Debugging with GDB: gdbserver man

gdbserver man

gdbserver comm prog [args…]

gdbserver –attach comm pid

gdbserver –multi comm

gdbserver is a program that allows you to run GDB on a different machine than the one which is running the program being debugged.

Usage (server (target) side)

First, you need to have a copy of the program you want to debug put onto the target system. The program can be stripped to save space if needed, as gdbserver doesn’t care about symbols. All symbol handling is taken care of by the GDB running on the host system.

To use the server, you log on to the target system, and run the gdbserver program. You must tell it (a) how to communicate with GDB, (b) the name of your program, and (c) its arguments. The general syntax is:

target> gdbserver comm program [args ...]

For example, using a serial port, you might say:

target> gdbserver /dev/com1 emacs foo.txt

This tells gdbserver to debug emacs with an argument of foo.txt, and to communicate with GDB via /dev/com1. gdbserver now waits patiently for the host GDB to communicate with it.

To use a TCP connection, you could say:

target> gdbserver host:2345 emacs foo.txt

This says pretty much the same thing as the last example, except that we are going to communicate with the host GDB via TCP. The host:2345 argument means that we are expecting to see a TCP connection from 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 existing TCP ports on the target system. This same port number must be used in the host GDBs target remote command, which will be described shortly. Note that if you chose a port number that conflicts with another service, gdbserver will print an error message and exit.

gdbserver can also attach to running programs. This is accomplished via the --attach argument. The syntax is:

target> gdbserver --attach comm 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.

To start gdbserver without supplying an initial command to run or process ID to attach, use the --multi command line option. In such case you should connect using target extended-remote to start the program you want to debug.

target> gdbserver --multi comm

Usage (host side)

You need an unstripped copy of the target program on your host system, since GDB needs to examine its symbol tables and such. Start up GDB as you normally would, with the target program as the first argument. (You may need to use the --baud option if the serial line is running at anything except 9600 baud.) That is gdb TARGET-PROG, or gdb --baud BAUD TARGET-PROG. After that, the only new command you need to know about is target remote (or target extended-remote). Its argument is either a device name (usually a serial device, like /dev/ttyb), or a HOST:PORT descriptor. 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’, where you previously started up gdbserver with the same port number. Note that for TCP connections, you must start up gdbserver prior to using the ‘target remote’ command, otherwise you may get an error that looks something like ‘Connection refused’.

gdbserver can also debug multiple inferiors at once, described in Inferiors Connections and Programs. In such case use the extended-remote GDB command variant:

(gdb) target extended-remote the-target:2345

The gdbserver option --multi may or may not be used in such case.

There are three different modes for invoking gdbserver:

Debug a specific program specified by its program name:
```
gdbserver comm prog [args…]
```
The comm parameter specifies how should the server communicate with GDB; it is either a device name (to use a serial line), a TCP port number (:1234), or - or stdio to use stdin/stdout of gdbserver. Specify the name of the program to debug in prog. Any remaining arguments will be passed to the program verbatim. When the program exits, GDB will close the connection, and gdbserver will exit.
Debug a specific program by specifying the process ID of a running program:
```
gdbserver --attach comm pid
```
The comm parameter is as described above. Supply the process ID of a running program in pid; GDB will do everything else. Like with the previous mode, when the process pid exits, GDB will close the connection, and gdbserver will exit.
Multi-process mode – debug more than one program/process:
```
gdbserver --multi comm
```
In this mode, GDB can instruct gdbserver which command(s) to run. Unlike the other 2 modes, GDB will not close the connection when a process being debugged exits, so you can debug several processes in the same session.

In each of the modes you may specify these options:

--help

List all options, with brief explanations.

--version

This option causes gdbserver to print its version number and exit.

--attach

gdbserver will attach to a running program. The syntax is:

target> gdbserver --attach comm 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.

--multi

To start gdbserver without supplying an initial command to run or process ID to attach, use this command line option. Then you can connect using target extended-remote and start the program you want to debug. The syntax is:

target> gdbserver --multi comm

--debug

Instruct gdbserver to display extra status information about the debugging process. This option is intended for gdbserver development and for bug reports to the developers.

--remote-debug

Instruct gdbserver to display remote protocol debug output. This option is intended for gdbserver development and for bug reports to the developers.

--debug-file=filename

Instruct gdbserver to send any debug output to the given filename. This option is intended for gdbserver development and for bug reports to the developers.

--debug-format=option1[,option2,...]

Instruct gdbserver to include extra information in each line of debugging output. See Other Command-Line Arguments for gdbserver.

--wrapper

Specify a wrapper to launch programs for debugging. The option should be followed by the name of the wrapper, then any command-line arguments to pass to the wrapper, then -- indicating the end of the wrapper arguments.

--once

By default, gdbserver keeps the listening TCP port open, so that additional connections are possible. However, if you start gdbserver with the --once option, it will stop listening for any further connection attempts after connecting to the first GDB session.