The LLDB Debugger

Remote debugging

Remote debugging refers to the act of debugging a process which is running on a different system, than the debugger itself. We shall refer to the system running the debugger as the local system, while the system running the debugged process will be the remote system.

To enable remote debugging, LLDB employs a client-server architecture. The client part runs on the local system and the remote system runs the server. The client and server communicate using the gdb-remote protocol, usually transported over TCP/IP. More information on the protocol can be found here and the LLDB-specific extensions are documented in docs/lldb-gdb-remote.txt file inside LLDB source repository. Besides the gdb-remote stub, the server part of LLDB also consists of a platform binary, which is responsible for performing advanced debugging operations, like copying files from/to the remote system and can be used to execute arbitrary shell commands on the remote system.

In order to reduce code complexity and improve remote debugging experience LLDB on Linux and OSX uses the remote debugging stub even when debugging a process locally. This is achieved by spawning a remote stub process locally and communicating with it over the loopback interface. In the case of local debugging this whole process is transparent to the user. The platform binary is not used in this case, since no file transfers are needed.

Preparation for remote debugging

While the process of actual debugging (stepping, backtraces, evaluating expressions) is same as in the local case, in the case of remote debugging, more preparation is needed as the required binaries cannot started on the remote system automatically. Also, if the remote system runs a different OS or architecture, the server component needs to be compiled separately.

Remote system

On Linux and Android, all required remote functionality is contained in the lldb-server binary. This binary combines the functionality of the platform and gdb-remote stub. A single binary facilitates deployment and reduces code size, since the two functions share a lot of code. The lldb-server binary is also statically linked with the rest of LLDB (unlike lldb, which dynamically links to by default), so it does not have any dependencies on the rest of lldb. On Mac OSX and iOS, the remote-gdb functionality is implemented by the debugserver binary, which you will need to deploy alongside lldb-server.

The binaries mentioned above need to be present on the remote system to enable remote debugging. You can either compile on the remote system directly or copy them from the local machine. If compiling locally and the remote architecture differs from the local one, you will need to cross-compile the correct version of the binaries. More information on cross-compiling LLDB can be found on the build page.

Once the binaries are in place, you just need to run the lldb-server in platform mode and specify the port it should listen on. For example, the command

remote% lldb-server platform --listen "*:1234" --server

will start the LLDB platform and wait for incoming connections from any address to port 1234. Specifying an address instead of * will only allow connections originating from that address. Adding a --server parameter to the command line will fork off a new process for every incoming connection, allowing multiple parallel debug sessions.

Local system

On the local system, you need to let LLDB know that you intend to do remote debugging. This is achieved through the platform command and its sub-commands. As a first step you need to choose the correct platform plug-in for your remote system. A list of available plug-ins can be obtained through platform list.

local% lldb
(lldb) platform list
Available platforms:
host: Local Mac OS X user platform plug-in.
remote-freebsd: Remote FreeBSD user platform plug-in.
remote-linux: Remote Linux user platform plug-in.
remote-netbsd: Remote NetBSD user platform plug-in.
remote-windows: Remote Windows user platform plug-in.
remote-android: Remote Android user platform plug-in.
remote-ios: Remote iOS platform plug-in.
remote-macosx: Remote Mac OS X user platform plug-in.
ios-simulator: iOS simulator platform plug-in.
darwin-kernel: Darwin Kernel platform plug-in.
tvos-simulator: Apple TV simulator platform plug-in.
watchos-simulator: Apple Watch simulator platform plug-in.
remote-tvos: Remote Apple TV platform plug-in.
remote-watchos: Remote Apple Watch platform plug-in.
remote-gdb-server: A platform that uses the GDB remote protocol as the communication transport.

The default platform is the platform host which is used for local debugging. Apart from this, the list should contain a number of plug-ins, for debugging different kinds of systems. The remote plug-ins are prefixed with "remote-". For example, to debug a remote Linux application:

(lldb) patform select remote-linux

After selecting the platform plug-in, you should receive a prompt which confirms the selected platform, and states that you are not connected. This is because remote plug-ins need to be connected to their remote platform counterpart to operate. This is achieved using the platform connect command. This command takes a number of arguments (as always, use the help command to find out more), but normally you only need to specify the address to connect to, e.g.:

(lldb) platform connect connect://remote:1234
  Platform: remote-linux
    Triple: x86_64-gnu-linux
  Hostname: remote
 Connected: yes
WorkingDir: /tmp

Note that the platform has a working directory of /tmp. This directory will be used as the directory that executables will be uploaded to by default when launching a process from local.

After this, you should be able to debug normally. You can use the process attach to attach to an existing remote process or target create, process launch to start a new one. The platform plugin will transparently take care of uploading or downloading the executable in order to be able to debug. If your application needs additional files, you can transfer them using the platform commands: get-file, put-file, mkdir, etc. The environment can be prepared further using the platform shell command.

Launching a locally built process on the remote machine

Install and run in the platform working directory

To launch a locally built process on the remote system in the platform working directory:

(lldb) file a.out
(lldb) run

This will cause LLDB to create a target with the "a.out" executable that you cross built. The "run" command will cause LLDB to upload "a.out" to the platform's current working directory only if the file has changed. The platform connection allows us to transfer files, but also allows us to get the MD5 checksum of the file on the other end and only upload the file if it has changed. LLDB will automatically launch a lldb-server in gdbremote mode to allow you to debug this executable, connect to it and start your debug session for you.

Changing the platform working directory

You can change the platform working directory while connected to the platform with:

(lldb) platform settings -w /usr/local/bin

And you can verify it worked using "platform status":

(lldb) platform status
  Platform: remote-linux
    Triple: x86_64-gnu-linux
  Hostname: remote
 Connected: yes
WorkingDir: /usr/local/bin

If we run again, the program will be installed into /usr/local/bin.

Install and run by specifying a remote install path

If you want the "a.out" executable to be installed into "/bin/a.out" instead of the platorm's current working directory, we can set the platform file specification using python:

(lldb) file a.out
(lldb) script['a.out'].SetPlatformFileSpec("/bin/a.out")
(lldb) run

Now when you run your program, the program will be uploaded to "/bin/a.out" instead of the the platform current working directory. Only the main executable is uploaded to the remote system by default when launching the application. If you have shared libraries that should also be uploaded, then you can add the locally build shared library to the current target and set its platform file specification:

(lldb) file a.out
(lldb) target module add /local/build/
(lldb) target module add /local/build/
(lldb) script[''].SetPlatformFileSpec("/usr/lib/")
(lldb) script[''].SetPlatformFileSpec("/usr/local/lib/")
(lldb) run

Attaching to a remote process

If you want to attach to a remote process, you can first list the processes on the remote system:

(lldb) platform process list
223 matching processes were found on "remote-linux"
PID    PARENT USER       TRIPLE                   NAME
====== ====== ========== ======================== ============================
68639  90652             x86_64-apple-macosx      lldb

Then attaching is as simple as specifying the remote process ID:

(lldb) attach 68639