zxdb: Fuchsia native debugger setup and troubleshooting


The debugger is for C/C++ code running on Fuchsia compiled in-tree for either CPU (ARM64 or x64). The state of other languages (like Rust) can be seen here.

This is the very detailed setup guide. Please see:

The debugger runs remotely only (you can't do self-hosted debug).


  • Variables in non-top stack frames aren't available as often as they could be.

  • Obviously many advanced features are missing.

Bugs (Googlers only)

Binary location (for SDK users)

The binary is tools/zxdb in the Fuchsia SDK. SDK users will have to do an extra step to set up your symbols. See “Running out-of-tree” below for more.

Compiling (for Fuchsia team members)

A Fuchsia “core” build includes (as of this writing) the necessary targets for the debugger. So this build configuration is sufficient:

fx --dir=out/x64 set core.x64

If you‘re compiling with another product, you may not get it by default. 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" ]


Preparation: Boot with networking

Boot the target system with networking support. For QEMU you'll need to set up a bridge interface so your target is visible (Googlers see go/zxdb-networking).

Then run:

fx run -N

Simple method

You can use the fx utility to start the debug agent and connect automatically.

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

to make the debug agent's package avilable for serving to the system. Otherwise you will get the message “Timed out trying to find the Debug Agent”. Once the server is running, launch the debugger in another terminal window:

fx debug

Manual method

In some cases you may want to run the debug agent and connect manually. To do so, follow these steps:

1. Run the debug agent on the target

On the target system pick a port and run the debug agent:

run fuchsia-pkg://fuchsia.com/debug_agent#meta/debug_agent.cmx --port=2345

If you get an error “Cannot create child process: ... failed to resolve ...” it means the debug agent can't be loaded. You may need to run fx serve or its equivalent in your environment to make it available.

You will want to note the target's IP address. Run ifconfig on the target to see this, or run fx netaddr on the host.

2. Run the client and connect

On the host system (where you do the build), run the client. Use the IP address of the target and the port you picked above in the connect command.

For QEMU, we recommend using IPv6 and link local addresses. These addresses have to be annotated with the interface they apply to, so make sure the address you use includes the appropriate interface (should be the name of the bridge device).

The address should look like fe80::5054:4d:fe63:5e7a%br0

fx zxdb



[zxdb] connect [fe80::5054:4d:fe63:5e7a%br0]:2345

(Substitute your build directory as-needed).

If you're connecting or running many times, there are command-line switches:

zxdb -c [fe80::5054:4d:fe63:5e7a%br0]:2345

See help connect for more examples, including IPv6 syntax.

Read the user guide

Once you're connected, the user guide has detailed instructions!


Running out-of-tree

You can run with kernels or user programs compiled elsewhere with some extra steps.

Be aware that we aren't yet treating the protocol as frozen. Ideally the debugger will be from the same build as the operating system itself (more precisely, it needs to match the debug_agent). But the protocol does not change very often so there is some flexibility.

When you run out-of-tree, you will need to tell zxdb where your symbols are on the local development box (Linux or Mac). Having symbols in the binary you pushed to the target device doesn't help. Use the -s command-line flag to tell zxdb about new symbol locations:

zxdb -s path/to/my_binary -s some/other_location

It's best if you build make a “.build-id” directory. You then pass the parent directory as a symbol dir. For example, the Fuchsia build itself makes a “.build-id” directory inside the build directory. You would run (assuming your build directory is “x64”) with:

out/x64/host_x64/zxdb -s out/x64

Some builds produce a file called “ids.txt” that lists build IDs and local paths to the corresponding binaries. This is the second-best option.

If you don‘t have that, you can just list the name of the file you’re debugging directly. You can pass multiple “-s” flags to list multiple symbol locations.

The -s flag accepts three possible things:

  • Directory names. If the given directory contains a “.build-id” subdirectory that will be used. Otherwise all ELF files in the given directory will be indexed.

  • File names ending in “.txt”. Zxdb will treat this as a “ids.txt” file mapping build IDs to binaries.

  • Any other file name will be treated as an ELF file with symbols.

Diagnosing symbol problems.

The sym-stat command will tell you status for symbols. With no running process, it will give stats on the different symbol locations you have specified. If your symbols aren't found, make sure these stats match your expectations:

[zxdb] sym-stat
Symbol index status

  Indexed  Source path
      950  /home/me/build/garnet/out/x64/ids.txt
        0  my_dir/my_file

If you see “0” in the “Indexed” column of the “Symbol index stats” that means that the debugger could not find where your symbols are. Try the -s flag (see “Running out-of-tree” above) to specify where your symbols are.

Symbol sources using the “.build-id” hierarchy will list “(folder)” for the indexed symbols since this type of source does not need to be indexed. To check if your hierarchy includes a given build ID, go to “.build-id” inside it, then to the folder with the first to characters of the build ID to see if there is a matching file.

When you have a running program, sym-stat will additionally print symbol information for each binary loaded into the process. If you're not getting symbols, find the entry for the binary or shared library in this list. If it says:

    Symbols loaded: No

then that means it couldn't find the symbolized binary on the local computer for the given build ID in any of the locations listed in “Symbol index status”. You may need to add a new location with -s.

If instead it says something like this:

    Symbols loaded: Yes
    Symbol file: /home/foo/bar/...
    Source files indexed: 1
    Symbols indexed: 0

where “Source files indexed” and “Symbols indexed” is 0 or a very low integer, that means that the debugger found a symbolized file but there are few or no symbols in it. Normally this means the binary was not built with symbols enabled or the symbols were stripped. Check your build, you should be passing the path to the unstripped binary and the original compile line should have a -g in it to get symbols.

Debugging the debugger and running the tests


For developers working on the debugger, you can activate the --debug-mode flag that will activate many logging statements for the debugger:

zxdb --debug-mode

You can also debug the client on GDB or LLDB on your host machine. You will want to run the unstripped binary: out/<yourbuild>/host_x64/exe.unstripped/zxdb. Since this path is different than the default, you will need to specify the location of ids.txt (in the root build directory) with -s on the command line.

There are tests for the debugger that run on the host. These are relevant if you're working on the debugger client.

fx run-host-tests zxdb_tests

or directly with


Debug Agent

Similar as with the client, the debug agent is programmed to log many debug statements when run with the --debug-mode flag:

run fuchsia-pkg://fuchsia.com/debug_agent#meta/debug_agent.cmx --debug-mode

It is also possible to attach the debugger to the debugger. The preferred way to do this is to make zxdb catch the debugger on launch using the filtering feature. This is done frequently by the debugger team. See the user guide for more details:

// Run the debugger that will attach to the "to-be-debugged" debug agent.
fx debug

// * Within zxdb.
[zxdb] set filters debug_agent

// Launch another debug agent manually
// * Within the target (requires another port).
run fuchsia-pkg://fuchsia.com/debug_agent#meta/debug_agent.cmx --port=5000 --debug-mode

// * Within the first zxdb:
Attached Process 1 [Running] koid=12345 debug_agent.cmx
  The process is currently in an initializing state. You can set pending
  breakpoints (symbols haven't been loaded yet) and "continue".
[zxdb] continue

// Now there is a running debug agent that is attached by the first zxdb run.
// You can also attach to it using another client (notice the port):
fx zxdb --connect [<IPv6 to target>]:5000 --debug-mode

// Now you have two running instances of the debugger!

NOTE: Only one debugger can be attached to the main job in order to auto-attach to new processes. Since you‘re using it for the first debugger, you won’t be able to launch components with the second one, only attach to them.

The debug agent tests are in


To run them:

fx run-tests debug_agent_tests

Other Languages

Rust mostly works but there are issues. Go currently is currently not supported.

Please contact brettw@ if you’re interested in helping! Even if you don't know how to write debugger code, just defining the proper behavior for Rust or Go would be helpful (the team has no experience with these languages).