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

The front-end lives at //zircon/tools/fidl/, with tests in //zircon/system/utest/fidl/.

The back-end and runtime library locations are based on the target:

Target Back-end Runtime Libraries
C //zircon/tools/fidl/lib/ //zircon/system/ulib/fidl/
C++ //garnet/go/src/fidl/compiler/backend/cpp/ //zircon/system/ulib/fidl/ & //sdk/lib/fidl/cpp/
Go //garnet/go/src/fidl/compiler/backend/golang/ //third_party/go/src/syscall/zx/fidl/
Rust //garnet/go/src/fidl/compiler/backend/rust/ //src/lib/fidl/rust/fidl/
Dart //topaz/bin/fidlgen_dart/ //topaz//public/dart/fidl/
JavaScript chromium:build/fuchsia/fidlgen_fs chromium:build/fuchsia/fidlgen_js/runtime

Other FIDL Tools

TBD: linter, formatter, gidl, difl, regen scripts, etc.

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 VSCode 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": {
        "*": "rust",
        "*": "cpp",
        "*.test.fidl.json.h.golden": "cpp",
        "*": "cpp",
        "*.test.fidl.json.llcpp.h.golden": "cpp",
        "*.test.fidl.json.h.go.golden": "go",
        "*.test.fidl.json_async.dart.golden": "dart",
        "*.test.fidl.json_test.dart.golden": "dart"

C++ Style Guide

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


Always place the initializer list on a line below the constructor.

// Don't do this.
SomeClass::SomeClass() : field_(1), another_field_(2) {}

// Correct.
    : field_(1), another_field_(2) {}


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

fx set core.x64 --with //bundles:tests --with //topaz/packages/tests:all --with //sdk:modular_testing

or, to ensure there's no breakage with lots of bindings etc.:

fx set terminal.x64 --with //bundles:kitchen_sink --with //vendor/google/bundles:buildbot


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

export ASAN_SYMBOLIZER_PATH="$FUCHSIA_DIR/prebuilt/third_party/clang/$HOST_PLATFORM/bin/llvm-symbolizer"

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.


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

# build fidlc
fx build zircon/tools

If you're doing extensive edit-compile-test cycles on fidlc, building with no optimizations can make a significant difference in the build speed. To optimize the build, change the opt_level setting in zircon/public/gn/config/levels.gni. But be careful: the kernel is not regularly tested with opt_level below 2, and it does not support -1 at all (this setting can cause kernel panics from stack overflows in the kernel).

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:

# build & run fidlc tests
fx build system/utest:host

# build & run fidl-coding-tables tests
# --with-base puts all zircon tests under /boot with the bringup.x64 target, or /system when using the core.x64 target
fx set bringup.x64 --with-base //garnet/packages/tests:zircon   # optionally append "--variant asan"
fx build
fx qemu -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-coding-tables-test

To regenerate the FIDL definitions used in unit testing, run: sh fx build zircon/tools $FUCHSIA_DIR/out/default.zircon/tools/fidlc \ --tables $FUCHSIA_DIR/zircon/system/utest/fidl/fidl/ \ --files $FUCHSIA_DIR/zircon/system/utest/fidl/fidl/extra_messages.test.fidl

To regenerate the fidlc JSON goldens, ensure fidlc is built and up to date, then run:

fx exec $FUCHSIA_DIR/zircon/tools/fidl/testdata/

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 (LLCPP, HLCPP, Rust, Go)


fx build garnet/go/src/fidl



Some example tests you can run:

fx run-host-tests fidlgen_cpp_test
fx run-host-tests fidlgen_golang_ir_test

To regenerate the goldens:

fx exec garnet/go/src/fidl/compiler/backend/typestest/


Some example tests you can run:

fx run-host-tests fidlgen_dart_backend_ir_test

To regenerate the goldens:

fx exec topaz/bin/fidlgen_dart/

C runtime

fx set core.x64 --with-base //garnet/packages/tests:zircon
fx build
fx qemu -kN

# On Fuchsia device's shell
$ runtests -t fidl-test
$ runtests -t fidl-simple-test

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

You might get lucky with fx qemu -kN -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-test fx qemu -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-simple-test

When the test completes, you're running in the QEMU emulator. To exit, use Ctrl-A x.

Alternatively, if you build including the shell you can run these commands from the shell:

Tab 1> fx set core.x64 --with-base //garnet/packages/tests:zircon
Tab 1> fx build && fx qemu -kN

Tab 2> fx shell
Tab 2(shell)> runtests -t fidl-simple-test
Tab 2(shell)> runtests -t fidl-test

Some of the C runtime tests can run on host: sh fx build zircon && fx run-host-tests fidl-test This only includes a few tests, so be sure to check the output to see if it is running the test you care about.

C++ runtime

You first need to have Fuchsia running in an emulator. Here are the steps:

Tab 1> fx build && fx serve-updates

Tab 2> fx qemu -kN

Tab 3> fx run-test fidl_tests

There are separate tests for LLCPP that can be run in the same way as fidl_tests:

  • fidl_llcpp_types_test
  • fidl_llcpp_conformance_test

Go runtime

You first need to have Fuchsia running in an emulator. Here are the steps:

Tab 1> fx build && fx serve-updates

Tab 2> fx qemu -kN

Tab 3> fx run-test go_fidl_tests
Tab 3> fx run-test fidl_go_conformance

As with normal Go tests, you can pass various flags to control execution, filter test cases, run benchmarks, etc. For instance:

Tab 3> fx run-test go_fidl_tests -- -test.v 'TestAllSuccessCases/.*union.*'

Rust runtime

You first need to have Fuchsia running in an emulator. Here are the steps:

Tab 1> fx build && fx serve-updates

Tab 2> fx qemu -kN

Tab 3> fx run-test rust_fidl_tests

Dart runtime

The Dart FIDL bindings tests are in //topaz/bin/fidl_bindings_test/.

You first need to have Fuchsia running in an emulator. Here are the steps:

Tab 1> fx build && fx serve-updates

Tab 2> fx qemu -kN

Tab 3> fx run-test fidl_bindings_test

Compatibility Test

Details about how the compatibility tests work and where the code is located can be found in the README at //garnet/bin/fidl_compatibility_test

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 with HLCPP, LLCPP, Rust, and Go:

Tab 3> fx set core.x64 --with-base //garnet/packages/tests:zircon --with //garnet/packages/tests:all
Tab 3> fx run-test fidl_compatibility_test

To run the compatiblity tests with Dart:

Tab 3> fx set core.x64 --with //topaz/packages/tests:all
Tab 3> fx run-test fidl_compatibility_test_topaz


To rebuild GIDL:

fx build host-tools/gidl

All Tests

Name Test Command Directories Covered
gidl parser fx run-host-tests gidl_parser_test tools/fidl/gidl/parser
fidlgen hlcpp fx run-host-tests fidlgen_cpp_test garnet/go/src/fidl/compiler/backend/cpp
fidlgen llcpp fx run-host-tests fidlgen_llcpp_test garnet/go/src/fidl/compiler/llcpp_backend
fidlgen overnet fx run-host-tests fidlgen_cpp_overnet_internal_test garnet/go/src/fidl/compiler/backend/cpp_overnet_internal
fidlgen golang fx run-host-tests fidlgen_golang_test garnet/go/src/fidl/compiler/backend/golang
fidlgen golang ir fx run-host-tests fidlgen_golang_ir_test garnet/go/src/fidl/compiler/backend/golang/ir
fidlgen rust fx run-host-tests fidlgen_rust_test garnet/go/src/fidl/compiler/backend/rust
fidlgen rust ir fx run-host-tests fidlgen_rust_ir_test garnet/go/src/fidl/compiler/backend/rust/ir
fidlgen syzkaller fx run-host-tests fidlgen_syzkaller_test garnet/go/src/fidl/compiler/backend/syzkaller
fidlgen syzkaller ir fx run-host-tests fidlgen_syzkaller_ir_test garnet/go/src/fidl/compiler/backend/syzkaller/ir
fidlgen type definitions fx run-host-tests fidlgen_types_test garnet/go/src/fidl/compiler/backend/types
fidl c runtime host test fx run-host-tests fidl-test zircon/system/ulib/fidl
c++ host unittests fx run-host-tests fidl_cpp_host_unittests sdk/lib/fidl
c++ bindings tests fx run-test fidl_tests sdk/lib/fidl
llcpp bindings tests fx run-test fidl_llcpp_types_test garnet/go/src/fidl/compiler/llcpp_backend
go bindings tests fx run-test go_fidl_tests third_party/go/syscall/zx/fidl third_party/go/syscall/zx/fidl/fidl_test
dart bindings tests fx run-test fidl_bindings_test topaz/public/dart/fidl
rust bindings fx run-test rust_fidl_tests src/lib/fidl/rust/fidl

The following requires: fx set bringup.x64 --with-base //garnet/packages/tests:zircon

Name Test Command Directories Covered
fidlc host test $FUCHSIA_DIR/out/default.zircon/host-x64-linux-clang/obj/system/utest/fidl-compiler/fidl-compiler-test.debug zircon/system/host/fidl
fidl coding tables test fx qemu -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-coding-tables-test zircon/system/host/fidl
fidl c runtime test fx qemu -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-test zircon/system/ulib/fidl
fidl c runtime test fx qemu -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-simple-test zircon/system/ulib/fidl
fidl c-llcpp interop test fx qemu -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-llcpp-interop-test zircon/system/ulib/fidl

All Regen Commands

Name Regen Commands Input Output
fidlc goldens fx exec $FUCHSIA_DIR/tools/fidl/testdata/ zircon/tools/fidl/goldens zircon/tools/fidl/testdata
fidlgen goldens fx exec $FUCHSIA_DIR/garnet/go/src/fidl/compiler/backend/typestest/ garnet/go/src/fidl/compiler/backend/goldens garnet/go/src/fidl/compiler/backend/goldens
dart fidlgen goldens fx exec $FUCHSIA_DIR/topaz/bin/fidlgen_dart/ garnet/go/src/fidl/compiler/backend/goldens topaz/bin/fidlgen_dart/goldens
dangerous identifiers garnet/tests/fidl-dangerous-identifiers/ garnet/tests/fidl-dangerous-identifiers/dangerous_identifiers.txt garnet/tests/fidl-dangerous-identifiers/cpp/ garnet/tests/fidl-dangerous-identifiers/fidl/
regen third party go fx exec $FUCHSIA_DIR/third_party/go/regen-fidl

Debugging (host)

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


Start GDB:

gdb --args out/default.zircon/host-x64-linux-clang/obj/tools/fidl/fidlc.debug --files test.fidl

Then, enter "r" to start the program.


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.zircon/host-x64-linux-asan/obj/tools/fidl/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.zircon/host-x64-linux-clang/obj/tools/fidl/fidlc.debug --files test.fidl


Language evolutions

One common task is to evolve the language, or introduce stricter checks in fidlc. These changes typically follow a three phase approach:

  1. Write the new compiler code in fidlc;
  2. Use this updated fidlc to compile all layers, including vendor/google, make changes as needed;
  3. When all is said and done, the fidlc changes can finally be merged.

All of this assumes that (a) code which wouldn't pass the new checks, or (b) code that has new features, is not introduced concurrently between step 2 and step 3. That typically is the case, however, it is ok to deal with breaking rollers once in a while.

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 third_party/go/regen-fidl


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


Why is fidlc in the zircon repo?


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