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Set up and run zxdb, Fuchsia's debugger for C++ and Rust code

Launching zxdb on Fuchsia is as simple as one command:

ffx debug connect

When the debugger is launched, it should show

Connecting (use "disconnect" to cancel)...
Connected successfully.
👉 To get started, try "status" or "help".
[zxdb]

It'll work 95% of the time, regardless of whether you work in-tree or out-of-tree, using an emulator or a hardware device. If it doesn't work as expected, please check the troubleshooting below.

Troubleshooting

The build type is compatible

Debuggers on Fuchsia depend on privileged syscalls, most notably zx_process_write_memory. These syscalls are only enabled when the kernel flag kernel.enable-debugging-syscalls is set to true, which means debuggers are not available for user and userdebug build types.

If you are building from source, most probably these syscalls are enabled.

zxdb and debug_agent are built

Zxdb depends on a target-side component called debug_agent. If an error message says "The plugin service selector 'core/debug_agent:expose:fuchsia.debugger.DebugAgent' did not match any services on the target", it means that debug_agent is not built. You can also check whether there's debug_agent and host_x64/zxdb in your build directory.

If you don't have the debugger in your build, add //bundles:tools to your "universe", either with:

fx <normal_stuff_you_use> --with //bundles:tools

Or you can edit your GN args directly by editing <build_dir>/args.gn and adding to the bottom:

universe_package_labels += [ "//bundles:tools" ]

ffx can talk with the device

Make sure that ffx can discover the device, either a emulator or a hardware device, and RCS is started on the device.

$ ffx target list
NAME       SERIAL       TYPE             STATE      ADDRS/IP                    RCS
demo-emu   <unknown>    core.qemu-x64    Product    [10.0.2.15,                 Y
                                                    fec0::90e:486e:b6b5:9780,
                                                    fec0::487b:fabd:20fa:43ee,
                                                    127.0.0.1]

Package server is running

For most build configurations, the debug agent will be in the "universe" (i.e. "available to use") but not in the base build so won't be on the system before boot. You will need to run:

fx serve

Debug symbols are registered {#set-symbol-location}

Zxdb will by default obtain the locations of the debug symbols from the symbol index. The registrations of debug symbols from in-tree and most out-of-tree environments are automated. In case these doesn't work out, there are three command-line flags in zxdb to provide additional symbol lookup locations for zxdb: --build-id-dir, --ids-txt, and a general --symbol-path. They all have the corresponding settings that can be manipulated using set or get.

For example, to add a ".build-id" directory, either use --build-id-dir flag:

ffx debug connect -- --build-id-dir some/other_location/.build-id

Or add it to the build-id-dirs list option in the interactive UI:

[zxdb] set build-id-dirs += some/other_location/.build-id

For in-tree development, ffx debug connect automatically sets up all necessary flags.

build-id-dir

Some builds produce a .build-id directory. Symbol files in it are already indexed according to their build IDs. For example, the Fuchsia build itself makes a .build-id directory inside the build directory, e.g., out/x64/.build-id. They can be added to zxdb by --build-id-dir command-line flag or build-id-dirs setting. This is the best option.

ids-txt

Instead of a .build-id directory, some builds produce a file called ids.txt that lists build IDs and local paths to the corresponding binaries. They can be added to zxdb by --ids-txt command-line flag or ids-txts setting. This is the second-best option.

symbol-path

In addition, --symbol-path flag can be used to add arbitrary files or directories to symbol index. If the path is pointing to a file, it will be treated as an ELF file and added to the symbol index. If it's a directory, all binaries under the given path are indexed.

Source code location is correctly set up

The Fuchsia build generates symbols relative to the build directory so relative paths look like ../../src/my_component/file.cc). The build directory is usually provided by the symbol index.

If your files are not being found with the default build directories, you will need to provide a build directory to locate the files. This build directory does not need have been used to build, it just needs to produce correct absolute paths when concatenated with the relative paths from the symbol file.

You can add additional build directories on the command line:

ffx debug connect -- --build-dir /home/me/fuchsia/out/x64

Or interactively from within the debugger:

[zxdb] set build-dirs += /home/me/fuchsia/out/x64

If debugger is finding the wrong file, you can replace the entire build directory list by omitting the +=:

[zxdb] set build-dirs /home/me/fuchsia/out/x64

If your build produces DWARF symbols with absolute file paths the files must be in that location on the local system. Absolute file paths in the symbols are not affected by the build search path. Clang users should use the -fdebug-prefix-map, which will also help with build hermeticity.