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.
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
userdebug build types.
If you are building from source, most probably these syscalls are enabled.
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
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" ]
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]
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:
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:
--ids-txt, and a general
--symbol-path. They all have the corresponding settings that can be manipulated using
For example, to add a “.build-id” directory, either use
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.
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.
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 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.
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.