Contributing to FIDL


The FIDL toolchain is composed of roughly three parts:

  1. Front-end, a.k.a. fidlc
    • Parses and validates .fidl files
    • Calculates size, alignment, and offset of various structures
    • Produces a JSON IR (Intermediate Representation)
  2. Back-end
    • Works off the IR (except the C back-end)
    • Produces target language specific code, which ties into the libraries for that language
  3. Runtime Libraries
    • Implement encoding/decoding/validation of messages
    • Method dispatching mechanics

Code location

Compiler front-end

The front-end lives at //tools/fidl/fidlc/, with tests in //tools/fidl/fidlc/tests.

Compiler back-ends

Target Codegen Runtime Libraries Tests
HLCPP (Old) /tools/fidl/fidlgen_hlcpp /sdk/lib/fidl/cpp (located alongside runtime libraries)
New C++ /tools/fidl/fidlgen_cpp /sdk/lib/fidl/cpp /src/lib/fidl/llcpp and /src/lib/fidl/cpp
Go /tools/fidl/fidlgen_go /third_party/go/src/syscall/zx/fidl (located alongside runtime libraries)
Rust /tools/fidl/fidlgen_rust /src/lib/fidl/rust (located alongside runtime libraries)

Supporting code for the target specific backends is located in /tools/fidl/lib/fidlgen.

Testing tools


GIDL is a tool used to create general "write once, generate for every backend" programs. Currently GIDL is used to generate encode or decode tests ("conformance tests") as well as benchmarks.

Path Description
/tools/fidl/gidl Source code and build templates for the GIDL tool itself.
/src/tests/fidl/conformance_suite Test definitions (.fidl and .gidl files) for conformance tests.
/sdk/ctf/tests/pkg/fidl/cpp/test/{test,handle}_util.h Runtime support for HLCPP conformance tests.
/src/lib/fidl/llcpp/tests/conformance/conformance_utils.h Runtime support for C++ wire types conformance tests.
/src/lib/fidl/rust/gidl_util Runtime support for Rust conformance tests.
/third_party/go/src/syscall/zx/fidl/fidl_test Runtime support for Go conformance tests.
/src/tests/benchmarks/fidl/benchmark_suite Benchmark definitions (.fidl and .gidl files).
/src/tests/benchmarks/fidl Runtime support for benchmarks.

The actual test targets for the conformance tests in each backend are generally defined alongside the corresponding tests for that backend. Refer to the Bindings tests section for details.


Compatibility tests are integration tests that run FIDL clients and servers from different bindings with each other in order to test that they are compatible. Compatibility tests are found at /src/tests/fidl/compatibility/.

Dangerous identifiers

Dangerous identifier tests are found in /src/tests/fidl/dangerous_identifiers.


Some other FIDL related areas are:

Path Contents
/tools/fidl/fidlgen_* Various other compiler back-ends.
/tools/fidl/fidlc/cmd/fidl-format FIDL formatter.
/tools/fidl/fidlc/cmd/fidl-lint FIDL linter.
/tools/fidl/fidldoc Generate documentation for FIDL.
/tools/fidl/fidlmerge Tool for generating code from FIDL JSON.
/tools/fidl/measure-tape Tool to max out pagination.
/tools/fidl/scripts Mostly one-off scripts for e.g. performing migrations that are kept for future reference.
/src/lib/fostr fidlmerge based tool to generate formatting code in C++.
/src/lib/fostr/build Build templates for the fostr formatting library.
/src/lib/fidl_codec Library for encoding/decoding FIDL messages (used by fidlcat).

Other FIDL tools

A number of FIDL tools are located in the fidl-misc repo. To clone this repo, run

git clone

It is then recommended to export the path to this directory, to make setting aliases easier:

export FIDLMISC_DIR=...

Common development tools

This is a crowdsourced section from the FIDL team on useful tools that they use for working on the FIDL codebase.


Most of the FIDL team uses VS Code for development. Some useful plugins and workflows:

  • The remote ssh feature works really well for doing remote work from your laptop.
    • Setting up tmux or screen is also helpful for remote work, to preserve history and manage multiple sessions in the shell.
  • The Fuchsia documentation has instructions for setting up language servers:
  • The rewrap extension is useful for automatically reflowing lines to a certain length (e.g. when editing markdown files).
  • To get automatic syntax highlighting for the bindings golden files, update the file.associations setting:

    "files.associations": {
          "*.json.golden": "json",
          "*.rs.golden": "rust",
          "*.cc.golden": "cpp",
          "*.h.golden": "cpp",
          "*.go.golden": "go",
          "*.dart.golden": "dart",

Commit message style guide

When writing a change message, follow the Commit message style guide.

C++ style guide

We follow the Fuchsia C++ Style Guide, with additional rules to further remove ambiguity around the application or interpretation of guidelines.


Comments must respect 80 columns line size limit, unlike code, which can extend to 100 lines size limit.

Lambda captures

  • If a lambda escapes the current scope, capture all variables explicitly.
  • If the lambda is local (does not escape the current scope), prefer using a default capture by reference ("[&]").

Seeing [&] is a strong signal that the lambda exists within the current scope only, and can be used to distinguish local from non-local lambdas.

// Correct.
std::set<const flat::Library*, LibraryComparator> dependencies;
auto add_dependency = [&](const flat::Library* dep_library) {
  if (!dep_library->HasAttribute("Internal")) {

General setup

Fuchsia setup

Read the Fuchsia Getting Started guide first.

fx set

If you are working on the FIDL toolchain, use:

fx set core.x64 --with //bundles/fidl:tests --with-base //src/dart:dart_jit_runner

The --with-base flag is necessary to run Dart tests and benchmarks.

If you are working on an LSC:

fx set terminal.x64 --with //bundles/kitchen_sink \


To symbolize backtraces, you'll need a symbolizer in scope:

export ASAN_SYMBOLIZER_PATH="$(find `pwd` -name llvm-symbolizer | grep clang | head -1)"

Fast test builds

FIDL is a very deep-seated part of the system, so modifying it often results in "rebuilding the world", with small changes potentially triggering tens of thousands of compilation actions and minutes long builds. This can be slow and cumbersome when you're just trying to build and run some FIDL tests, rather than the system as a whole.

The fx tool supports on-demand, narrow-scope build and package publishing to help with this exact situation. To enable it, set the following environment variable in your ~/.bashrc (or equivalent):

export FUCHSIA_DISABLED_incremental=0

Compiling and running tests

We provide mostly one-liners to run tests for the various parts. When in doubt, refer to the "Test:" comment in the git commit message; we do our best to describe the commands used to validate our work there.

Tests are run using the fidldev tool. Examples assume that the fidldev script is somewhere on your PATH, e.g. by adding an alias:

alias fidldev=$FIDLMISC_DIR/fidldev/


# optional; builds fidlc for the host with ASan <>
fx set core.x64 --variant=host_asan

fx build host_x64/fidlc

If you're doing extensive edit-compile-test cycles on fidlc, building with fewer optimizations can make a significant difference in the build speed. To do this, change the optimization setting in zircon/public/gn/config/levels.gni from default to debug or none.

To avoid accidentally committing this change, run:

git update-index --skip-worktree zircon/public/gn/config/levels.gni

If you want to allow the changes to be committed again, run:

git update-index --no-skip-worktree zircon/public/gn/config/levels.gni

fidlc tests

fidlc tests are at:

To build and run fidlc tests:

fx test //tools/fidl/fidlc

If you prefer to use ninja directly:

fx_build_dir=$(cat .fx-build-dir) \
    fidlc_tests_target=$(fx ninja -C $fx_build_dir -t targets all | grep -e 'unstripped.*fidlc-test:' | awk -F : '{ print $1; }') \
    fx ninja -C $fx_build_dir $fidlc_tests_target && ./$fx_build_dir/$fidlc_tests_target

To run a specific suite of tests, use the --gtest_filter with an appropriate pattern. For instance:

fx_build_dir=$(cat .fx-build-dir) \
    fidlc_tests_target=$(fx ninja -C $fx_build_dir -t targets all | grep -e 'unstripped.*fidlc-test:' | awk -F : '{ print $1; }') \
    fx ninja -C $fx_build_dir $fidlc_tests_target && ./$fx_build_dir/$fidlc_tests_target --gtest_filter 'EnumsTests.*'

fidlc debugging

To easily run tests in a debug build, set your environment slightly differently:

fx set core.x64 --variant=host_asan --with //bundles/fidl:tests
export ASAN_SYMBOLIZER_PATH="$(find `pwd` -name llvm-symbolizer | grep clang | head -1)"

Once properly set up, you can run tests using the commands listed previously, with or without filtering.

To step through a test, you can use GDB:

fx_build_dir=$(cat .fx-build-dir) \
    fidlc_tests_target=$(fx ninja -C $fx_build_dir -t targets all | grep -e 'unstripped.*fidlc-test:' | awk -F : '{ print $1; }') \
    fx ninja -C $fx_build_dir $fidlc_tests_target && fx gdb --args ./$fx_build_dir/$fidlc_tests_target --gtest_filter 'AliasTests.invalid_recursive_alias'

fidlc test style guide

All fidlc compiler tests written in C++ must conform to the following rules:

  • Tests written using the TEST macro must have an UpperCamelCased group name of the format <CATEGORY>Tests. and an UpperCamelCased test case name. For example: TEST(BitsTests, GoodValidBits) {....
  • Test case names should not begin or end with "Test" since it's redundant.
  • Test case names that test parsing and/or compilation must be prefixed with one of the following:
    • Good: when the test case is expected to pass. Ex: GoodValidMethod.
    • Bad: when the test case is expected to pass. Ex: BadMustBeDense.
    • Warn: when the test case is expected to pass, but with reporter warnings. Warnings are intended for temporary use when introducing a new check, so tests prefixed with Warn should be changed to Good or Bad when the check is removed. Ex: WarnTooManyProvidedLibraries.

Additionally, test cases which expect compilation failures should use the ASSERT_ERRORED_DURING_COMPILE and ASSERT_ERRORED_TWICE_DURING_COMPILE macros in cases when one and two errors are expected, respectively.

fidlc goldens

To regenerate the fidlc JSON goldens:

fidldev regen fidlc

These "golden" files are examples of what kind of JSON IR fidlc produces and are used to track changes. It is required to regenerate the golden files each time the JSON IR is changed in any way, otherwise the json_generator_tests fails.

fidlgen (New C++, HLCPP, Rust, Go, Dart)


fx build tools/fidl



Some example tests you can run:

fx test fidlgen_hlcpp_golden_tests
fx test fidlgen_golang_lib_tests
fx test dart-bindings-test
fidldev test --no-regen fidlgen

To regenerate the goldens:

fidldev regen fidlgen



fx build host_x64/fidlgen_banjo

Run tests:

fx build host_x64/fidlgen_banjo_unittests


fidldev supports tests for each of the bindings. Some of the bindings tests run on device and require having Fuchsia running in an emulator. Here are the steps:

Tab 1> fx build && fx serve

Tab 2> fx qemu -kN

The -k flag enables KVM. It is not required, but the emulator is much slower without it. The -N flag enables networking.

The bindings tests can then be run with fidldev:

fidldev test --no-regen hlcpp
fidldev test --no-regen llcpp
fidldev test --no-regen c
fidldev test --no-regen go
fidldev test --no-regen rust
fidldev test --no-regen dart

Alternatively, run fidldev with no arguments to test files that have changed:

fidldev test

To run a specific test or to pass flags to a specific test, run fidldev with the --dry-run, --no-build, --no-regen flags to obtain the desired test commands.

Compatibility test

Details about how the compatibility tests work and where the code is located can be found in the README at //src/tests/fidl/compatibility.

To run the compatibility tests, you first need to have Fuchsia running in an emulator:

Tab 1> fx build && fx serve

Tab 2> fx qemu -kN

To run the compatibility tests:

Tab 3> fx set core.x64 --with //src/tests/fidl/compatibility
Tab 3> fx test fidl-compatibility-test


To rebuild GIDL:

fx build host-tools/gidl

Measure tape

fx set core.x64 --with //tools/fidl/measure-tape/src:host
fx build

All tests

This section gives the full fx test commands to run all FIDL-related tests. Use these instead of fidldev test if you want to run a specific test.

Bindings tests

On device tests generally have greater coverage than host tests, due to support for only running a subset of features on host. However, host tests can be useful for debugging issues that prevent boot of the device.

On device
Name Test Command Coverage
c runtime test, coding tables fx test fidl_c_tests //sdk/lib/fidl_base
walker, misc fx test fidl-walker-tests //sdk/lib/fidl_base
walker tests w/ handle closing checks fx test fidl-handle-closing-tests //sdk/lib/fidl_base
hlcpp bindings tests including conformance tests fx test fidl_hlcpp_unit_test_package fidl_hlcpp_conformance_test_package //sdk/lib/fidl
New C++ wire tests fx test //src/lib/fidl/llcpp //sdk/lib/fidl/cpp/wire
New C++ tests fx test //src/lib/fidl/cpp //src/lib/fidl/cpp
go bindings tests fx test go-fidl-tests //third_party/go/syscall/zx/fidl //third_party/go/syscall/zx/fidl/fidl_test //src/tests/fidl/go_bindings_test
rust bindings tests fx test //src/lib/fidl/rust //src/lib/fidl/rust
Name Test Command Coverage
walker, misc fx test --host fidl-walker-host-tests //sdk/lib/fidl_base
hlcpp unittests fx test --host fidl_hlcpp_unit_tests //sdk/lib/fidl
hlcpp conformance tests fx test --host fidl_hlcpp_conformance_tests //sdk/lib/fidl
C++ wire types conformance tests fx test --host fidl_llcpp_conformance_tests //sdk/lib/fidl/cpp/wire
C++ natural types conformance tests fx test --host fidl_cpp_conformance_tests //src/lib/fidl/cpp
rust conformance tests fx test --host fidl_rust_conformance_tests //src/lib/fidl/rust
rust fidl lib tests fx test --host fidl_rust_lib_tests //src/lib/fidl/rust
go conformance tests fx test --host fidl_go_conformance_tests //third_party/go/syscall/zx/fidl
go fidl tests (extended) fx test --host go_extended_fidl_test //third_party/go/syscall/zx/fidl
go unsafevalue test fx test --host go_unsafevalue_test //third_party/go/syscall/zx/fidl/internal/unsafevalue

fidlgen tests

Name Test Command Coverage
fidlgen type definitions fx test fidlgen_lib_test //tools/fidl/lib/fidlgen
fidlgen C++ specific IR fx test fidlgen_cpp_ir_test //tools/fidl/lib/fidlgen_cpp
fidlgen hlcpp fx test fidlgen_hlcpp_golden_tests //tools/fidl/fidlgen_hlcpp
fidlgen new C++ fx test fidlgen_cpp_golden_tests //tools/fidl/fidlgen_cpp
fidlgen golang fx test fidlgen_go_{lib,golden}_tests //tools/fidl/fidlgen_golang
fidlgen rust fx test fidlgen_rust_{lib,golden}_tests //tools/fidl/fidlgen_rust
fidlgen syzkaller fx test fidlgen_syzkaller_golden_tests //tools/fidl/fidlgen_syzkaller


Name Test Command Coverage
fidlc compiler fx test fidlc-test
fx test fidlc_golden_tests
gidl parser fx test gidl_parser_test //tools/fidl/gidl/parser
measure tape test fx test measure-tape_test //tools/fidl/measure-tape
Rust IR parser fx build //src/devices/tools/fidlgen_banjo/tests/parser

All benchmarks

Benchmarks can either be run directly or through one of two test runners: fuchsia_benchmarks (old), SL4F (new).

Benchmarks on chromeperf are currently generated through the fuchsia_benchmarks runner but are transitioning to SL4F. During this transition, benchmarks should be integrated in both systems.

Directly running benchmarks

Ensure that the benchmarks are included in your build:

fx set core.x64 --with //src/tests/benchmarks

You will need to fx build and restart qemu for the packages to be available.

Available benchmarks:

Name Benchmark Command Notes
Go Benchmarks fx shell /bin/go_fidl_microbenchmarks
Rust Benchmarks fx shell /bin/rust_fidl_microbenchmarks /tmp/myresultsfile Results can be viewed with fx shell cat /tmp/myresultsfile/
C++ wire types benchmarks fx shell /bin/llcpp_fidl_microbenchmarks
lib/fidl Benchmarks fx shell /bin/lib_fidl_microbenchmarks
Roundtrip Benchmarks fx shell /bin/roundtrip_fidl_benchmarks

Running all benchmarks with SL4F benchmark runner

This runs benchmarks the same way they are run on CQ. SL4F requires the terminal.x64 product. Use fx set to switch products:

fx set terminal.x64 --with //bundles/buildbot/terminal

To run all FIDL tests, use:

fx test --e2e fidl_microbenchmarks_test

All regen commands

This section gives the fx check-goldens commands to regenerate all FIDL-related golden files. This is what fidldev regen uses under the hood.

Name Regen commands Input Output
(all goldens) fx check-goldens
fidlc goldens fx check-goldens fidlc tools/fidl/fidlc/testdata tools/fidl/fidlc/goldens
fidlgen goldens fx check-goldens $TOOL tools/fidl/fidlc/testdata tools/fidl/$TOOL/goldens
fidldoc goldens fx check-goldens fidldoc tools/fidl/fidlc/testdata tools/fidl/fidldoc/goldens
gidl goldens fx check-goldens gidl src/tests/fidl/conformance_suite/golden{.gidl,.test.fidl} tools/fidl/gidl/goldens
third party go fx exec $FUCHSIA_DIR/third_party/go/regen-fidl

Compiling with ninja

In some cases, GN can build many unneeded targets. You can build a specific target with ninja instead of GN. In most cases, you can grep for the binary name to determine the ninja invocation.

For example, you can grep for fidlgen_cpp:

fx ninja -C out/default -t targets all | grep -e 'fidlgen_cpp:'

This example outputs a list of ninja targets, including host_x64/fidlgen_cpp. Therefore, to build fidlgen_cpp run the following ninja command:

fx ninja -C out/default host_x64/fidlgen_cpp

Debugging (host)

There are several ways of debugging issues in host binaries. This section gives instructions for the case where fidlc --files test.fidl is crashing:


First, cd to the build directory. You can also stay in $FUCHSIA_DIR, but then you need to run dir out/default in GDB for it to find source files.

cd $FUCHSIA_DIR/out/default

Next, start GDB. The copy on your system might work, but the prebuilt fx gdb is more likely to work with build artifacts in the Fuchsia project. See fx gnu --help for the full list of prebuilt GNU tools.

fx gdb --args host_x64/exe.unstripped/fidlc --files test.fidl

Then, enter "r" to start the program. For additional uses, and a convenient quick reference we've found this GDB Cheat Sheet very useful.


Ensure you are compiling with ASan enabled:

fx set core.x64 --variant=host_asan
fx build host_x64/fidlc

Then run out/default/host_x64/fidlc --files test.fidl. That binary should be the same as out/default/host_x64-asan/fidlc.


On Google Linux machines, you may need to install a standard version of Valgrind instead of using the pre-installed binary:

sudo apt-get install valgrind


valgrind -v -- out/default/host_x64/exe.unstripped/fidlc --files test.fidl


Go and

To update all the saved fidlgen files, run the following command, which automatically searches for and generates the necessary go files:

fx exec $FUCHSIA_DIR/third_party/go/regen-fidl


Why is the C back-end different than all other back-ends?

The current C bindings are deprecated. See for more information on the future of using FIDL in C.

Why aren't all back-ends in one tool?

We'd actually like all back-ends to be in separate tools!

Down the road, we plan to have a script over all the various tools (fidlc, fidlfmt, the various back-ends) to make all things accessible easily, and manage the chaining of these things. For instance, it should be possible to generate Go bindings in one command such as:

fidl gen --library my_library.fidl --binding go --out-dir go/src/my/library

Or format a library in place with:

fidl fmt --library my_library.fidl -i