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Specifying a Debugging Target

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).

Active 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.

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).

Commands for managing targets

target type parameters
Connects the GDB host environment to a target machine or process. A target is typically a protocol for talking to debugging facilities. You use the argument type to specify the type or protocol of the target machine.

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 does not repeat if you press RET again after executing the command.

help target
Displays the names of all targets available. To display targets currently selected, use either info target or info files (see section Commands to specify files).

help target name
Describe a particular target, including any parameters necessary to select it.

Here are some common targets (available, or not, depending on the GDB configuration):

target exec program
An executable file. `target exec program' is the same as `exec-file program'.

target core filename
A core dump file. `target core filename' is the same as `core-file filename'.

target remote dev
Remote serial target in GDB-specific protocol. The argument dev specifies what serial device to use for the connection (e.g. `/dev/ttya'). See section Remote debugging.

target sim
CPU simulator. See section Simulated CPU target.

target udi keyword
Remote AMD29K target, using the AMD UDI protocol. The keyword argument specifies which 29K board or simulator to use. See section The UDI protocol for AMD29K.

target amd-eb dev speed PROG
Remote PC-resident AMD EB29K board, attached over serial lines. dev is the serial device, as for 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.

target hms
A Hitachi SH, H8/300, or H8/500 board, attached via serial line to your host. Use special commands device and speed to control the serial line and the communications speed used. See section GDB and Hitachi microprocessors.

target nindy devicename
An Intel 960 board controlled by a Nindy Monitor. devicename is the name of the serial device to use for the connection, e.g. `/dev/ttya'. See section GDB with a remote i960 (Nindy).

target st2000 dev speed
A Tandem ST2000 phone switch, running Tandem's STDBUG protocol. dev is the name of the device attached to the ST2000 serial line; speed is the communication line speed. The arguments are not used if GDB is configured to connect to the ST2000 using TCP or Telnet. See section GDB with a Tandem ST2000.

target vxworks machinename
A VxWorks system, attached via TCP/IP. The argument machinename is the target system's machine name or IP address. See section GDB and VxWorks.

Different targets are available on different configurations of GDB; your configuration may have more or fewer targets.

Remote debugging

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.

The GDB remote serial protocol

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

  1. A startup routine to set up the C runtime environment; these usually have a name like `crt0'. The startup routine may be supplied by your hardware supplier, or you may have to write your own.

  2. You probably need a C subroutine library to support your program's subroutine calls, notably managing input and output.

  3. A way of getting your program to the other machine--for example, a download program. These are often supplied by the hardware manufacturer, but you may have to write your own from hardware documentation.

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 the host,
GDB already understands how to use this protocol; when everything else is set up, you can simply use the `target remote' command (see section Specifying a Debugging Target).

On the target,
you must link with your program a few special-purpose subroutines that implement the GDB remote serial protocol. The file containing these subroutines is called a debugging stub.

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:

For SPARC architectures.

For Motorola 680x0 architectures.

For Intel 386 and compatible architectures.

The `README' file in the GDB distribution may list other recently added stubs.

What the stub can do for you

The debugging stub for your architecture supplies these three subroutines:

This routine arranges for handle_exception to run when your program stops. You must call this subroutine explicitly near the beginning of your program.

This is the central workhorse, but your program never calls it explicitly--the setup code arranges for 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.

Use this auxiliary subroutine to make your program contain a breakpoint. Depending on the particular situation, this may be the only way for GDB to get control. For instance, if your target machine has some sort of interrupt button, you won't need to call this; pressing the interrupt button transfers control to 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 breakpoint from your own program--simply running `target remote' from the host GDB session gets 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.

What you must do for the stub

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.

int getDebugChar()
Write this subroutine to read a single character from the serial port. It may be identical to getchar for your target system; a different name is used to allow you to distinguish the two if you wish.

void putDebugChar(int)
Write this subroutine to write a single character to the serial port. It may be identical to putchar for 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 SIGINT).

Other routines you need to supply are:

void exceptionHandler (int exception_number, void *exception_address)
Write this function to install exception_address in the exception handling tables. You need to do this because the stub does not have any way of knowing what the exception handling tables on your target system are like (for example, the processor's table might be in ROM, containing entries which point to a table in RAM). exception_number is the exception number which should be changed; its meaning is architecture-dependent (for example, different numbers might represent divide by zero, misaligned access, etc). When this exception occurs, control should be transferred directly to exception_address, and the processor state (stack, registers, and so on) should be just as it is when a processor exception occurs. So if you want to use a jump instruction to reach exception_address, it should be a simple jump, not a jump to subroutine.

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 help from exceptionHandler.

void flush_i_cache()
Write this subroutine to flush the instruction cache, if any, on your target machine. If there is no instruction cache, this subroutine may be a no-op.

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)
This is the standard library function 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 varies 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.

Putting it all together

In summary, when your program is ready to debug, you must follow these steps.

  1. Make sure you have the supporting low-level routines (see section What you must do for the stub):
    getDebugChar, putDebugChar,
    flush_i_cache, memset, exceptionHandler.

  2. Insert these lines near the top of your program:


  3. For the 680x0 stub only, you need to provide a variable called 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.

  4. Compile and link together: your program, the GDB debugging stub for your target architecture, and the supporting subroutines.

  5. Make sure you have a serial connection between your target machine and the GDB host, and identify the serial port used for this on the host.

  6. Download your program to your target machine (or get it there by whatever means the manufacturer provides), and start it.

  7. To start remote debugging, run GDB on the host machine, and specify as an executable file the program that is running in the remote machine. This tells GDB how to find your program's symbols and the contents of its pure text.

    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 manyfarms:

    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 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.

Communication protocol

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:

Requests the values of CPU registers.

Sets the values of CPU registers.

Read count bytes at location addr.

Write count bytes at location addr.

Resume execution at the current address (or at addr if supplied).

Step the target program for one instruction, from either the current program counter or from addr if supplied.

Kill the target program.

Report the most recent signal. To allow you to take advantage of the GDB signal handling commands, one of the functions of the debugging stub is to report CPU traps as the corresponding POSIX signal values.

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 shows you its current state.

Using the gdbserver program

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 GDB locally! 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 gdbserver. 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.

GDB and gdbserver communicate via either a serial line or a TCP connection, using the standard GDB remote serial protocol.

On the target machine,
you need to have a copy of the program you want to debug. gdbserver does 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 with it.

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, 23 is reserved for telnet).(3) You must use the same port number with the host GDB target remote command.

On the GDB host machine,
you need an unstripped copy of your program, since GDB needs symbols and debugging information. Start up GDB as usual, using the name of the local copy of your program as the first argument. (You may also need the `--baud' option if the serial line is running at anything other than 9600 bps.) After that, use target remote to 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 the target remote command. Otherwise you may get an error whose text depends on the host system, but which usually looks something like `Connection refused'.

GDB with a remote i960 (Nindy)

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:

Startup with Nindy

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).

Options for Nindy

These are the startup options for beginning your GDB session with a Nindy-960 board attached:

-r port
Specify the serial port name of a serial interface to be used to connect to the target system. This option is only available when GDB is configured for the Intel 960 target architecture. You may specify port as any of: a full pathname (e.g. `-r /dev/ttya'), a device name in `/dev' (e.g. `-r ttya'), or simply the unique suffix for a specific tty (e.g. `-r a').

(An uppercase letter "O", not a zero.) Specify that GDB should use the "old" Nindy monitor protocol to connect to the target system. This option is only available when GDB is configured for the Intel 960 target architecture.

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.

Specify that GDB should first send a 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.

Nindy reset command

For a Nindy target, this command sends a "break" to the remote target system; this is only useful if the target has been equipped with a circuit to perform a hard reset (or some other interesting action) when a break is detected.

The UDI protocol for AMD29K

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
Select the UDI interface to a remote a29k board or simulator, where keyword is an entry in the AMD configuration file `udi_soc'. This file contains keyword entries which specify parameters used to connect to a29k targets. If the `udi_soc' file is not in your working directory, you must set the environment variable `UDICONF' to its pathname.

The EBMON protocol for AMD29K

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.

Communications setup

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

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 keeps 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 serial line.

EB29K cross-debugging

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.

Remote log

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.

GDB with a Tandem ST2000

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 hostname:portnumber.

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 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:

st2000 command
Send a command to the STDBUG monitor. See the manufacturer's manual for available commands.

Connect the controlling terminal to the STDBUG command monitor. When you are done interacting with STDBUG, typing either of two character sequences gets you back to the GDB command prompt: RET~. (Return, followed by tilde and period) or RET~C-d (Return, followed by tilde and control-D).

GDB and VxWorks

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 manual.

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 gdb (or vxgdb, depending on your installation).

GDB comes up showing the prompt:


Connecting to VxWorks

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:

(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 target command again.

VxWorks download

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. 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 table.)

Running tasks

You can also attach to an existing task using the attach command as follows:

(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.

GDB and Hitachi microprocessors

GDB needs to know these things to talk to your Hitachi SH, H8/300, or H8/500:

  1. that you want to use `target hms', the remote debugging interface for Hitachi microprocessors, or `target e7000', the in-circuit emulator for the Hitachi SH and the Hitachi 300H. (`target hms' is the default when GDB is configured specifically for the Hitachi SH, H8/300, or H8/500.)

  2. what serial device connects your host to your Hitachi board (the first serial device available on your host is the default).

  3. what speed to use over the serial device.

Connecting to Hitachi boards

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.

Using the E7000 in-circuit emulator

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
Use this form if your E7000 is connected to a serial port. The port argument identifies what serial port to use (for example, `com2'). The third argument is the line speed in bits per second (for example, `9600').

target e7000 hostname
If your E7000 is installed as a host on a TCP/IP network, you can just specify its hostname; GDB uses telnet to connect.

Special GDB commands for Hitachi micros

Some GDB commands are available only on the H8/300 or the H8/500 configurations:

set machine h8300
set machine h8300h
Condition GDB for one of the two variants of the H8/300 architecture with `set machine'. You can use `show machine' to check which variant is currently in effect.

set memory mod
show memory
Specify which H8/500 memory model (mod) you are using with `set memory'; check which memory model is in effect with `show memory'. The accepted values for mod are small, big, medium, and compact.

GDB and remote MIPS boards

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
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.

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
On some GDB host configurations, you can specify a TCP connection (for instance, to a serial line managed by a terminal concentrator) instead of a serial port, using the syntax `hostname:portnumber'.

GDB also supports these special commands for MIPS targets:

set mipsfpu double
set mipsfpu single
set mipsfpu none
show mipsfpu
If your target board does not support the MIPS floating point coprocessor, you should use the command `set mipsfpu none' (if you need this, you may wish to put the command in your .gdbinit file). This tells GDB how to find the return value of functions which return floating point values. It also allows GDB to avoid saving the floating point registers when calling functions on the board. If you are using a floating point coprocessor with only single precision floating point support, as on the R4650 processor, use the command `set mipsfpu single'. The default double precision floating point coprocessor may be selected using `set mipsfpu double'.

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 `show mipsfpu'.

set remotedebug n
show remotedebug
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 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
show timeout
show retransmit-timeout
You can control the timeout used while waiting for a packet, in the MIPS remote protocol, with the set timeout seconds command. 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 seconds command. The default is 3 seconds. You can inspect both values with show timeout and 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.

Simulated CPU target

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
Debug programs on a simulated CPU (which CPU depends on the GDB configuration)

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:

Counts clock-ticks in the simulator.

Counts instructions run in the simulator.

Execution time in 60ths of a second.

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.

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