an \fBassign\fP command is turned off.
Use these variables with care,
because they severely limit the debugger's
usefulness
for detecting errors.
.RE
.IP "\fBunalias\fP \fIname\fP" 10
.NX R "unalias command (dbx)"
Removes the alias with the specified name.
.IP "\fBunset\fP \fIname\fP"
.NX R "unset command (dbx)"
Deletes the debugger variable associated with the name you specify.
.SS Vector Environment Commands
The 
.PN dbx
debugger provides some special commands that you can use to debug a
program that uses vectors. The following list explains these commands:
.NXR "vector environment" "dbx commands for"
.IP "\fBcallv\fP \fIprocedure/function\fP[\fB(\fIparameters\fB)\fP]"
Executes the object code associated with the named procedure or function.
The
.B callv
command causes
.PN dbx
to save the vector environment before it executes the procedure. The
debugger restores the vector environment after the procedure exits.
.IP "\fBfmask\fP[\fB/""\fIhex constant\fB""\fR] \fIvector_register\fP"
.ns
.IP "\fBtmask\fP[\fB/""\fIhex constant\fB""\fR] \fIvector_register\fP"
.NX R "fmask command (dbx)"
.NX R "tmask command (dbx)"
Displays the contents of the specified vector data register using the
vector mask register (\fB$vmr\fR) or the hexadecimal constant you
specify as a mask value. Replace \fIvector_register\fR with the name of
a vector register to specify which vector data register the debugger
displays.
.IP
The \fBfmask\fR command causes the debugger to apply a false mask to the
register value. The \fBtmask\fR command causes the debugger to apply a
true mask to the register value.
.IP "\fBnextv\fP"
.NX R "nextv command (dbx)"
Executes to the next vector instruction at the current or higher
procedure level. The
.PN dbx
debugger ignores vector instructions at lower procedure levels when
executing this command. Thus, execution continues through lower level
procedures, but stops prior to the next instruction at the current or
higher procedure level.
.IP "\fBstepv\fP"
.NX R "stepv command (dbx)"
Executes the next vector instruction. The debugger stops execution
prior to the next vector instruction regardless of the procedure level
of that instruction.
.SS Machine Level Commands
You can use machine level commands to debug any program, regardless of
whether the program object file contains extended symbol table
information.
.PP
You can specify symbolic addresses by preceding the name with an 
ampersand (&).
You denote registers by \fB$r\fIN\fR where \fIN\fR is the number of the register. You
denote vector registers by \fB$v\fIN\fR where \fIN\fR is the number of the register.
Addresses may be expressions made up of other addresses and
the operators plus (+), minus (\-), and indirection (unary asterisk, *).
.PP
The following describes the
.PN dbx
machine level commands:
.IP "\fIaddress\fP \fB,\fP \fIaddress\fP\fB/\fP [\fImode\fP]"
.ns
.IP "\fIaddress\fP \fB/\fP [\fIcount\fP] [\fImode\fP]"
.ns
.IP "\fIat source-line-number\fP \fB/\fP [\fIcount\fP] [\fImode\fP]"
Display the contents of memory starting at the first \fIaddress\fP 
and continuing up to the second \fIaddress\fP.
.IP 
The second form causes
.PN dbx
to display the contents of memory starting at
\fIaddress\fP and continuing until \fIcount\fP items are displayed.
If you type a period (.) in the address field, 
.PN dbx
uses the address following
the one it most recently displayed.
.IP 
If you specify \fIat source-line-number\fP, 
.PN dbx
displays the contents of memory associated with the specified source line.
You can specify how many items the debugger displays using the
\fIcount\fP argument.
.IP 
The \fImode\fP on each format specifies how 
.PN dbx
displays memory;
if you omit it, the debugger uses the previous mode.
The initial mode is X.
.IP 
You can specify the following modes:
.nr In 5
.in +\n(Inn
.ta \n(Inn
.sp 1
.ti -\n(Inn
\&\fBb\fP	\c
Displays a byte in octal
.ti -\n(Inn
\&\fBc\fP	\c
Displays a byte as a character
.ti -\n(Inn
\&\fBd\fP	\c
Displays a short word in decimal
.ti -\n(Inn
\&\fBD\fP	\c
Displays a long word in decimal
.ti -\n(Inn
\&\fBf\fP	\c
Displays a single precision real number
.ti -\n(Inn
\&\fBg\fP	\c
Displays a double precision real number
.ti -\n(Inn
\&\fBi\fP	\c
Displays the machine instruction
.ti -\n(Inn
\&\fBo\fP	\c
Displays a short word in octal
.ti -\n(Inn
\&\fBO\fP	\c
Displays a long word in octal
.ti -\n(Inn
\&\fBs\fP	\c
Displays a string of characters terminated by a null byte
.ti -\n(Inn
\&\fBx\fP	\c
Displays a short word in hexadecimal
.ti -\n(Inn
\&\fBX\fP	\c
Displays a long word in hexadecimal
.IP "\fBnexti\fP"
.NX R "nexti command (dbx)"
Executes to the next machine instruction. If the machine instruction
that 
.PN dbx
executes contains a call to a procedure or function, the
debugger executes the entire procedure or function. Program execution
stops following the return from the procedure; that is, execution stops prior 
to execution of the machine instruction that follows the call.
.IP "\fBstepi\fP"
.NX R "stepi command (dbx)"
Executes one machine instruction. If the machine instruction contains a call to a
procedure or function, the
debugger stops at the first line of the procedure or function.
.IP "\fBstopi\fP [\fBat\fP] [\fIaddress\fP] [\fBif\fP \fIcondition\fP]"
.NX R "stopi command (dbx)"
Stops at the specified machine address or when the specified condition is
met. If you specify both \fIaddress\fP and \fIcondition\fP, the debugger stops
at the specified address only when the specified condition is true.
.IP "\fBtracei\fP [\fIaddress\fP] [\fBif\fP \fIcondition\fP]"
.ns
.IP "\fBtracei\fP [\fIvariable\fP] [\fBat\fP \fIaddress\fP] [\fBif\fP \fIcondition\fP]"
.NX R "tracei command (dbx)"
Displays tracing information during program execution. The first argument
specifies an address at which tracing begins or the name of a variable
to trace. 
.IP
If you specify tracing a variable at a specific address, 
.PN dbx
displays the value of that variable when it reaches the address you
specify.
.IP 
The \fIcondition\fP argument is a Boolean expression that 
.PN dbx
evaluates prior to displaying any tracing information. If the condition
is false, 
.PN dbx 
does not display the information.
.SS Miscellaneous Commands
.NXR "dbx debugger" "miscellaneous commands"
Use the following to get help about 
.PN dbx ,
exit from the debugger, issue shell commands, and read commands from
a file:
.IP \fB!\fIstring\fR
.ns
.IP \fB!\fIinteger\fR
Executes a command from the history list. You can specify the command
name in the \fIstring\fR argument. If you specify an integer,
.PN dbx
executes the command having that number in the history list. For example,
the following executes the third command in the history list:
.EX
dbx> \fB!3\fP
reread
.EE
The debugger echoes the command before execution. 
.IP "\fBhelp\fP"
.NX R "help command (dbx)"
Displays a synopsis of 
.PN dbx
commands.
.IP "\fBhi[story]\fP"
.NX R "history command (dbx)"
Displays a list of the previous commands you issued. By default,
.PN dbx
displays the previous 20 commands. You can change the number of commands
.PN dbx
keeps in the history list by modifying the 
.B $historywindow 
debugger
variable. (For information on changing debugger variables, see 
Command Aliases And Variables.) 
.IP "\fBprintf \fIformat, arg1, arg2,...\fP"
.NX R "printf command (dbx)"
Formats a complex structure for display as specified. You use the same
format characters for this command as for the 
.PN printf
subroutine. For information on specifying the format, see
.MS printf 3s .
.IP "\fBquit\fP"
Exits from 
.PN dbx .
.IP "\fBrecord input\fR [\fIfilename\fR]
Records all commands you enter at the 
.PN dbx
prompt. The debugger stores the commands in the specified file. You can
replay the commands by naming the file on the
.B source
command line.
.IP
If you omit \fIfilename\fR, the debugger terminates the existing
recording session. If recording is not active, 
.PN dbx
ignores the command.
.NX R "sh command (dbx)"
.IP "\fBsh \fIcommand-line\fR
Passes the command line to the shell for execution.
The SHELL environment variable determines which shell is used.
.IP "\fBsource\fP \fIfilename\fP"
Reads 
.PN dbx
commands from the specified file.
.SH Restrictions
If you have a program consisting of several object files and
each is built from source files that include header files,
the symbolic information for the header files is reproduced
in each object code file.  Since one debugger startup usually
is done for each link, having the 
.PN ld
linker 
reorganize the symbol information will not save much time,
although it will reduce some of the disk space used.  
.PP
This
problem results from the unrestricted semantics of
.PN #include 
statements in C.  For example, an include
file can contain static declarations that are separate
entities for each file in which they are included.
If your image is too large for 
.PN dbx
to run, compile with the
.B \-g
option only those files that you are interested in debugging. However,
even with Modula-2, there is a substantial amount of duplication of
symbol information necessary for inter-module type checking.
.PP
The following restrictions apply to debugging FORTRAN programs:
.IP \(bu 4
Inability to assign values to 
logical*2, complex, and double complex variables.
.IP \(bu 4
Inability
to represent parameter constants that are not type integer
or real. 
.IP \(bu 4
Peculiar representations of the values of dummy
procedures.  (The value shown for a dummy procedure is actually
the first few bytes of the procedure text.  To find the
location of the procedure, use an ampersand (&) to use
the address of the variable.)
.PP
The
.PN dbx
debugger does not allow you to run a program you do not own
unless you are root.  If you are not root, the following message
might be displayed on your screen
when you issue the 
.PN run
command:
.EX
can't-write-to-process
.EE
This message is displayed when the
.PN dbx 
debugger tries to set breakpoints because of restrictions
on the 
.PN ptrace
system call.  The 
.PN dbx
debugger always tries set a breakpoint
on exit. If you repeat the
.PN run
command, your program
runs without breakpoints. 
.PP
The
.B printf
debugger command does not support the
.PN %s
conversion specification.
.SH Files
.TP 20
.PN a.out
Object file
.TP
.PN \&.dbxinit
Initialization file
.SH See Also
cc(1), pc(1), ptrace(1), vcc(1), signal(3), printf(3s)