Debugging with GDB

Node:Server, Next:NetWare, Up:Remote Debugging

### 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`).1 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.

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 9600bps.) 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`.

#### Footnotes

1. If you choose a port number that conflicts with another service, `gdbserver` prints an error message and exits.