Connecting - Debugging with GDB

Next: File Transfer, Up: Remote Debugging

20.1 Connecting to a Remote Target

This section describes how to connect to a remote target, including the types of connections and their differences, how to set up executable and symbol files on the host and target, and the commands used for connecting to and disconnecting from the remote target.

20.1.1 Types of Remote Connections

gdb supports two types of remote connections, target remote mode and target extended-remote mode. Note that many remote targets support only target remote mode. There are several major differences between the two types of connections, enumerated here:

Result of detach or program exit

With target remote mode: When the debugged program exits or you detach from it, gdb disconnects from the target. When using gdbserver, gdbserver will exit.

With target extended-remote mode: When the debugged program exits or you detach from it, gdb remains connected to the target, even though no program is running. You can rerun the program, attach to a running program, or use monitor commands specific to the target.

When using gdbserver in this case, it does not exit unless it was invoked using the --once option. If the --once option was not used, you can ask gdbserver to exit using the monitor exit command (see Monitor Commands for gdbserver).

Specifying the program to debug

For both connection types you use the file command to specify the program on the host system. If you are using gdbserver there are some differences in how to specify the location of the program on the target.

With target remote mode: You must either specify the program to debug on the gdbserver command line or use the --attach option (see Attaching to a Running Program).

With target extended-remote mode: You may specify the program to debug on the gdbserver command line, or you can load the program or attach to it using gdb commands after connecting to gdbserver.

You can start gdbserver without supplying an initial command to run or process ID to attach. To do this, use the --multi command line option. Then you can connect using target extended-remote and start the program you want to debug (see below for details on using the run command in this scenario). Note that the conditions under which gdbserver terminates depend on how gdb connects to it (target remote or target extended-remote). The --multi option to gdbserver has no influence on that.

The run command

With target remote mode: The run command is not supported. Once a connection has been established, you can use all the usual gdb commands to examine and change data. The remote program is already running, so you can use commands like step and continue.

With target extended-remote mode: The run command is supported. The run command uses the value set by set remote exec-file (see set remote exec-file) to select the program to run. Command line arguments are supported, except for wildcard expansion and I/O redirection (see Arguments).

If you specify the program to debug on the command line, then the run command is not required to start execution, and you can resume using commands like step and continue as with target remote mode.

Attaching

With target remote mode: The gdb command attach is not supported. To attach to a running program using gdbserver, you must use the --attach option (see Running gdbserver).

With target extended-remote mode: To attach to a running program, you may use the attach command after the connection has been established. If you are using gdbserver, you may also invoke gdbserver using the --attach option (see Running gdbserver).

20.1.2 Host and Target Files

gdb, running on the host, needs access to symbol and debugging information for your program running on the target. This requires access to an unstripped copy of your program, and possibly any associated symbol files. Note that this section applies equally to both target remote mode and target extended-remote mode.

Some remote targets (see qXfer executable filename read, and see Host I/O Packets) allow gdb to access program files over the same connection used to communicate with gdb. With such a target, if the remote program is unstripped, the only command you need is target remote (or target extended-remote).

If the remote program is stripped, or the target does not support remote program file access, start up gdb using the name of the local unstripped copy of your program as the first argument, or use the file command. Use set sysroot to specify the location (on the host) of target libraries (unless your gdb was compiled with the correct sysroot using --with-sysroot). Alternatively, you may use set solib-search-path to specify how gdb locates target libraries.

The symbol file and target libraries must exactly match the executable and libraries on the target, with one exception: the files on the host system should not be stripped, even if the files on the target system are. Mismatched or missing files will lead to confusing results during debugging. On gnu/Linux targets, mismatched or missing files may also prevent gdbserver from debugging multi-threaded programs.

20.1.3 Remote Connection Commands

gdb can communicate with the target over a serial line, a local Unix domain socket, or over an IP network using TCP or UDP. In each case, gdb uses the same protocol for debugging your program; only the medium carrying the debugging packets varies. The target remote and target extended-remote commands establish a connection to the target. Both commands accept the same arguments, which indicate the medium to use:

target remote serial-device

target extended-remote serial-device

Use serial-device to communicate with the target. For example, to use a serial line connected to the device named /dev/ttyb:

          target remote /dev/ttyb

If you're using a serial line, you may want to give gdb the ‘--baud’ option, or use the set serial baud command (see set serial baud) before the target command.

target remote local-socket

target extended-remote local-socket

Use local-socket to communicate with the target. For example, to use a local Unix domain socket bound to the file system entry /tmp/gdb-socket0:

          target remote /tmp/gdb-socket0

Note that this command has the same form as the command to connect to a serial line. gdb will automatically determine which kind of file you have specified and will make the appropriate kind of connection. This feature is not available if the host system does not support Unix domain sockets.

target remote host:port

target remote [host]:port

target remote tcp:host:port

target remote tcp:[host]:port

target remote tcp4:host:port

target remote tcp6:host:port

target remote tcp6:[host]:port

target extended-remote host:port

target extended-remote [host]:port

target extended-remote tcp:host:port

target extended-remote tcp:[host]:port

target extended-remote tcp4:host:port

target extended-remote tcp6:host:port

target extended-remote tcp6:[host]:port

Debug using a TCP connection to port on host. The host may be either a host name, a numeric IPv4 address, or a numeric IPv6 address (with or without the square brackets to separate the address from the port); port must be a decimal number. The host could be the target machine itself, if it is directly connected to the net, or it might be a terminal server which in turn has a serial line to the target.

For example, to connect to port 2828 on a terminal server named manyfarms:

          target remote manyfarms:2828

To connect to port 2828 on a terminal server whose address is 2001:0db8:85a3:0000:0000:8a2e:0370:7334, you can either use the square bracket syntax:

          target remote [2001:0db8:85a3:0000:0000:8a2e:0370:7334]:2828

or explicitly specify the IPv6 protocol:

          target remote tcp6:2001:0db8:85a3:0000:0000:8a2e:0370:7334:2828

This last example may be confusing to the reader, because there is no visible separation between the hostname and the port number. Therefore, we recommend the user to provide IPv6 addresses using square brackets for clarity. However, it is important to mention that for gdb there is no ambiguity: the number after the last colon is considered to be the port number.

If your remote target is actually running on the same machine as your debugger session (e.g. a simulator for 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.

target remote udp:host:port

target remote udp:[host]:port

target remote udp4:host:port

target remote udp6:[host]:port

target extended-remote udp:host:port

target extended-remote udp:[host]:port

target extended-remote udp4:host:port

target extended-remote udp6:host:port

target extended-remote udp6:[host]:port

Debug using UDP packets to port on host. 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.

target remote | command

target extended-remote | command

Run command in the background and communicate with it using a pipe. The command is a shell command, to be parsed and expanded by the system's command shell, /bin/sh; it should expect remote protocol packets on its standard input, and send replies on its standard output. You could use this to run a stand-alone simulator that speaks the remote debugging protocol, to make net connections using programs like ssh, or for other similar tricks.

If command closes its standard output (perhaps by exiting), gdb will try to send it a SIGTERM signal. (If the program has already exited, this will have no effect.)

Whenever gdb is waiting for the remote program, if you type the interrupt character (often Ctrl-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)

In target remote mode, 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.

In target extended-remote mode, typing n will leave gdb connected to the target.

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 in target remote mode, gdb is free to connect to another target. In target extended-remote mode, gdb is still connected to the target.
disconnect: The disconnect command closes the connection to the target, and 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 arbitrary commands directly to the remote monitor. Since gdb doesn't care about the commands it sends like this, this command is the way to extend gdb—you can add new commands that only the external monitor will understand and implement.