Go to the previous, next section.
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
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 uses 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.
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
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.
target command does not repeat if you press RET again
after executing the command.
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 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 The EBMON protocol for AMD29K.
speedto 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 runs on the remote system to communicate with GDB.
Other remote targets may be available in your
configuration of GDB; use
help target 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:
On certain remote targets, you can use an auxiliary program
gdbserver instead of linking a stub into your program.
See section Using the
gdbserver program, for details.
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:
The `README' file in the GDB distribution may list other recently added stubs.
The debugging stub for your architecture supplies these three subroutines:
handle_exceptionto run when your program stops. You must call this subroutine explicitly near the beginning of your program.
handle_exceptionto 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
handle_exception---in effect, 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
breakpointfrom your own program--simply running `target remote' from the host GDB session gets control.
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.
First of all you need to tell the stub how to communicate with the serial port.
getcharfor your target system; a different name is used to allow you to distinguish the two if you wish.
putcharfor your target system; a different name is used to allow you to distinguish the two if you wish.
If you want GDB to be able to stop your program while it is
running, you need to use an interrupt-driven serial driver, and arrange
for it to stop when it receives a
^C (`\003', the control-C
character). That is the character which GDB uses to tell the
remote system to stop.
Getting the debugging target to return the proper status to GDB
probably requires changes to the standard stub; one quick and dirty way
is to just execute a breakpoint instruction (the "dirty" part is that
GDB reports a
SIGTRAP instead of a
Other routines you need to supply are:
void exceptionHandler (int exception_number, void *exception_address)
For the 386, exception_address should be installed as an interrupt
gate so that interrupts are masked while the handler runs. The gate
should be at privilege level 0 (the most privileged level). The
SPARC and 68k stubs are able to mask interrupts themself without
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)
memsetthat sets an area of memory to a known value. If you have one of the free versions of
memsetcan 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 varies from one stub to another,
but in general the stubs are likely to use any of the common library
gcc generates as inline code.
In summary, when your program is ready to debug, you must follow these steps.
exceptionHook. Normally you just use
void (*exceptionHook)() = 0;
but if before calling
set_debug_traps, you set it to point to a
function in your program, that function is called when
GDB continues after stopping on a trap (for example, bus
error). The function indicated by
exceptionHook is called with
one parameter: an
int which is the exception number.
Then establish communication using the
target remote command.
Its argument specifies how to communicate with the target
machine--either via a devicename attached to a direct serial line, or a
TCP port (usually to a terminal server which in turn has a serial line
to the target). For example, to use a serial line connected to the
device named `/dev/ttyb':
target remote /dev/ttyb
To use a TCP connection, use an argument of the form
host:port. For example, to connect to port 2828 on a
terminal server named
target remote manyfarms:2828
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
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:
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:
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
set remotedebug off turns it off, and
remotedebug shows you its current state.
gdbserver is a control program for Unix-like systems, which
allows you to connect your program with a remote GDB via
target remote---but without linking in the usual debugging stub.
gdbserver is not a complete replacement for the debugging stubs,
because it requires essentially the same operating-system facilities
that GDB itself does. In fact, a system that can run
gdbserver to connect to a remote GDB could also run
gdbserver is sometimes useful nevertheless,
because it is a much smaller program than GDB itself. It is
also easier to port than all of GDB, so you may be able to get
started more quickly on a new system by using
Finally, if you develop code for real-time systems, you may find that
the tradeoffs involved in real-time operation make it more convenient to
do as much development work as possible on another system, for example
by cross-compiling. You can use
gdbserver to make a similar
choice for debugging.
gdbserver communicate via either a serial line
or a TCP connection, using the standard GDB remote serial
gdbserverdoes not need your program's symbol table, so you can strip the program if necessary to save space. GDB on the host system does all the symbol handling.
To use the server, you must tell it how to communicate with GDB; the name of your program; and the arguments for your program. The syntax is:
target> gdbserver comm program [ args ... ]
comm is either a device name (to use a serial line) or a TCP hostname and portnumber. For example, to debug Emacs with the argument `foo.txt' and communicate with GDB over the serial port `/dev/com1':
target> gdbserver /dev/com1 emacs foo.txt
gdbserver waits passively for the host GDB to communicate
To use a TCP connection instead of a serial line:
target> gdbserver host:2345 emacs foo.txt
The only difference from the previous example is the first argument,
specifying that you are communicating with the host GDB via
TCP. The `host:2345' argument means that
gdbserver is to
expect a TCP connection from machine `host' to local TCP port 2345.
(Currently, the `host' part is ignored.) You can choose any number
you want for the port number as long as it does not conflict with any
TCP ports already in use on the target system (for example,
telnet).(3) You must use the same port number with the host GDB
target remote command.
target remoteto establish communications with
gdbserver. Its argument is either a device name (usually a serial device, like `/dev/ttyb'), or a TCP port descriptor in the form
host:PORT. For example:
(gdb) target remote /dev/ttyb
communicates with the server via serial line `/dev/ttyb', and
(gdb) target remote the-target:2345
communicates via a TCP connection to port 2345 on host `the-target'.
For TCP connections, you must start up
gdbserver prior to using
target remote command. Otherwise you may get an error whose
text depends on the host system, but which usually looks something like
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:
targetcommand 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,
target (see section Commands for managing targets).
These are the startup options for beginning your GDB session with a Nindy-960 board attached:
tty(e.g. `-r a').
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.
BREAKsignal 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.
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
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
AMD distributes a 29K development board meant to fit in a PC, together
with a DOS-hosted monitor program called
EBMON. As a shorthand
term, this development system is called the "EB29K". To use
GDB from a Unix system to run programs on the EB29K board, you
must first connect a serial cable between the PC (which hosts the EB29K
board) 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
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
cu to communicate with the PC; for example,
cu -s 9600 -l /dev/ttya
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'.
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
tip program (done in the example by
~. at the
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 does not download it over the
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 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
To stop debugging the remote program, use the GDB
To return control of the PC to its console, use
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.
CTTY con to return command input to the main DOS console,
and type ~. to leave
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
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. dev is normally
the name of a serial device, such as `/dev/ttya', connected to the
ST2000 via a serial line. You can instead specify dev as a TCP
connection (for example, to a serial line attached via a terminal
concentrator) using the syntax
attach commands are not defined for
this target; you must load your program into the ST2000 as you normally
would for standalone operation. GDB reads 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:
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. (It may be
installed with the name
vxgdb, to distinguish it from a
GDB for debugging programs on the host itself.)
The following information on connecting to VxWorks was current when this manual was produced; newer releases of VxWorks may use revised procedures.
To use GDB with VxWorks, you must rebuild your VxWorks kernel
to include the remote debugging interface routines in the VxWorks
library `rdb.a'. To do this, define
INCLUDE_RDB in the
VxWorks configuration file `configAll.h' and rebuild your VxWorks
kernel. The resulting kernel contains `rdb.a', and spawns the
source debugging task
tRdbTask when VxWorks is booted. For more
information on configuring and remaking VxWorks, see the manufacturer's
Once you have included `rdb.a' 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, run
depending on your installation).
GDB comes up showing the prompt:
The GDB command
target lets you connect to a VxWorks target on the
network. To connect to a target whose host name is "
(vxgdb) target vxworks tt
GDB displays messages like these:
Attaching remote machine across net... Connected to tt.
GDB then attempts 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 displays a message such as:
prog.o: No such file or directory.
When this happens, add the appropriate directory to the search path with
the GDB command
path, and execute the
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
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. If both systems have NFS mounted the same
filesystems, you can avoid these problems by using absolute paths.
Otherwise, 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. For instance, a program
`prog.o' may reside in `vxpath/vw/demo/rdb' in VxWorks
and in `hostpath/vw/demo/rdb' on the host. To load this
program, type this on VxWorks:
-> cd "vxpath/vw/demo/rdb"
Then, in GDB, type:
(vxgdb) cd hostpath/vw/demo/rdb (vxgdb) load prog.o
GDB displays a response similar to this:
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 makes GDB 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
You can also attach to an existing task using the
attach command as
(vxgdb) attach task
where task is the VxWorks hexadecimal task ID. The task can be running or suspended when you attach to it. Running tasks are 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
asynctsr to communicate with the development board
through a PC serial port. You must also use the DOS
to set up the serial port on the DOS side.
You can use the E7000 in-circuit emulator to develop code for either the Hitachi SH or the H8/300H. Use one of these forms of the `target e7000' command to connect GDB to your E7000:
target e7000 port speed
target e7000 hostname
Some GDB commands are available only on the H8/300 or the H8/500 configurations:
set machine h8300
set machine h8300h
set memory mod
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'.
Use these GDB commands to specify the connection to your target board:
target mips port
gdbwith 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
loadcommand to download it. You can then use all the usual GDB commands.
For example, this sequence connects to the target board through a serial port, and loads and runs a program called prog through the debugger:
host$ gdb prog GDB is free software and ... (gdb) target mips /dev/ttyb (gdb) load prog (gdb) run
target mips hostname:portnumber
GDB also supports these special commands for MIPS targets:
set mipsfpu double
set mipsfpu single
set mipsfpu none
In previous versions the only choices were double precision or no floating point, so `set mipsfpu on' will select double precision and `set mipsfpu off' will select no floating point.
As usual, you can inquire about the
mipsfpu variable with
set remotedebug n
remotedebugvariable. If you set it to
1using `set remotedebug 1', every packet is displayed. If you set it to
2, every character is displayed. You can check the current value at any time with the command `show remotedebug'.
set timeout seconds
set retransmit-timeout seconds
set timeout secondscommand. The default is 5 seconds. Similarly, you can control the timeout used while waiting for an acknowledgement of a packet with the
set retransmit-timeout secondscommand. The default is 3 seconds. You can inspect both values with
show retransmit-timeout. (These commands are only available when GDB is configured for `--target=mips-idt-ecoff'.)
The timeout set by
set timeout does not apply when GDB
is waiting for your program to stop. In that case, GDB waits
forever because it has no way of knowing how long the program is going
to run before stopping.
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.
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
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:
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 suspends only after at least 5000 simulated clock ticks.
Go to the previous, next section.