pub package Build Status

Binding generator for Wasmjs flavor of FFI bindings.

Note: wasmjsgen only supports parsing C headers.

Status: Alpha, not all features are supported, but small apps can be built with it, see this example.

The parameter and return types share native FFI names, i.e. Pointer<Uint8> in order to allow consuming code to remain unchanged and switch from native FFI to WASM via conditional imports.

The below Readme was modified from the ffigen Readme and hasn't been fully updated yet.


For some header file example.h:

int sum(int a, int b);

Add configurations to Pubspec File:

  allocate: 'my_malloc'
  output: 'generated_bindings.dart'
      - 'example.h'

NOTE: that wasmjsgen adds allocate, dealloate and reallocate functions that are used to manage memory. At the very least you will need to specify _allocateif you want to sendString`s from Dart to Wasm.

Output (generated_bindings.dart).

class NativeLibrary {
  /// The symbol lookup function.
  T lookup<T>(String name) {
    return _wasmInstance.functions[name] as T;

  wasm_interop.Memory get memory {
    return _wasmInstance.memories['memory']!;

  dart_typed.Uint8List get memView {
    return _wasmInstance.memories['memory']!.buffer.asUint8List();

  [ .. ]
  // --- dividePercision ---
  /// Divides 2 floats, returns a pointer to double.
  Pointer<Double> dividePercision(
    Pointer<Float> a,
    Pointer<Float> b,
  ) {
    return Pointer.fromAddress(

  late final int Function(
  ) _dividePercision = lookup('dividePercision');
  [ .. ]

Using this package

  • Add wasmjsgen under dev_dependencies in your pubspec.yaml.
  • Install LLVM (see Installing LLVM).
  • Configurations must be provided in pubspec.yaml or in a custom YAML file (see configurations).
  • Run the tool- dart run wasmjsgen.

Jump to FAQ.

Installing LLVM

package:wasmjsgen uses LLVM. Install LLVM (9+) in the following way.


  1. Install libclangdev - sudo apt-get install libclang-dev.


  1. Install Visual Studio with C++ development support.
  2. Install LLVM or winget install -e --id LLVM.LLVM.


  1. Install Xcode.
  2. Install LLVM - brew install llvm.

From FFIgen Readme


Configurations can be provided in 2 ways-

  1. In the project's pubspec.yaml file under the key wasmjsgen.
  2. Via a custom YAML file, then specify this file while running - dart run wasmjsgen --config config.yaml

The following configuration options are available-

Key Explaination Example
Output path of the generated bindings.
output: 'generated_bindings.dart'
llvm-path Path to llvm folder.
ffigen will sequentially search for `lib/` on linux, `lib/libclang.dylib` on macOs and `bin\libclang.dll` on windows, in the specified paths.

Complete path to the dynamic library can also be supplied.
Required if ffigen is unable to find this at default locations.
  - '/usr/local/opt/llvm'
  - 'C:\Program Files\llvm`
  - '/usr/lib/llvm-11'
  # Specify exact path to dylib
  - '/usr/lib64/'
The header entry-points and include-directives. Glob syntax is allowed.
If include-directives are not specified ffigen will generate everything directly/transitively under the entry-points.
    - 'folder/**.h'
    - 'folder/specific_header.h'
    - '**index.h'
    - '**/clang-c/**'
    - '/full/path/to/a/header.h'
Name of generated class.
name: 'SQLite'
Dart Doc for generated class.
description: 'Bindings to SQLite'
compiler-opts Pass compiler options to clang. You can also pass these via the command line tool.
  - '-I/usr/lib/llvm-9/include/'

and/or via the command line -

dart run ffigen --compiler-opts "-I/headers
-L 'path/to/folder name/file'"
compiler-opts-automatic -> macos -> include-c-standard-library Tries to automatically find and add C standard library path to compiler-opts on macos.
Default: true
    include-c-standard-library: false






Filters for declarations.
Default: all are included.

Options -
- Include/Exclude declarations.
- Rename declarations.
- Rename enum and struct members.
- Expose symbol-address for functions and globals.
  include: # 'exclude' is also available.
    # Matches using regexp.
    - [a-z][a-zA-Z0-9]*
    # '.' matches any character.
    - prefix.*
    # Matches with exact name
    - someFuncName
    # Full names have higher priority.
    - anotherName
    # Regexp groups based replacement.
    'clang_(.*)': '$1'
    'clang_dispose': 'dispose'
    # Removes '_' from beginning.
    '_(.*)': '$1'
    # Used to expose symbol address.
      - myFunc
    # Removes prefix underscores
    # from all structures.
    '_(.*)': '$1'
    '.*': # Matches any struct.
      # Removes prefix underscores
      # from members.
      '_(.*)': '$1'
    # Regexp groups based replacement.
    'CXType_(.*)': '$1'
    '(.*)': # Matches any enum.
      # Removes '_' from beginning
      # enum member name.
      '_(.*)': '$1'
    # Full names have higher priority.
      # $1 keeps only the 1st
      # group i.e only '(.*)'.
      'CXType(.*)': '$1'
    - aGlobal
    # Removes '_' from
    # beginning of a name.
    '_(.*)': '$1'
typedefs Filters for referred typedefs.

Options -
- Include/Exclude (referred typedefs only).
- Rename typedefs.

Note: Typedefs that are not referred to anywhere will not be generated.
    # Typedefs starting with `p` are not generated.
    - 'p.*'
    # Removes '_' from beginning of a typedef.
    '_(.*)': '$1'
functions -> expose-typedefs Generate the typedefs to Native and Dart type of a function
Default: Inline types are used and no typedefs to Native/Dart type are generated.
      # Match function name.
      - 'myFunc'
       # Do this to expose types for all function.
      - '.*'
      # If you only use exclude, then everything
      # not excluded is generated.
      - 'dispose'
structs -> pack Override the @Packed(X) annotation for generated structs.

Options - none, 1, 2, 4, 8, 16
You can use RegExp to match with the generated names.

Note: Ffigen can only reliably identify packing specified using __attribute__((__packed__)). However, structs packed using `#pragma pack(...)` or any other way could potentially be incorrect in which case you can override the generated annotations.
    # Matches with the generated name.
    'NoPackStruct': none # No packing
    '.*': 1 # Pack all structs with value 1
comments Extract documentation comments for declarations.
The style and length of the comments recognized can be specified with the following options-
style: doxygen(default) | any
length: brief | full(default)
If you want to disable all comments you can also pass
comments: false.
  style: any
  length: full
structs -> dependency-only

unions -> dependency-only
If `opaque`, generates empty `Opaque` structs/unions if they were not included in config (but were added since they are a dependency) and only passed by reference(pointer).
Options - full(default) | opaque
  dependency-only: opaque
  dependency-only: opaque
sort Sort the bindings according to name.
Default: false, i.e keep the order as in the source files.
sort: true
use-supported-typedefs Should automatically map typedefs, E.g uint8_t => Uint8, int16_t => Int16 etc.
Default: true
use-supported-typedefs: true
dart-bool Should generate dart `bool` instead of Uint8 for c99 bool in functions.
Default: true
dart-bool: true
use-dart-handle Should map `Dart_Handle` to `Handle`.
Default: true
use-dart-handle: true
preamble Raw header of the file, pasted as-it-is.
preamble: |
  // ignore_for_file: camel_case_types, non_constant_identifier_names
typedef-map Map typedefs to Native Types.
Values can only be Void, Uint8, Int8, Uint16, Int16, Uint32, Int32, Uint64, Int64, IntPtr, Float and Double.
  'my_custom_type': 'IntPtr'
  'size_t': 'Int64'
size-map Size of integers to use (in bytes).
The defaults (see example) may not be portable on all OS. Do not change these unless absolutely sure.
# These are optional and also default,
# Omitting any and the default
# will be used.
  char: 1
  unsigned char: 1
  short: 2
  unsigned short: 2
  int: 4
  unsigned int: 4
  long: 8
  unsigned long: 8
  long long: 8
  unsigned long long: 8
  enum: 4


  1. Multi OS support for types such as long. Issue #7

Trying out examples

  1. cd examples/<example_u_want_to_run>, Run dart pub get.
  2. Run dart run ffigen.

Running Tests

  1. Dynamic library for some tests need to be built before running the examples.
  2. cd test/native_test.
  3. Run dart build_test_dylib.dart.

Run tests from the root of the package with dart run test.

Note: If llvm is not installed in one of the default locations, tests may fail.


Can ffigen be used for removing underscores or renaming declarations?

Ffigen supports regexp based renaming, the regexp must be a full match, for renaming you can use regexp groups ($1 means group 1).

E.g - For renaming clang_dispose_string to string_dispose. We can can match it using clang_(.*)_(.*) and rename with $2_$1.

Here's an example of how to remove prefix underscores from any struct and its members.

    '_(.*)': '$1' # Removes prefix underscores from all structures.
    '.*': # Matches any struct.
      '_(.*)': '$1' # Removes prefix underscores from members.

How to generate declarations only from particular headers?

The default behaviour is to include everything directly/transitively under each of the entry-points specified.

If you only want to have declarations directly particular header you can do so using include-directives. You can use glob matching to match header paths.

    - 'path/to/my_header.h'
    - '**my_header.h' # This glob pattern matches the header path.

Can ffigen filter declarations by name?

Ffigen supports including/excluding declarations using full regexp matching.

Here's an example to filter functions using names

    - 'clang.*' # Include all functions starting with clang.
    - '.*dispose': # Exclude all functions ending with dispose.

This will include clang_help. But will exclude clang_dispose.

Note: exclude overrides include.

How does ffigen handle C Strings?

Ffigen treats char* just as any other pointer,(Pointer<Int8>). To convert these to/from String, you can use package:ffi. Use ptr.cast<Utf8>().toDartString() to convert char* to dart string and "str".toNativeUtf8() to convert string to char*.

How does ffigen handle C99 bool data type?

Although dart:ffi doesn't have a NativeType for bool, they can be implemented as Uint8. Ffigen generates dart bool for function parameters and return type by default. To disable this, and use int instead, set dart-bool: false in configurations.

How are unnamed enums handled?

Unnamed enums are handled separately, under the key unnamed-enums, and are generated as top level constants.

Here's an example that shows how to include/exclude/rename unnamed enums

    - 'CX_.*'
    - '.*Flag'
    'CXType_(.*)': '$1'

Why are some struct/union declarations generated even after excluded them in config?

This happens when an excluded struct/union is a dependency to some included declaration. (A dependency means a struct is being passed/returned by a function or is member of another struct in some way)

Note: If you supply structs -> dependency-only as opaque ffigen will generate these struct dependencies as Opaque if they were only passed by reference(pointer).

  dependency-only: opaque
  dependency-only: opaque

How to expose the native pointers?

By default the native pointers are private, but you can use the symbol-address subkey for functions/globals and make them public by matching with its name. The pointers are then accesible via nativeLibrary.addresses.

Example -

      - 'myFunc' # Match function name.
      - '.*' # Do this to expose all function pointers.
    exclude: # If you only use exclude, then everything not excluded is generated.
      - 'dispose'

How to get typedefs to Native and Dart type of a function?

By default these types are inline. But you can use the expose-typedef subkey for functions to generate them. This will expose the Native and Dart type. E.g - for a function named hello, the generated typedefs are named as NativeHello and DartHello.

Example -

      - 'myFunc' # Match function name.
      - '.*' # Do this to expose types for all function.
    exclude: # If you only use exclude, then everything not excluded is generated.
      - 'dispose'

How are Structs/Unions/Enums that are reffered to via typedefs handled?

Named declarations use their own names even when inside another typedef. However, unnamed declarations inside typedefs take the name of the first typedef that refers to them.

Why are some typedefs not generated?

The following typedefs are not generated -

  • They are not referred to anywhere in the included declarations.
  • They refer to a struct/union having the same name as itself.
  • They refer to a boolean, enum, inline array, Handle or any unsupported type.


A bindings generator for dart.