dart-use-ffigen

Generating FFI Bindings using package:ffigen

Contents

• Introduction
• Constraints
• FFIgen Overview
• Step-by-Step Workflow
• Concrete Example: Binding a C Library
• Verification Checklist

Introduction

Automate and standardize the generation of FFI bindings using package:ffigen (FfiGenerator). Writing FFI bindings by hand is error-prone, brittle, and highly discouraged.

Constraints

• No Hand-Written FFI Bindings: If native headers (.h files) exist or are generated by a build step, never write manual DynamicLibrary.lookup, @Native external functions, or raw struct classes. Always use FfiGenerator to generate them.
• Generator Location: The generator script should be located at tool/ffigen.dart within the target package root.
• Header Location: If the native header files are third-party, they should be located in third_party/ within the target package (otherwise placing them in a src/ directory at the package root is also acceptable). If the headers are not in one of these standard locations, notify the user that it would be cleaner to move the header files to the standard location (e.g., third_party/).
• Targeted Inclusion Filters: Avoid importing an entire native library unless specifically needed. Always apply precise inclusion lists using positive matches to minimize the size and cognitive load of the generated code (e.g., using Functions.includeSet or filtering matches in include closures).
• Output Setup: If the generated FFI bindings interface with a third-party library (or reference third-party headers), the generated files must always be placed under lib/src/third_party/. The primary generated FFI bindings file must strictly use the .g.dart extension (e.g. sqlite3.g.dart).
• Preamble & License Headers: Always supply a premium preamble in the Output class to specify the license. This must match the native third-party library's license, explicitly include the copyright header of the target native header file, and contain an automatic generation warning (e.g. // Generated by package:ffigen. Do not edit manually.).
• No Unnecessary Commits of Stale Bindings: Ensure you run the generator script and check if the generated files have changed before finishing your task. Always verify the package by running dart analyze.
• Record Usage and Tree Shaking: If the package is integrated into standard runtime execution or compiles native assets via native hooks:
  • Enable recorded usage on all functions by setting recordUse: (_) => true under Functions.
  • Specify the recordUseMapping target in Output (which must strictly be a .g.dart file under lib/src/third_party/, e.g. lib/src/third_party/sqlite3.record_use_mapping.g.dart) to register bindings for symbol tree shaking.

FFIgen Overview

To construct the programmatic generator, use the core configuration objects imported from package:ffigen/ffigen.dart:

1. FfiGenerator

The parent class that orchestrates the configuration, parsing, and code generation.

FfiGenerator({
  Headers headers = const Headers(),
  Enums enums = Enums.excludeAll,
  Functions functions = Functions.excludeAll,
  Globals globals = Globals.excludeAll,
  Integers integers = const Integers(),
  Macros macros = Macros.excludeAll,
  Structs structs = Structs.excludeAll,
  Typedefs typedefs = Typedefs.excludeAll,
  Unions unions = Unions.excludeAll,
  UnnamedEnums unnamedEnums = UnnamedEnums.excludeAll,
  ObjectiveC? objectiveC,
  required Output output,
}).generate();

2. Headers

Configures Clang header parsing targets and compiler flags.

• entryPoints: A list of target header Uri inputs.
• include: A filter function bool Function(Uri header) that handles transitive header imports.
• compilerOptions: Custom preprocessor/include compiler flags to pass directly to libclang.
• ignoreSourceErrors: Set to true to silence errors occurring inside third-party headers during parsing.

3. Functions

Specifies which native C/C++ functions to expose in Dart.

• include: A matcher function (e.g. (decl) => {'my_func'}.contains(decl.originalName) or Functions.includeSet({'my_func'})).
• isLeaf: Declares functions as leaf functions ((decl) => true) if they do not call back into Dart or block thread execution.
• recordUse: Enables metadata generation for native asset tree shaking (essential in dart-lang/native). Set to (_) => true.

4. Output

Configures target generated files.

• dartFile: Target Uri where the primary FFI bindings will be written.
• recordUseMapping: Target Uri for recorded usage metadata maps (crucial for linking-time tree shaking).
• preamble: Text inserted at the top of the generated file (licensing, annotations).
• format: Set to true to run the Dart formatter automatically.

Step-by-Step Workflow

Step 1: Check/Add Dependencies

Open the package's pubspec.yaml and verify the dev_dependencies contains ffigen. Use the Dart MCP server or look up the latest version on pub.dev (e.g., ^20.1.1).

You can add it automatically using the CLI:

dart pub add dev:ffigen

Step 2: Formulate Paths Dynamically

Create a programmatic generator script under the package's tool/ directory (e.g., tool/ffigen.dart). Resolve paths relative to Platform.script to make sure it runs successfully from any working directory:

final packageRoot = Platform.script.resolve('../');
final headerFile = packageRoot.resolve('third_party/library.h');
final targetBindings = packageRoot.resolve('lib/src/third_party/bindings.g.dart');

Step 3: Write the Script (tool/ffigen.dart)

Define void main() and run FfiGenerator with dynamic options (see complete example below).

Step 4: Run Code Generation

Execute the script from the terminal inside the target package folder:

dart run tool/ffigen.dart

Step 5: Static Analysis

Verify that the generated bindings are correct and resolve any analysis issues. FFIgen automatically runs the Dart formatter on the output file (via format: true configuration), so manual formatting is not required.

1. Run the static analyzer inside the target package:

   dart analyze
   

2. Addressing Warnings/Lints: If dart analyze reports style or lint warnings inside the generated file, append the corresponding warning codes to the ignore_for_file: list in your generator script's preamble configuration (e.g., adding camel_case_types, non_constant_identifier_names, etc.). Do not modify the package's global rules.
3. Addressing Compilation Errors: If dart analyze reports actual compiler or analysis errors (not warnings) inside the generated file, do not attempt to edit the generated file manually. Report these error details directly to the user so they can file an issue on the repository at github.com/dart-lang/native.

Concrete Example: Binding a C Library

Let's assume we are working with the SQLite package under pkgs/code_assets/example/sqlite, which embeds SQLite C library sources inside third_party/sqlite/ and accesses it via FFI.

The C Header File (third_party/sqlite/sqlite3.h)

// The author disclaims copyright to this source code.

#ifndef SQLITE3_H_
#define SQLITE3_H_

const char *sqlite3_libversion(void);

#endif // SQLITE3_H_

BEFORE: Manual FFI Binding (The Anti-Pattern)

A developer might attempt to handcraft this integration. It is fragile, blocks tree-shaking metadata, and is highly prone to ABI and structural mapping issues:

// lib/src/sqlite3_manual.dart
import 'dart:ffi' as ffi;
import 'package:ffi/ffi.dart';

// Flaw 1: Hardcoded DynamicLibrary lookup blocks integration with modern native asset compilation.
final ffi.DynamicLibrary _dylib = ffi.DynamicLibrary.open('libsqlite3.so');

// Flaw 2: Manual function type matching requires writing redundant dynamic lookup boilerplate and lacks tree-shaking metadata.
typedef _sqlite3_libversion_C = ffi.Pointer<ffi.Char> Function();
typedef _sqlite3_libversion_Dart = ffi.Pointer<ffi.Char> Function();

final _sqlite3_libversion_Dart sqlite3LibVersion = _dylib
    .lookup<ffi.NativeFunction<_sqlite3_libversion_C>>('sqlite3_libversion')
    .asFunction();

AFTER: Generating via FFIgen (The Correct Pattern)

Create a programmatic script at tool/ffigen.dart:

// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:io';
import 'package:ffigen/ffigen.dart';

void main() {
  // Resolve paths dynamically relative to Platform.script
  final packageRoot = Platform.script.resolve('../');
  final entryHeader = packageRoot.resolve('third_party/sqlite/sqlite3.h');
  final bindingsOutput = packageRoot.resolve('lib/src/third_party/sqlite3.g.dart');
  final treeShakeMapping = packageRoot.resolve('lib/src/third_party/sqlite3.record_use_mapping.g.dart');

  FfiGenerator(
    headers: Headers(
      entryPoints: [entryHeader],
    ),
    functions: Functions(
      include: (decl) => {'sqlite3_libversion'}.contains(decl.originalName),
      // Essential for package optimization and tree-shaking
      recordUse: (_) => true,
    ),
    output: Output(
      dartFile: bindingsOutput,
      recordUseMapping: treeShakeMapping,
      format: true,
      preamble: '''

// AUTO-GENERATED FILE - DO NOT MODIFY.
// Generated via ffigen.
// To regenerate: dart run tool/ffigen.dart

// ignore_for_file: type=lint, unused_import, unused_element, deprecated_member_use_from_same_package, experimental_member_use
''',


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