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Debugging remote programs

Connecting to a remote target

On the GDB host machine, you will need an unstripped copy of your program, since GDB needs symobl and debugging information. Start up GDB as usual, using the name of the local copy of your program as the first argument.

If you're using a serial line, you may want to give GDB the `--baud' option, or use the set remotebaud command (see section Remote configuration) before the target command.

After that, use target remote to establish communications with the target machine. Its argument specifies how to communicate--either via a devicename attached to a direct serial line, or a TCP or UDP port (possibly 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 or tcp:host:port. For example, to connect to port 2828 on a terminal server named manyfarms:

target remote manyfarms:2828

If your remote target is actually running on the same machine as your debugger session (e.g. a simulator of your target running on the same host), you can omit the hostname. For example, to connect to port 1234 on your local machine:

target remote :1234

Note that the colon is still required here.

To use a UDP connection, use an argument of the form udp:host:port. For example, to connect to UDP port 2828 on a terminal server named manyfarms:

target remote udp:manyfarms:2828

When using a UDP connection for remote debugging, you should keep in mind that the `U' stands for "Unreliable". UDP can silently drop packets on busy or unreliable networks, which will cause havoc with your debugging session.

Now you can use all the usual commands to examine and change data and to step and continue the remote program.

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.

detach
When you have finished debugging the remote program, you can use the detach command to release it from GDB control. Detaching from the target normally resumes its execution, but the results will depend on your particular remote stub. After the detach command, GDB is free to connect to another target.
disconnect
The disconnect command behaves like detach, except that the target is generally not resumed. It will wait for GDB (this instance or another one) to connect and continue debugging. After the disconnect command, GDB is again free to connect to another target.
monitor cmd
This command allows you to send commands directly to the remote monitor.

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 usual 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).(6) You must use the same port number with the host GDB target remote command. On some targets, gdbserver can also attach to running programs. This is accomplished via the --attach argument. The syntax is:
target> gdbserver comm --attach pid
pid is the process ID of a currently running process. It isn't necessary to point gdbserver at a binary for the running process. You can debug processes by name instead of process ID if your target has the pidof utility:
target> gdbserver comm --attach `pidof PROGRAM`
In case more than one copy of PROGRAM is running, or PROGRAM has multiple threads, most versions of pidof support the -s option to only return the first process ID.
On the host machine,
connect to your target (see section Connecting to a remote 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'. You don't need to use the load command in GDB when using gdbserver, since the program is already on the target. However, if you want to load the symbols (as you normally would), do that with the file command, and issue it before connecting to the server; otherwise, you will get an error message saying "Program is already running", since the program is considered running after the connection.

Using the gdbserve.nlm program

gdbserve.nlm is a control program for NetWare systems, which allows you to connect your program with a remote GDB via target remote.

GDB and gdbserve.nlm communicate via a serial line, using the standard GDB remote serial protocol.

On the target machine,
you need to have a copy of the program you want to debug. gdbserve.nlm 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:
load gdbserve [ BOARD=board ] [ PORT=port ]
              [ BAUD=baud ] program [ args ... ]
board and port specify the serial line; baud specifies the baud rate used by the connection. port and node default to 0, baud defaults to 9600bps. For example, to debug Emacs with the argument `foo.txt'and communicate with GDB over serial port number 2 or board 1 using a 19200bps connection:
load gdbserve BOARD=1 PORT=2 BAUD=19200 emacs foo.txt
On the GDB host machine, connect to your target (see section Connecting to a remote target).

Remote configuration

This section documents the configuration options available when debugging remote programs. For the options related to the File I/O extensions of the remote protocol, see section The `system' function call.

set remoteaddresssize bits
Set the maximum size of address in a memory packet to the specified number of bits. GDB will mask off the address bits above that number, when it passes addresses to the remote target. The default value is the number of bits in the target's address.
show remoteaddresssize
Show the current value of remote address size in bits.
set remotebaud n
Set the baud rate for the remote serial I/O to n baud. The value is used to set the speed of the serial port used for debugging remote targets.
show remotebaud
Show the current speed of the remote connection.
set remotebreak
If set to on, GDB sends a BREAK signal to the remote when you press the Ctrl-C key to interrupt the program running on the remote. If set to off, GDB sends the `Strl-C' character instead. The default is off, since most remote systems expect to see `Ctrl-C' as the interrupt signal.
show remotebreak
Show whether GDB sends BREAK or `Ctrl-C' to interrupt the remote program.
set remotedebug
Control the debugging of the remote protocol. When enabled, each packet sent to or received from the remote target is displayed. The defaults is off.
show remotedebug
Show the current setting of the remote protocol debugging.
set remotedevice device
Set the name of the serial port through which to communicate to the remote target to device. This is the device used by GDB to open the serial communications line to the remote target. There's no default, so you must set a valid port name for the remote serial communications to work. (Some varieties of the target command accept the port name as part of their arguments.)
show remotedevice
Show the current name of the serial port.
set remotelogbase base
Set the base (a.k.a. radix) of logging serial protocol communications to base. Supported values of base are: ascii, octal, and hex. The default is ascii.
show remotelogbase
Show the current setting of the radix for logging remote serial protocol.
set remotelogfile file
Record remote serial communications on the named file. The default is not to record at all.
show remotelogfile.
Show the current setting of the file name on which to record the serial communications.
set remotetimeout num
Set the timeout limit to wait for the remote target to respond to num seconds. The default is 2 seconds.
show remotetimeout
Show the current number of seconds to wait for the remote target responses. @anchor{set remote hardware-watchpoint-limit} @anchor{set remote hardware-breakpoint-limit}
set remote hardware-watchpoint-limit limit
set remote hardware-breakpoint-limit limit
Restrict GDB to using limit remote hardware breakpoint or watchpoints. A limit of -1, the default, is treated as unlimited.
set remote fetch-register-packet
set remote set-register-packet
set remote P-packet
set remote p-packet
Determine whether GDB can set and fetch registers from the remote target using the `P' packets. The default depends on the remote stub's support of the `P' packets (GDB queries the stub when this packet is first required).
show remote fetch-register-packet
show remote set-register-packet
show remote P-packet
show remote p-packet
Show the current setting of using the `P' packets for setting and fetching registers from the remote target.
set remote binary-download-packet
set remote X-packet
Determine whether GDB sends downloads in binary mode using the `X' packets. The default is on.
show remote binary-download-packet
show remote X-packet
Show the current setting of using the `X' packets for binary downloads.
set remote read-aux-vector-packet
Set the use of the remote protocol's `qPart:auxv:read' (target auxiliary vector read) request. This request is used to fetch the remote target's auxiliary vector, see section Operating system auxiliary information. The default setting depends on the remote stub's support of this request (GDB queries the stub when this request is first required). See section General Query Packets, for more information about this request.
show remote read-aux-vector-packet
Show the current setting of use of the `qPart:auxv:read' request.
set remote symbol-lookup-packet
Set the use of the remote protocol's `qSymbol' (target symbol lookup) request. This request is used to communicate symbol information to the remote target, e.g., whenever a new shared library is loaded by the remote (see section Commands to specify files). The default setting depends on the remote stub's support of this request (GDB queries the stub when this request is first required). See section General Query Packets, for more information about this request.
show remote symbol-lookup-packet
Show the current setting of use of the `qSymbol' request.
set remote verbose-resume-packet
Set the use of the remote protocol's `vCont' (descriptive resume) request. This request is used to resume specific threads in the remote target, and to single-step or signal them. The default setting depends on the remote stub's support of this request (GDB queries the stub when this request is first required). This setting affects debugging of multithreaded programs: if `vCont' cannot be used, GDB might be unable to single-step a specific thread, especially under set scheduler-locking off; it is also impossible to pause a specific thread. See section Packets, for more details.
show remote verbose-resume-packet
Show the current setting of use of the `vCont' request
set remote software-breakpoint-packet
set remote hardware-breakpoint-packet
set remote write-watchpoint-packet
set remote read-watchpoint-packet
set remote access-watchpoint-packet
set remote Z-packet
These commands enable or disable the use of `Z' packets for setting breakpoints and watchpoints in the remote target. The default depends on the remote stub's support of the `Z' packets (GDB queries the stub when each packet is first required). The command set remote Z-packet, kept for back-compatibility, turns on or off all the features that require the use of `Z' packets.
show remote software-breakpoint-packet
show remote hardware-breakpoint-packet
show remote write-watchpoint-packet
show remote read-watchpoint-packet
show remote access-watchpoint-packet
show remote Z-packet
Show the current setting of `Z' packets usage.
set remote get-thread-local-storage-address
This command enables or disables the use of the `qGetTLSAddr' (Get Thread Local Storage Address) request packet. The default depends on whether the remote stub supports this request. See section General Query Packets, for more details about this packet.
show remote get-thread-local-storage-address
Show the current setting of `qGetTLSAddr' packet usage.

Implementing a remote stub

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.)

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. 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:

i386-stub.c
For Intel 386 and compatible architectures.
m68k-stub.c
For Motorola 680x0 architectures.
sh-stub.c
For Renesas SH architectures.
sparc-stub.c
For SPARC architectures.
sparcl-stub.c
For Fujitsu SPARCLITE 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:

set_debug_traps
This routine arranges for handle_exception to run when your program stops. You must call this subroutine explicitly near the beginning of your program.
handle_exception
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.
breakpoint
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 themselves without help from exceptionHandler.
void flush_i_cache()
On SPARC and SPARCLITE only, 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 defined 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:
    set_debug_traps();
    breakpoint();
    
  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 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.
  8. Start GDB on the host, and connect to the target (see section Connecting to a remote target).
    target remote udp:manyfarms:2828
    
    When using a UDP connection for remote debugging, you should keep in mind that the `U' stands for "Unreliable". UDP can silently drop packets on busy or unreliable networks, which will cause havoc with your debugging session.


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