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A target is the execution environment occupied by your program.
Often, GDB runs in the same host environment as your program; in
that case, the debugging target is specified as a side effect when you
use the file
or core
commands. When you need more
flexibility--for example, running GDB on a physically separate
host, or controlling a standalone system over a serial port or a
realtime system over a TCP/IP connection--you
can use the target
command to specify one of the target types
configured for GDB (see section Commands for managing targets).
There are three classes of targets: processes, core files, and executable files. GDB can work concurrently on up to three active targets, one in each class. This allows you to (for example) start a process and inspect its activity without abandoning your work on a core file.
For example, if you execute `gdb a.out', then the executable file
a.out
is the only active target. If you designate a core file as
well--presumably from a prior run that crashed and coredumped--then
GDB has two active targets and will use them in tandem, looking
first in the corefile target, then in the executable file, to satisfy
requests for memory addresses. (Typically, these two classes of target
are complementary, since core files contain only a program's
read-write memory--variables and so on--plus machine status, while
executable files contain only the program text and initialized data.)
When you type run
, your executable file becomes an active process
target as well. When a process target is active, all GDB commands
requesting memory addresses refer to that target; addresses in an
active core file or
executable file target are obscured while the process
target is active.
Use the core-file
and exec-file
commands to select a
new core file or executable target (see section Commands to specify files). To specify as a target a process that is already running, use
the attach
command (see section Debugging an already-running process).
target type parameters
Further parameters are interpreted by the target protocol, but typically include things like device names or host names to connect with, process numbers, and baud rates.
The target
command will not repeat if you press RET again
after executing the command.
help target
info target
or info files
(see section Commands to specify files).
help target name
Here are some common targets (available, or not, depending on the GDB configuration):
target exec program
target core filename
target remote dev
target sim
target udi keyword
target amd-eb dev speed PROG
target remote
;
speed allows you to specify the linespeed; and PROG is the
name of the program to be debugged, as it appears to DOS on the PC.
See section GDB with a remote EB29K.
target hms
device
and speed
to control the serial
line and the communications speed used.
See section GDB and Hitachi Microprocessors.
target nindy devicename
target st2000 dev speed
target vxworks machinename
Different targets are available on different configurations of GDB; your configuration may have more or fewer targets.
If you are trying to debug a program running on a machine that cannot run GDB in the usual way, it is often useful to use remote debugging. For example, you might use remote debugging on an operating system kernel, or on a small system which does not have a general purpose operating system powerful enough to run a full-featured debugger.
Some configurations of GDB have special serial or TCP/IP interfaces to make this work with particular debugging targets. In addition, GDB comes with a generic serial protocol (specific to GDB, but not specific to any particular target system) which you can use if you write the remote stubs--the code that will run on the remote system to communicate with GDB.
Other remote targets may be available in your
configuration of GDB; use help targets
to list them.
To debug a program running on another machine (the debugging target machine), you must first arrange for all the usual prerequisites for the program to run by itself. For example, for a C program, you need
The next step is to arrange for your program to use a serial port to communicate with the machine where GDB is running (the host machine). In general terms, the scheme looks like this:
The debugging stub is specific to the architecture of the remote machine; for example, use `sparc-stub.c' to debug programs on SPARC boards.
These working remote stubs are distributed with GDB:
sparc-stub.c
m68k-stub.c
i386-stub.c
The `README' file in the GDB distribution may list other recently added stubs.
The debugging stub for your architecture supplies these three subroutines:
set_debug_traps
handle_exception
to run when your
program stops. You must call this subroutine explicitly near the
beginning of your program.
handle_exception
handle_exception
to
run when a trap is triggered.
handle_exception
takes control when your program stops during
execution (for example, on a breakpoint), and mediates communications
with GDB on the host machine. This is where the communications
protocol is implemented; handle_exception
acts as the GDB
representative on the target machine; it begins by sending summary
information on the state of your program, then continues to execute,
retrieving and transmitting any information GDB needs, until you
execute a GDB command that makes your program resume; at that point,
handle_exception
returns control to your own code on the target
machine.
breakpoint
handle_exception
---in efect, to GDB. On some machines,
simply receiving characters on the serial port may also trigger a trap;
again, in that situation, you don't need to call breakpoint
from
your own program--simply running `target remote' from the host
GDB session will get control.
Call breakpoint
if none of these is true, or if you simply want
to make certain your program stops at a predetermined point for the
start of your debugging session.
The debugging stubs that come with GDB are set up for a particular chip architecture, but they have no information about the rest of your debugging target machine. To allow the stub to work, you must supply these special low-level subroutines:
int getDebugChar()
getchar
for your target system; a
different name is used to allow you to distinguish the two if you wish.
void putDebugChar(int)
putchar
for your target system; a
different name is used to allow you to distinguish the two if you wish.
void flush_i_cache()
On target machines that have instruction caches, GDB requires this function to make certain that the state of your program is stable.
You must also make sure this library routine is available:
void *memset(void *, int, int)
memset
that sets an area of
memory to a known value. If you have one of the free versions of
libc.a
, memset
can be found there; otherwise, you must
either obtain it from your hardware manufacturer, or write your own.
If you do not use the GNU C compiler, you may need other standard
library subroutines as well; this will vary from one stub to another,
but in general the stubs are likely to use any of the common library
subroutines which gcc
generates as inline code.
In summary, when your program is ready to debug, you must follow these steps.
getDebugChar
,putDebugChar
,flush_i_cache
,memset
.
set_debug_traps(); breakpoint();
Then establish communication using the target remote
command.
Its argument is the name of the device you're using to control the
target machine. For example:
target remote /dev/ttyb
if the serial line is connected to the device named `/dev/ttyb'.
Now you can use all the usual commands to examine and change data and to step and continue the remote program.
To resume the remote program and stop debugging it, use the detach
command.
Whenever GDB is waiting for the remote program, if you type the interrupt character (often C-C), GDB attempts to stop the program. This may or may not succeed, depending in part on the hardware and the serial drivers the remote system uses. If you type the interrupt character once again, GDB displays this prompt:
Interrupted while waiting for the program. Give up (and stop debugging it)? (y or n)
If you type y, GDB abandons the remote debugging session. (If you decide you want to try again later, you can use `target remote' again to connect once more.) If you type n, GDB goes back to waiting.
The stub files provided with GDB implement the target side of the communication protocol, and the GDB side is implemented in the GDB source file `remote.c'. Normally, you can simply allow these subroutines to communicate, and ignore the details. (If you're implementing your own stub file, you can still ignore the details: start with one of the existing stub files. `sparc-stub.c' is the best organized, and therefore the easiest to read.)
However, there may be occasions when you need to know something about the protocol--for example, if there is only one serial port to your target machine, you might want your program to do something special if it recognizes a packet meant for GDB.
All GDB commands and responses (other than acknowledgements, which are single characters) are sent as a packet which includes a checksum. A packet is introduced with the character `$', and ends with the character `#' followed by a two-digit checksum:
$packet info#checksum
checksum is computed as the modulo 256 sum of the packet info characters.
When either the host or the target machine receives a packet, the first response expected is an acknowledgement: a single character, either `+' (to indicate the package was received correctly) or `-' (to request retransmission).
The host (GDB) sends commands, and the target (the debugging stub incorporated in your program) sends data in response. The target also sends data when your program stops.
Command packets are distinguished by their first character, which identifies the kind of command.
These are the commands currently supported:
g
G
maddr,count
Maddr,count:...
c
caddr
s
saddr
k
?
If you have trouble with the serial connection, you can use the command
set remotedebug
. This makes GDB report on all packets sent
back and forth across the serial line to the remote machine. The
packet-debugging information is printed on the GDB standard output
stream. set remotedebug off
turns it off, and show
remotedebug
will show you its current state.
Nindy is a ROM Monitor program for Intel 960 target systems. When GDB is configured to control a remote Intel 960 using Nindy, you can tell GDB how to connect to the 960 in several ways:
target
command at any point during your GDB
session. See section Commands for managing targets.
If you simply start gdb
without using any command-line
options, you are prompted for what serial port to use, before you
reach the ordinary GDB prompt:
Attach /dev/ttyNN -- specify NN, or "quit" to quit:
Respond to the prompt with whatever suffix (after `/dev/tty')
identifies the serial port you want to use. You can, if you choose,
simply start up with no Nindy connection by responding to the prompt
with an empty line. If you do this and later wish to attach to Nindy,
use target
(see section Commands for managing targets).
These are the startup options for beginning your GDB session with a Nindy-960 board attached:
-r port
tty
(e.g. `-r a').
-O
Warning: if you specify `-O', but are actually trying to connect to a target system that expects the newer protocol, the connection fails, appearing to be a speed mismatch. GDB repeatedly attempts to reconnect at several different line speeds. You can abort this process with an interrupt.
-brk
BREAK
signal to the target
system, in an attempt to reset it, before connecting to a Nindy target.
Warning: Many target systems do not have the hardware that this requires; it only works with a few boards.
The standard `-b' option controls the line speed used on the serial port.
reset
GDB supports AMD's UDI ("Universal Debugger Interface")
protocol for debugging the a29k processor family. To use this
configuration with AMD targets running the MiniMON monitor, you need the
program MONTIP
, available from AMD at no charge. You can also
use GDB with the UDI conformant a29k simulator program
ISSTIP
, also available from AMD.
target udi keyword
To use GDB from a Unix system to run programs on AMD's EB29K board in a PC, you must first connect a serial cable between the PC and a serial port on the Unix system. In the following, we assume you've hooked the cable between the PC's `COM1' port and `/dev/ttya' on the Unix system.
The next step is to set up the PC's port, by doing something like this in DOS on the PC:
C:\> MODE com1:9600,n,8,1,none
This example--run on an MS DOS 4.0 system--sets the PC port to 9600 bps, no parity, eight data bits, one stop bit, and no "retry" action; you must match the communications parameters when establishing the Unix end of the connection as well.
To give control of the PC to the Unix side of the serial line, type the following at the DOS console:
C:\> CTTY com1
(Later, if you wish to return control to the DOS console, you can use
the command CTTY con
---but you must send it over the device that
had control, in our example over the `COM1' serial line).
From the Unix host, use a communications program such as tip
or
cu
to communicate with the PC; for example,
cu -s 9600 -l /dev/ttya
The cu
options shown specify, respectively, the linespeed and the
serial port to use. If you use tip
instead, your command line
may look something like the following:
tip -9600 /dev/ttya
Your system may require a different name where we show
`/dev/ttya' as the argument to tip
. The communications
parameters, including which port to use, are associated with the
tip
argument in the "remote" descriptions file--normally the
system table `/etc/remote'.
Using the tip
or cu
connection, change the DOS working
directory to the directory containing a copy of your 29K program, then
start the PC program EBMON
(an EB29K control program supplied
with your board by AMD). You should see an initial display from
EBMON
similar to the one that follows, ending with the
EBMON
prompt `#'---
C:\> G: G:\> CD \usr\joe\work29k G:\USR\JOE\WORK29K> EBMON Am29000 PC Coprocessor Board Monitor, version 3.0-18 Copyright 1990 Advanced Micro Devices, Inc. Written by Gibbons and Associates, Inc. Enter '?' or 'H' for help PC Coprocessor Type = EB29K I/O Base = 0x208 Memory Base = 0xd0000 Data Memory Size = 2048KB Available I-RAM Range = 0x8000 to 0x1fffff Available D-RAM Range = 0x80002000 to 0x801fffff PageSize = 0x400 Register Stack Size = 0x800 Memory Stack Size = 0x1800 CPU PRL = 0x3 Am29027 Available = No Byte Write Available = Yes # ~.
Then exit the cu
or tip
program (done in the example by
typing ~.
at the EBMON
prompt). EBMON
will keep
running, ready for GDB to take over.
For this example, we've assumed what is probably the most convenient
way to make sure the same 29K program is on both the PC and the Unix
system: a PC/NFS connection that establishes "drive G:
" on the
PC as a file system on the Unix host. If you do not have PC/NFS or
something similar connecting the two systems, you must arrange some
other way--perhaps floppy-disk transfer--of getting the 29K program
from the Unix system to the PC; GDB will not download it over the
serial line.
Finally, cd
to the directory containing an image of your 29K
program on the Unix system, and start GDB---specifying as argument the
name of your 29K program:
cd /usr/joe/work29k gdb myfoo
Now you can use the target
command:
target amd-eb /dev/ttya 9600 MYFOO
In this example, we've assumed your program is in a file called
`myfoo'. Note that the filename given as the last argument to
target amd-eb
should be the name of the program as it appears to DOS.
In our example this is simply MYFOO
, but in general it can include
a DOS path, and depending on your transfer mechanism may not resemble
the name on the Unix side.
At this point, you can set any breakpoints you wish; when you are ready
to see your program run on the 29K board, use the GDB command
run
.
To stop debugging the remote program, use the GDB detach
command.
To return control of the PC to its console, use tip
or cu
once again, after your GDB session has concluded, to attach to
EBMON
. You can then type the command q
to shut down
EBMON
, returning control to the DOS command-line interpreter.
Type CTTY con
to return command input to the main DOS console,
and type ~. to leave tip
or cu
.
The target amd-eb
command creates a file `eb.log' in the
current working directory, to help debug problems with the connection.
`eb.log' records all the output from EBMON
, including echoes
of the commands sent to it. Running `tail -f' on this file in
another window often helps to understand trouble with EBMON
, or
unexpected events on the PC side of the connection.
To connect your ST2000 to the host system, see the manufacturer's manual. Once the ST2000 is physically attached, you can run
target st2000 dev speed
to establish it as your debugging environment.
The load
and attach
commands are not defined for
this target; you must load your program into the ST2000 as you normally
would for standalone operation. GDB will read debugging information
(such as symbols) from a separate, debugging version of the program
available on your host computer.
These auxiliary GDB commands are available to help you with the ST2000 environment:
st2000 command
connect
GDB enables developers to spawn and debug tasks running on networked
VxWorks targets from a Unix host. Already-running tasks spawned from
the VxWorks shell can also be debugged. GDB uses code that runs on
both the UNIX host and on the VxWorks target. The program
gdb
is installed and executed on the UNIX host.
The following information on connecting to VxWorks was current when this manual was produced; newer releases of VxWorks may use revised procedures.
The remote debugging interface (RDB) routines are installed and executed on the VxWorks target. These routines are included in the VxWorks library `rdb.a' and are incorporated into the system image when source-level debugging is enabled in the VxWorks configuration.
If you wish, you can define INCLUDE_RDB
in the VxWorks
configuration file `configAll.h' to include the RDB interface
routines and spawn the source debugging task tRdbTask
when
VxWorks is booted. For more information on configuring and remaking
VxWorks, see the manufacturer's manual.
Once you have included the RDB interface in your VxWorks system image and set your Unix execution search path to find GDB, you are ready to run GDB. From your UNIX host, type:
% gdb
GDB will come up showing the prompt:
(gdb)
The GDB command target
lets you connect to a VxWorks target on the
network. To connect to a target whose host name is "tt
", type:
(gdb) target vxworks tt
GDB will display a message similar to the following:
Attaching remote machine across net... Success!
GDB will then attempt to read the symbol tables of any object modules loaded into the VxWorks target since it was last booted. GDB locates these files by searching the directories listed in the command search path (see section Your program's environment); if it fails to find an object file, it will display a message such as:
prog.o: No such file or directory.
This will cause the target
command to abort. When this happens,
you should add the appropriate directory to the search path, with the
GDB command path
, and execute the target
command
again.
If you have connected to the VxWorks target and you want to debug an
object that has not yet been loaded, you can use the GDB load
command to download a file from UNIX to VxWorks incrementally. The
object file given as an argument to the load
command is actually
opened twice: first by the VxWorks target in order to download the code,
then by GDB in order to read the symbol table. This can lead to
problems if the current working directories on the two systems differ.
It is simplest to set the working directory on both systems to the
directory in which the object file resides, and then to reference the
file by its name, without any path. Thus, to load a program
`prog.o', residing in `wherever/vw/demo/rdb', on VxWorks type:
-> cd "wherever/vw/demo/rdb"
On GDB type:
(gdb) cd wherever/vw/demo/rdb (gdb) load prog.o
GDB will display a response similar to the following:
Reading symbol data from wherever/vw/demo/rdb/prog.o... done.
You can also use the load
command to reload an object module
after editing and recompiling the corresponding source file. Note that
this will cause GDB to delete all currently-defined breakpoints,
auto-displays, and convenience variables, and to clear the value
history. (This is necessary in order to preserve the integrity of
debugger data structures that reference the target system's symbol
table.)
You can also attach to an existing task using the attach
command as
follows:
(gdb) attach task
where task is the VxWorks hexadecimal task ID. The task can be running or suspended when you attach to it. If running, it will be suspended at the time of attachment.
Use the special gdb
command `device port' if you
need to explicitly set the serial device. The default port is the
first available port on your host. This is only necessary on Unix
hosts, where it is typically something like `/dev/ttya'.
gdb
has another special command to set the communications
speed: `speed bps'. This command also is only used from Unix
hosts; on DOS hosts, set the line speed as usual from outside GDB with
the DOS mode command (for instance, `mode
com2:9600,n,8,1,p' for a 9600 bps connection).
The `device' and `speed' commands are available only when you
use a Unix host to debug your Hitachi microprocessor programs. If you
use a DOS host,
GDB depends on an auxiliary terminate-and-stay-resident program
called asynctsr
to communicate with the development board
through a PC serial port. You must also use the DOS mode
command
to set up the serial port on the DOS side.
GDB can use the MIPS remote debugging protocol to talk to a MIPS board attached to a serial line. This is available when you configure GDB with `--target=mips-idt-ecoff'.
To run a program on the board, start up gdb
with the
name of your program as the argument. To connect to the board, use the
command `target mips port', where port is the name of
the serial port connected to the board. If the program has not already
been downloaded to the board, you may use the load
command to
download it. You can then use all the usual GDB commands.
You can see some debugging information about communications with the board
by setting the remotedebug
variable. If you set it to 1 using
`set remotedebug 1' every packet will be displayed. If you set it
to 2 every character will be displayed. You can check the current value
at any time with the command `show remotedebug'.
If your target board does not support the MIPS floating point coprocessor, you should use the command `set mipsfpu off' (you may wish to put this in your .gdbinit file). This will tell GDB how to find the return value of functions which return floating point values, and tell it to call functions on the board without saving the floating point registers.
For some configurations, GDB includes a CPU simulator that you can use instead of a hardware CPU to debug your programs. Currently, a simulator is available when GDB is configured to debug Zilog Z8000 or Hitachi microprocessor targets.
For the Z8000 family, `target sim' simulates either the Z8002 (the unsegmented variant of the Z8000 architecture) or the Z8001 (the segmented variant). The simulator recognizes which architecture is appropriate by inspecting the object code.
target sim
After specifying this target, you can debug programs for the simulated
CPU in the same style as programs for your host computer; use the
file
command to load a new program image, the run
command
to run your program, and so on.
As well as making available all the usual machine registers (see
info reg
), this debugging target provides three additional items
of information as specially named registers:
cycles
insts
time
You can refer to these values in GDB expressions with the usual conventions; for example, `b fputc if $cycles>5000' sets a conditional breakpoint that will suspend only after at least 5000 simulated clock ticks.
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