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/ //garnet/public/lib/fidl/rust/fidl/
Dart //topaz/bin/fidlgen_dart/ //topaz//public/dart/fidl/
JavaScipt 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.

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 run -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

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_cpp_ir_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 bringup.x64 --with-base //garnet/packages/tests:zircon
fx build
fx run -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-test
fx run -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: sh Tab 1> fx set core.x64 --with-base //garnet/packages/tests:zircon Tab 1> fx build && fx run -kN Tab 2> fx shell Tab 2(shell)> runtests -t fidl-simple-test Tab 2(shell)> runtests -t fidl-test

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 run -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 run -kN

Tab 3> fx run-test go_fidl_tests

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/.*xunion.*'

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 run -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 run -kN

Tab 3> fx run-test fidl_bindings_test

Compatibility Test

The language bindings compatibility test is located in //topaz/bin/fidl_compatibility_test, and is launched from a shell script.

To build this test, use:

fx build topaz/bin/fidl_compatibility_test/dart:fidl_compatibility_test_server_dart

To run this test, you first need to have a Fuchsia instance running in an emulator:

fx run -N

Then, copy the script to the device, and run it:

fx cp `find topaz -name` /tmp/
fx shell /tmp/

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 hlcpp ir fx run-host-tests fidlgen_cpp_ir_test garnet/go/src/fidl/compiler/backend/cpp/ir
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
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

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 run -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-coding-tables-test zircon/system/host/fidl
fidl c runtime test fx run -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-test zircon/system/ulib/fidl
fidl c runtime test fx run -k -c zircon.autorun.boot=/boot/bin/runtests+-t+fidl-simple-test zircon/system/ulib/fidl
fidl c-llcpp interop test fx run -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
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
gidl conformance test generation fx exec $FUCHSIA_DIR/tools/fidl/gidl-conformance-suite/ tools/fidl/gidl-conformance-suite third_party/go/src/syscall/zx/fidl/conformance/impl.go third_party/go/src/syscall/zx/fidl/fidl_test/conformance_test.go sdk/lib/fidl/cpp/ topaz/bin/fidl_bindings_test/test/test/conformance_test_types.dart topaz/bin/fidl_bindings_test/test/test/conformance_test.dart
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
regen c-llcpp interop test bindings fx exec $FUCHSIA_DIR/zircon/system/utest/fidl-llcpp-interop/ zircon/system/utest/fidl-llcpp-interop/*.test.fidl zircon/system/utest/fidl-llcpp-interop/generated/
regen llcpp fidl::Bind test bindings fx exec $FUCHSIA_DIR/zircon/system/utest/fidl/ zircon/system/utest/fidl/llcpp.test.fidl zircon/system/utest/fidl/generated/
regen fidl-async test bindings fx exec $FUCHSIA_DIR/zircon/system/ulib/fidl-async/test/ zircon/system/ulib/fidl-async/test/simple.test.fidl zircon/system/ulib/fidl-async/test/generated/
checked in production llcpp bindings fx build -k 0 tools/fidlgen_llcpp_zircon:update FIDL definitions in zircon/system/fidl/ "gen/" folder relative to the corresponding FIDL definition

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