# Mojom Interface Definition Language (IDL)
This document is a subset of the [Mojo documentation](/mojo/README.md).

[TOC]

## Overview

Mojom is the IDL for Mojo interfaces. Given a `.mojom` file, the
[bindings
generator](https://cs.chromium.org/chromium/src/mojo/public/tools/bindings/) can
output bindings for any supported language: **C++**, **JavaScript**, or
**Java**.

For a trivial example consider the following hypothetical Mojom file we write to
`//services/widget/public/mojom/frobinator.mojom`:

```
module widget.mojom;

interface Frobinator {
  Frobinate();
};
```

This defines a single [interface](#Interfaces) named `Frobinator` in a
[module](#Modules) named `widget.mojom` (and thus fully qualified in Mojom as
`widget.mojom.Frobinator`.) Note that many interfaces and/or other types of
definitions (structs, enums, *etc.*) may be included in a single Mojom file.

If we add a corresponding GN target to
`//services/widget/public/mojom/BUILD.gn`:

```
import("mojo/public/tools/bindings/mojom.gni")

mojom("mojom") {
  sources = [
    "frobinator.mojom",
  ]
}
```

and then build this target:

```
ninja -C out/r services/widget/public/mojom
```

we'll find several generated sources in our output directory:

```
out/r/gen/services/widget/public/mojom/frobinator.mojom.cc
out/r/gen/services/widget/public/mojom/frobinator.mojom.h
out/r/gen/services/widget/public/mojom/frobinator.mojom-shared.h
etc...
```

Each of these generated source modules includes a set of definitions
representing the Mojom contents in C++. You can also build or depend on suffixed
target names to get bindings for other languages. For example,

```
ninja -C out/r services/widget/public/mojom:mojom_js
ninja -C out/r services/widget/public/mojom:mojom_java
```

would generate JavaScript and Java bindings respectively, in the same generated
output directory.

For more details regarding the generated
outputs please see
[documentation for individual target languages](#Generated-Code-For-Target-Languages).

## Mojom Syntax

Mojom IDL allows developers to define **structs**, **unions**, **interfaces**,
**constants**, and **enums**, all within the context of a **module**. These
definitions are used to generate code in the supported target languages at build
time.

Mojom files may **import** other Mojom files in order to reference their
definitions.

### Primitive Types
Mojom supports a few basic data types which may be composed into structs or used
for message parameters.

| Type                          | Description
|-------------------------------|-------------------------------------------------------|
| `bool`                        | Boolean type (`true` or `false`.)
| `int8`, `uint8`               | Signed or unsigned 8-bit integer.
| `int16`, `uint16`             | Signed or unsigned 16-bit integer.
| `int32`, `uint32`             | Signed or unsigned 32-bit integer.
| `int64`, `uint64`             | Signed or unsigned 64-bit integer.
| `float`, `double`             | 32- or 64-bit floating point number.
| `string`                      | UTF-8 encoded string.
| `array<T>`                    | Array of any Mojom type *T*; for example, `array<uint8>` or `array<array<string>>`.
| `array<T, N>`                 | Fixed-length array of any Mojom type *T*. The parameter *N* must be an integral constant.
| `map<S, T>`                   | Associated array mapping values of type *S* to values of type *T*. *S* may be a `string`, `enum`, or numeric type.
| `handle`                      | Generic Mojo handle. May be any type of handle, including a wrapped native platform handle.
| `handle<message_pipe>`        | Generic message pipe handle.
| `handle<shared_buffer>`       | Shared buffer handle.
| `handle<data_pipe_producer>`  | Data pipe producer handle.
| `handle<data_pipe_consumer>`  | Data pipe consumer handle.
| `handle<platform>`            | A native platform/OS handle.
| *`pending_remote<InterfaceType>`*             | Any user-defined Mojom interface type. This is sugar for a strongly-typed message pipe handle which should eventually be used to make outgoing calls on the interface.
| *`pending_receiver<InterfaceType>`*            | A pending receiver for any user-defined Mojom interface type. This is sugar for a more strongly-typed message pipe handle which is expected to receive request messages and should therefore eventually be bound to an implementation of the interface.
| *`pending_associated_remote<InterfaceType>`*  | An associated interface handle. See [Associated Interfaces](#Associated-Interfaces)
| *`pending_associated_receiver<InterfaceType>`* | A pending associated receiver. See [Associated Interfaces](#Associated-Interfaces)
| *T*?                          | An optional (nullable) value. Primitive numeric types (integers, floats, booleans, and enums) are not nullable. All other types are nullable.

### Modules

Every Mojom file may optionally specify a single **module** to which it belongs.

This is used strictly for aggregating all defined symbols therein within a
common Mojom namespace. The specific impact this has on generated bindings code
varies for each target language. For example, if the following Mojom is used to
generate bindings:

```
module business.stuff;

interface MoneyGenerator {
  GenerateMoney();
};
```

Generated C++ bindings will define a class interface `MoneyGenerator` in the
`business::stuff` namespace, while Java bindings will define an interface
`MoneyGenerator` in the `org.chromium.business.stuff` package. JavaScript
bindings at this time are unaffected by module declarations.

**NOTE:** By convention in the Chromium codebase, **all** Mojom files should
declare a module name with at least (and preferably exactly) one top-level name
as well as an inner `mojom` module suffix. *e.g.*, `chrome.mojom`,
`business.mojom`, *etc.*

This convention makes it easy to tell which symbols are generated by Mojom when
reading non-Mojom code, and it also avoids namespace collisions in the fairly
common scenario where you have a real C++ or Java `Foo` along with a
corresponding Mojom `Foo` for its serialized representation.

### Imports

If your Mojom references definitions from other Mojom files, you must **import**
those files. Import syntax is as follows:

```
import "services/widget/public/mojom/frobinator.mojom";
```

Import paths are always relative to the top-level directory.

Note that circular imports are **not** supported.

### Structs

Structs are defined using the **struct** keyword, and they provide a way to
group related fields together:

``` cpp
struct StringPair {
  string first;
  string second;
};
```

Struct fields may be comprised of any of the types listed above in the
[Primitive Types](#Primitive-Types) section.

Default values may be specified as long as they are constant:

``` cpp
struct Request {
  int32 id = -1;
  string details;
};
```

What follows is a fairly
comprehensive example using the supported field types:

``` cpp
struct StringPair {
  string first;
  string second;
};

enum AnEnum {
  kYes,
  kNo
};

interface SampleInterface {
  DoStuff();
};

struct AllTheThings {
  // Note that these types can never be marked nullable!
  bool boolean_value;
  int8 signed_8bit_value = 42;
  uint8 unsigned_8bit_value;
  int16 signed_16bit_value;
  uint16 unsigned_16bit_value;
  int32 signed_32bit_value;
  uint32 unsigned_32bit_value;
  int64 signed_64bit_value;
  uint64 unsigned_64bit_value;
  float float_value_32bit;
  double float_value_64bit;
  AnEnum enum_value = AnEnum.kYes;

  // Strings may be nullable.
  string? maybe_a_string_maybe_not;

  // Structs may contain other structs. These may also be nullable.
  StringPair some_strings;
  StringPair? maybe_some_more_strings;

  // In fact structs can also be nested, though in practice you must always make
  // such fields nullable -- otherwise messages would need to be infinitely long
  // in order to pass validation!
  AllTheThings? more_things;

  // Arrays may be templated over any Mojom type, and are always nullable:
  array<int32> numbers;
  array<int32>? maybe_more_numbers;

  // Arrays of arrays of arrays... are fine.
  array<array<array<AnEnum>>> this_works_but_really_plz_stop;

  // The element type may be nullable if it's a type which is allowed to be
  // nullable.
  array<AllTheThings?> more_maybe_things;

  // Fixed-size arrays get some extra validation on the receiving end to ensure
  // that the correct number of elements is always received.
  array<uint64, 2> uuid;

  // Maps follow many of the same rules as arrays. Key types may be any
  // non-handle, non-collection type, and value types may be any supported
  // struct field type. Maps may also be nullable.
  map<string, int32> one_map;
  map<AnEnum, string>? maybe_another_map;
  map<StringPair, AllTheThings?>? maybe_a_pretty_weird_but_valid_map;
  map<StringPair, map<int32, array<map<string, string>?>?>?> ridiculous;

  // And finally, all handle types are valid as struct fields and may be
  // nullable. Note that interfaces and interface requests (the "Foo" and
  // "Foo&" type syntax respectively) are just strongly-typed message pipe
  // handles.
  handle generic_handle;
  handle<data_pipe_consumer> reader;
  handle<data_pipe_producer>? maybe_writer;
  handle<shared_buffer> dumping_ground;
  handle<message_pipe> raw_message_pipe;
  pending_remote<SampleInterface>? maybe_a_sample_interface_client_pipe;
  pending_receiver<SampleInterface> non_nullable_sample_pending_receiver;
  pending_receiver<SampleInterface>? nullable_sample_pending_receiver;
  pending_associated_remote<SampleInterface> associated_interface_client;
  pending_associated_receiver<SampleInterface> associated_pending_receiver;
  pending_associated_receiver<SampleInterface>? maybe_another_pending_receiver;
};
```

For details on how all of these different types translate to usable generated
code, see
[documentation for individual target languages](#Generated-Code-For-Target-Languages).

### Unions

Mojom supports tagged unions using the **union** keyword. A union is a
collection of fields which may take the value of any single one of those fields
at a time. Thus they provide a way to represent a variant value type while
minimizing storage requirements.

Union fields may be of any type supported by [struct](#Structs) fields. For
example:

```cpp
union ExampleUnion {
  string str;
  StringPair pair;
  int64 id;
  array<uint64, 2> guid;
  SampleInterface iface;
};
```

For details on how unions like this translate to generated bindings code, see
[documentation for individual target languages](#Generated-Code-For-Target-Languages).

### Enumeration Types

Enumeration types may be defined using the **enum** keyword either directly
within a module or nested within the namespace of some struct or interface:

```
module business.mojom;

enum Department {
  kSales = 0,
  kDev,
};

struct Employee {
  enum Type {
    kFullTime,
    kPartTime,
  };

  Type type;
  // ...
};
```

C++ constant-style enum value names are preferred as specified in the
[Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html#Enumerator_Names).

Similar to C-style enums, individual values may be explicitly assigned within an
enum definition. By default, values are based at zero and increment by
1 sequentially.

The effect of nested definitions on generated bindings varies depending on the
target language. See [documentation for individual target languages](#Generated-Code-For-Target-Languages).

### Constants

Constants may be defined using the **const** keyword either directly within a
module or nested within the namespace of some struct or interface:

```
module business.mojom;

const string kServiceName = "business";

struct Employee {
  const uint64 kInvalidId = 0;

  enum Type {
    kFullTime,
    kPartTime,
  };

  uint64 id = kInvalidId;
  Type type;
};
```

The effect of nested definitions on generated bindings varies depending on the
target language. See [documentation for individual target languages](#Generated-Code-For-Target-Languages).

### Features

Features can be declared with a `name` and `default_state` and can be attached
in mojo to interfaces or methods using the `RuntimeFeature` attribute. If the
feature is disabled at runtime, the method will crash and the interface will
refuse to be bound / instantiated. Features cannot be serialized to be sent over
IPC at this time.

```
module experimental.mojom;

feature kUseElevators {
  const string name = "UseElevators";
  const bool default_state = false;
}

[RuntimeFeature=kUseElevators]
interface Elevator {
  // This interface cannot be bound or called if the feature is disabled.
}

interface Building {
  // This method cannot be called if the feature is disabled.
  [RuntimeFeature=kUseElevators]
  CallElevator(int floor);

  // This method can be called.
  RingDoorbell(int volume);
}
```

### Interfaces

An **interface** is a logical bundle of parameterized request messages. Each
request message may optionally define a parameterized response message. Here's
an example to define an interface `Foo` with various kinds of requests:

```
interface Foo {
  // A request which takes no arguments and expects no response.
  MyMessage();

  // A request which has some arguments and expects no response.
  MyOtherMessage(string name, array<uint8> bytes);

  // A request which expects a single-argument response.
  MyMessageWithResponse(string command) => (bool success);

  // A request which expects a response with multiple arguments.
  MyMessageWithMoarResponse(string a, string b) => (int8 c, int8 d);
};
```

Anything which is a valid struct field type (see [Structs](#Structs)) is also a
valid request or response argument type. The type notation is the same for both.

### Attributes

Mojom definitions may have their meaning altered by **attributes**, specified
with a syntax similar to Java or C# attributes. There are a handle of
interesting attributes supported today.

* **`[Sync]`**:
  The `Sync` attribute may be specified for any interface method which expects a
  response. This makes it so that callers of the method can wait synchronously
  for a response. See [Synchronous
  Calls](/mojo/public/cpp/bindings/README.md#Synchronous-Calls) in the C++
  bindings documentation. Note that sync methods are only actually synchronous
  when called from C++.

* **`[NoInterrupt]`**:
  When a thread is waiting for a reply to a `Sync` message, it's possible to be
  woken up to dispatch other unrelated incoming `Sync` messages. This measure
  helps to avoid deadlocks. If a `Sync` message is also marked as `NoInterrupt`
  however, this behavior is disabled: instead the calling thread will only wake
  up for the precise message being waited upon. This attribute must be used with
  extreme caution, because it can lead to deadlocks otherwise.

* **`[Default]`**:
  The `Default` attribute may be used to specify an enumerator value or union
  field that will be used if an `Extensible` enumeration or union does not
  deserialize to a known value on the receiver side, i.e. the sender is using a
  newer version of the enum or union. This allows unknown values to be mapped to
  a well-defined value that can be appropriately handled.

  Note: The `Default` field for a union must be of nullable or integral type.
  When a union is defaulted to this field, the field takes on the default value
  for its type: null for nullable types, and zero/false for integral types.

* **`[Extensible]`**:
  The `Extensible` attribute may be specified for any enum or union definition.
  For enums, this essentially disables builtin range validation when receiving
  values of the enum type in a message, allowing older bindings to tolerate
  unrecognized values from newer versions of the enum.

  If an enum value within an extensible enum definition is affixed with the
  `Default` attribute, out-of-range values for the enum will deserialize to that
  default value. Only one enum value may be designated as the `Default`.

  Similarly, a union marked `Extensible` will deserialize to its `Default` field
  when an unrecognized field is received. Extensible unions MUST specify exactly
  one `Default` field, and the field must be of nullable or integral type. When
  defaulted to this field, the value is always null/zero/false as appropriate.

  An `Extensible` enumeration REQUIRES that a `Default` value be specified,
  so all new extensible enums should specify one.

* **`[Native]`**:
  The `Native` attribute may be specified for an empty struct declaration to
  provide a nominal bridge between Mojo IPC and legacy `IPC::ParamTraits` or
  `IPC_STRUCT_TRAITS*` macros. See [Repurposing Legacy IPC
  Traits](/docs/mojo_ipc_conversion.md#repurposing-and-invocations) for more
  details. Note support for this attribute is strictly limited to C++ bindings
  generation.

* **`[MinVersion=N]`**:
  The `MinVersion` attribute is used to specify the version at which a given
  field, enum value, interface method, or method parameter was introduced.
  See [Versioning](#Versioning) for more details. `MinVersion` does not apply
  to interfaces, structs or enums, but to the fields of those types.
  `MinVersion` is not a module-global value, but it is ok to pretend it is by
  skipping versions when adding fields or parameters.

* **`[Stable]`**:
  The `Stable` attribute specifies that a given mojom type or interface
  definition can be considered stable over time, meaning it is safe to use for
  things like persistent storage or communication between independent
  version-skewed binaries. Stable definitions may only depend on builtin mojom
  types or other stable definitions, and changes to such definitions MUST
  preserve backward-compatibility through appropriate use of versioning.
  Backward-compatibility of changes is enforced in the Chromium tree using a
  strict presubmit check. See [Versioning](#Versioning) for more details on
  backward-compatibility constraints.

* **`[Uuid=<UUID>]`**:
  Specifies a UUID to be associated with a given interface. The UUID is intended
  to remain stable across all changes to the interface definition, including
  name changes. The value given for this attribute should be a standard UUID
  string representation as specified by RFC 4122. New UUIDs can be generated
  with common tools such as `uuidgen`.

* **`[RuntimeFeature=feature]`**
  The `RuntimeFeature` attribute should reference a mojo `feature`. If this
  feature is enabled (e.g. using `--enable-features={feature.name}`) then the
  interface behaves entirely as expected. If the feature is not enabled the
  interface cannot be bound to a concrete receiver or remote - attempting to do
  so will result in the receiver or remote being reset() to an unbound state.
  Note that this is a different concept to the build-time `EnableIf` directive.
  `RuntimeFeature` is currently only supported for C++ bindings and has no
  effect for, say, Java or TypeScript bindings (see https://crbug.com/1278253).

* **`[EnableIf=value]`**:
  The `EnableIf` attribute is used to conditionally enable definitions when the
  mojom is parsed. If the `mojom` target in the GN file does not include the
  matching `value` in the list of `enabled_features`, the definition will be
  disabled. This is useful for mojom definitions that only make sense on one
  platform. Note that the `EnableIf` attribute can only be set once per
  definition and cannot be set at the same time as `EnableIfNot`. Also be aware
  that only one condition can be tested, `EnableIf=value,xyz` introduces a new
  `xyz` attribute. `xyz` is not part of the `EnableIf` condition that depends
  only on the feature `value`. Complex conditions can be introduced via
  enabled_features in `build.gn` files.

* **`[EnableIfNot=value]`**:
  The `EnableIfNot` attribute is used to conditionally enable definitions when
  the mojom is parsed. If the `mojom` target in the GN file includes the
  matching `value` in the list of `enabled_features`, the definition will be
  disabled. This is useful for mojom definitions that only make sense on all but
  one platform. Note that the `EnableIfNot` attribute can only be set once per
  definition and cannot be set at the same time as `EnableIf`.

* **`[ServiceSandbox=value]`**:
  The `ServiceSandbox` attribute is used in Chromium to tag which sandbox a
  service hosting an implementation of interface will be launched in. This only
  applies to `C++` bindings. `value` should match a constant defined in an
  imported `sandbox.mojom.Sandbox` enum (for Chromium this is
  `//sandbox/policy/mojom/sandbox.mojom`), such as `kService`.

* **`[RequireContext=enum]`**:
  The `RequireContext` attribute is used in Chromium to tag interfaces that
  should be passed (as remotes or receivers) only to privileged process
  contexts. The process context must be an enum that is imported into the
  mojom that defines the tagged interface. `RequireContext` may be used in
  future to DCHECK or CHECK if remotes are made available in contexts that
  conflict with the one provided in the interface definition. Process contexts
  are not the same as the sandbox a process is running in, but will reflect
  the set of capabilities provided to the service.

* **`[AllowedContext=enum]`**:
  The `AllowedContext` attribute is used in Chromium to tag methods that pass
  remotes or receivers of interfaces that are marked with a `RequireContext`
  attribute. The enum provided on the method must be equal or better (lower
  numerically) than the one required on the interface being passed. At present
  failing to specify an adequate `AllowedContext` value will cause mojom
  generation to fail at compile time. In future DCHECKs or CHECKs might be
  added to enforce that method is only called from a process context that meets
  the given `AllowedContext` value. The enum must of the same type as that
  specified in the interface's `RequireContext` attribute. Adding an
  `AllowedContext` attribute to a method is a strong indication that you need
   a detailed security review of your design - please reach out to the security
   team.

* **`[SupportsUrgent]`**:
  The `SupportsUrgent` attribute is used in conjunction with
  `mojo::UrgentMessageScope` in Chromium to tag messages as having high
  priority. The IPC layer notifies the underlying scheduler upon both receiving
  and processing an urgent message. At present, this attribute only affects
  channel associated messages in the renderer process.

## Generated Code For Target Languages

When the bindings generator successfully processes an input Mojom file, it emits
corresponding code for each supported target language. For more details on how
Mojom concepts translate to a given target language, please refer to the
bindings API documentation for that language:

* [C++ Bindings](/mojo/public/cpp/bindings/README.md)
* [JavaScript Bindings](/mojo/public/js/README.md)
* [Java Bindings](/mojo/public/java/bindings/README.md)

## Message Validation

Regardless of target language, all interface messages are validated during
deserialization before they are dispatched to a receiving implementation of the
interface. This helps to ensure consistent validation across interfaces without
leaving the burden to developers and security reviewers every time a new message
is added.

If a message fails validation, it is never dispatched. Instead a **connection
error** is raised on the binding object (see
[C++ Connection Errors](/mojo/public/cpp/bindings/README.md#Connection-Errors),
[Java Connection Errors](/mojo/public/java/bindings/README.md#Connection-Errors),
or
[JavaScript Connection Errors](/mojo/public/js/README.md#Connection-Errors) for
details.)

Some baseline level of validation is done automatically for primitive Mojom
types.

### Non-Nullable Objects

Mojom fields or parameter values (*e.g.*, structs, interfaces, arrays, *etc.*)
may be marked nullable in Mojom definitions (see
[Primitive Types](#Primitive-Types).) If a field or parameter is **not** marked
nullable but a message is received with a null value in its place, that message
will fail validation.

### Enums

Enums declared in Mojom are automatically validated against the range of legal
values. For example if a Mojom declares the enum:

``` cpp
enum AdvancedBoolean {
  kTrue = 0,
  kFalse = 1,
  kFileNotFound = 2,
};
```

and a message is received with the integral value 3 (or anything other than 0,
1, or 2) in place of some `AdvancedBoolean` field or parameter, the message will
fail validation.

*** note
NOTE: It's possible to avoid this type of validation error by explicitly marking
an enum as [Extensible](#Attributes) if you anticipate your enum being exchanged
between two different versions of the binding interface. See
[Versioning](#Versioning).
***

### Other failures

There are a  host of internal validation errors that may occur when a malformed
message is received, but developers should not be concerned with these
specifically; in general they can only result from internal bindings bugs,
compromised processes, or some remote endpoint making a dubious effort to
manually encode their own bindings messages.

### Custom Validation

It's also possible for developers to define custom validation logic for specific
Mojom struct types by exploiting the
[type mapping](/mojo/public/cpp/bindings/README.md#Type-Mapping) system for C++
bindings. Messages rejected by custom validation logic trigger the same
validation failure behavior as the built-in type validation routines.

## Associated Interfaces

As mentioned in the [Primitive Types](#Primitive-Types) section above, pending_remote
and pending_receiver fields and parameters may be marked as `associated`. This
essentially means that they are piggy-backed on some other interface's message
pipe.

Because individual interface message pipes operate independently there can be no
relative ordering guarantees among them. Associated interfaces are useful when
one interface needs to guarantee strict FIFO ordering with respect to one or
more other interfaces, as they allow interfaces to share a single pipe.

Currently associated interfaces are only supported in generated C++ bindings.
See the documentation for
[C++ Associated Interfaces](/mojo/public/cpp/bindings/README.md#Associated-Interfaces).

## Versioning

### Overview

*** note
**NOTE:** You don't need to worry about versioning if you don't care about
backwards compatibility. Today, all parts of the Chrome browser are
updated atomically and there is not yet any possibility of any two
Chrome processes communicating with two different versions of any given Mojom
interface. On Chrome OS, there are several places where versioning is required.
For example,
[ARC++](https://developer.android.com/chrome-os/intro)
uses versioned mojo to send IPC to the Android container.
Likewise, the
[Lacros](/docs/lacros.md)
browser uses versioned mojo to talk to the ash system UI.
***

Services extend their interfaces to support new features over time, and clients
want to use those new features when they are available. If services and clients
are not updated at the same time, it's important for them to be able to
communicate with each other using different snapshots (versions) of their
interfaces.

This document shows how to extend Mojom interfaces in a backwards-compatible
way. Changing interfaces in a non-backwards-compatible way is not discussed,
because in that case communication between different interface versions is
impossible anyway.

### Versioned Structs

You can use the `MinVersion` [attribute](#Attributes) to indicate from which
version a struct field is introduced. Assume you have the following struct:

``` cpp
struct Employee {
  uint64 employee_id;
  string name;
};
```

and you would like to add birthday and nickname fields. You can add them as
optional types with a `MinVersion` like so:

``` cpp
struct Employee {
  uint64 employee_id;
  string name;
  [MinVersion=1] Date? birthday;
  [MinVersion=1] string? nickname;
};
```

*** note
**NOTE:** Mojo object or handle types added with a `MinVersion` **MUST** be
optional (nullable) or primitive. See [Primitive Types](#Primitive-Types) for
details on nullable values.
***

By default, fields belong to version 0. New fields must be appended to the
struct definition (*i.e*., existing fields must not change **ordinal value**)
with the `MinVersion` attribute set to a number greater than any previous
existing versions.

The value of `MinVersion` is unrelated to ordinals. The choice of a particular
version number is arbitrary. All its usage means is that a field isn't present
before the numbered version.

*** note
**NOTE:** do not change existing fields in versioned structs, as this is
not backwards-compatible. Instead, rename the old field to make its
deprecation clear and add a new field with a new `MinVersion` number.
***

**Ordinal value** refers to the relative positional layout of a struct's fields
(and an interface's methods) when encoded in a message. Implicitly, ordinal
numbers are assigned to fields according to lexical position. In the example
above, `employee_id` has an ordinal value of 0 and `name` has an ordinal value
of 1.

Ordinal values can be specified explicitly using `**@**` notation, subject to
the following hard constraints:

* For any given struct or interface, if any field or method explicitly specifies
    an ordinal value, all fields or methods must explicitly specify an ordinal
    value.
* For an *N*-field struct, the set of explicitly assigned ordinal values must be
    limited to the range *[0, N-1]*. Structs should include placeholder fields
    to fill the ordinal positions of removed fields (for example "Unused_Field"
    or "RemovedField", etc).

You may reorder fields, but you must ensure that the ordinal values of existing
fields remain unchanged. For example, the following struct remains
backwards-compatible:

``` cpp
struct Employee {
  uint64 employee_id@0;
  [MinVersion=1] Date? birthday@2;
  string name@1;
  [MinVersion=1] string? nickname@3;
};
```

### Versioned Interfaces

There are two dimensions on which an interface can be extended

**Appending New Parameters To Existing Methods**
:   Parameter lists are treated as structs internally, so all the rules of
    versioned structs apply to method parameter lists. The only difference is
    that the version number is scoped to the whole interface rather than to any
    individual parameter list.

``` cpp
// Old version:
interface HumanResourceDatabase {
  QueryEmployee(uint64 id) => (Employee? employee);
};

// New version:
interface HumanResourceDatabase {
  QueryEmployee(uint64 id, [MinVersion=1] bool retrieve_finger_print)
      => (Employee? employee,
          [MinVersion=1] array<uint8>? finger_print);
};
```

Similar to [versioned structs](#Versioned-Structs), when you pass the parameter
list of a request or response method to a destination using an older version of
an interface, unrecognized fields are silently discarded.

    Please note that adding a response to a message which did not previously
    expect a response is a not a backwards-compatible change.

**Appending New Methods**
:   Similarly, you can reorder methods with explicit ordinal values as long as
    the ordinal values of existing methods are unchanged.

For example:

``` cpp
// Old version:
interface HumanResourceDatabase {
  QueryEmployee(uint64 id) => (Employee? employee);
};

// New version:
interface HumanResourceDatabase {
  QueryEmployee(uint64 id) => (Employee? employee);

  [MinVersion=1]
  AttachFingerPrint(uint64 id, array<uint8> finger_print)
      => (bool success);
};
```

If a method call is not recognized, it is considered a validation error and the
receiver will close its end of the interface pipe. For example, if a client on
version 1 of the above interface sends an `AttachFingerPrint` request to an
implementation of version 0, the client will be disconnected.

Bindings target languages that support versioning expose means to query or
assert the remote version from a client handle (*e.g.*, an
`mojo::Remote<T>` in C++ bindings.)

See
[C++ Versioning Considerations](/mojo/public/cpp/bindings/README.md#Versioning-Considerations)
and
[Java Versioning Considerations](/mojo/public/java/bindings/README.md#Versioning-Considerations)

### Versioned Enums

**By default, enums are non-extensible**, which means that generated message
validation code does not expect to see new values in the future. When an unknown
value is seen for a non-extensible enum field or parameter, a validation error
is raised.

If you want an enum to be extensible in the future, you can apply the
`[Extensible]` [attribute](#Attributes):

``` cpp
[Extensible]
enum Department {
  kSales,
  kDev,
};
```

And later you can extend this enum without breaking backwards compatibility:

``` cpp
[Extensible]
enum Department {
  kSales,
  kDev,
  [MinVersion=1] kResearch,
};
```

*** note
**NOTE:** For versioned enum definitions, the use of a `[MinVersion]` attribute
is strictly for documentation purposes. It has no impact on the generated code.
***

With extensible enums, bound interface implementations may receive unknown enum
values and will need to deal with them gracefully. See
[C++ Versioning Considerations](/mojo/public/cpp/bindings/README.md#Versioning-Considerations)
for details.

### Renaming versioned structs
It's possible to rename versioned structs by using the `[RenamedFrom]` attribute.
RenamedFrom

``` cpp
module asdf.mojom;

// Old version:
[Stable]
struct OldStruct {
};

// New version:
[Stable, RenamedFrom="asdf.mojom.OldStruct"]
struct NewStruct {
};
```

## Component targets

If there are multiple components depending on the same mojom target within one binary,
the target will need to be defined as `mojom_component` instead of `mojom`.
Since `mojom` targets are generated `source_set` targets and `mojom_component` targets
are generated `component` targets, you would use `mojom_component` in the same cases
where you would use `component` for non-mojom files.
*** note
**NOTE**: by default, components for both blink and non-blink bindings are generated.
Use the `disable_variants` target parameter to generate only non-blink bindings.
You can also generate a `source_set` for one of the variants by defining
[export_*](https://source.chromium.org/chromium/chromium/src/+/main:mojo/public/tools/bindings/mojom.gni;drc=739b9fbce50310c1dd2b59c279cd90a9319cb6e8;l=318)
parameters for the `mojom_component` target.
***

## Grammar Reference

Below is the (BNF-ish) context-free grammar of the Mojom language:

```
MojomFile = StatementList
StatementList = Statement StatementList | Statement
Statement = ModuleStatement | ImportStatement | Definition

ModuleStatement = AttributeSection "module" Identifier ";"
ImportStatement = "import" StringLiteral ";"
Definition = Struct Union Interface Enum Feature Const

AttributeSection = <empty> | "[" AttributeList "]"
AttributeList = <empty> | NonEmptyAttributeList
NonEmptyAttributeList = Attribute
                      | Attribute "," NonEmptyAttributeList
Attribute = Name
          | Name "=" Name
          | Name "=" Literal

Struct = AttributeSection "struct" Name "{" StructBody "}" ";"
       | AttributeSection "struct" Name ";"
StructBody = <empty>
           | StructBody Const
           | StructBody Enum
           | StructBody StructField
StructField = AttributeSection TypeSpec Name Ordinal Default ";"

Union = AttributeSection "union" Name "{" UnionBody "}" ";"
UnionBody = <empty> | UnionBody UnionField
UnionField = AttributeSection TypeSpec Name Ordinal ";"

Interface = AttributeSection "interface" Name "{" InterfaceBody "}" ";"
InterfaceBody = <empty>
              | InterfaceBody Const
              | InterfaceBody Enum
              | InterfaceBody Method
Method = AttributeSection Name Ordinal "(" ParameterList ")" Response ";"
ParameterList = <empty> | NonEmptyParameterList
NonEmptyParameterList = Parameter
                      | Parameter "," NonEmptyParameterList
Parameter = AttributeSection TypeSpec Name Ordinal
Response = <empty> | "=>" "(" ParameterList ")"

TypeSpec = TypeName "?" | TypeName
TypeName = BasicTypeName
         | Array
         | FixedArray
         | Map
         | InterfaceRequest
BasicTypeName = Identifier | "associated" Identifier | HandleType | NumericType
NumericType = "bool" | "int8" | "uint8" | "int16" | "uint16" | "int32"
            | "uint32" | "int64" | "uint64" | "float" | "double"
HandleType = "handle" | "handle" "<" SpecificHandleType ">"
SpecificHandleType = "message_pipe"
                   | "shared_buffer"
                   | "data_pipe_consumer"
                   | "data_pipe_producer"
                   | "platform"
Array = "array" "<" TypeSpec ">"
FixedArray = "array" "<" TypeSpec "," IntConstDec ">"
Map = "map" "<" Identifier "," TypeSpec ">"
InterfaceRequest = Identifier "&" | "associated" Identifier "&"

Ordinal = <empty> | OrdinalValue

Default = <empty> | "=" Constant

Enum = AttributeSection "enum" Name "{" NonEmptyEnumValueList "}" ";"
     | AttributeSection "enum" Name "{" NonEmptyEnumValueList "," "}" ";"
NonEmptyEnumValueList = EnumValue | NonEmptyEnumValueList "," EnumValue
EnumValue = AttributeSection Name
          | AttributeSection Name "=" Integer
          | AttributeSection Name "=" Identifier

; Note: `feature` is a weak keyword and can appear as, say, a struct field name.
Feature = AttributeSection "feature" Name "{" FeatureBody "}" ";"
       | AttributeSection "feature" Name ";"
FeatureBody = <empty>
           | FeatureBody FeatureField
FeatureField = AttributeSection TypeSpec Name Default ";"

Const = "const" TypeSpec Name "=" Constant ";"

Constant = Literal | Identifier ";"

Identifier = Name | Name "." Identifier

Literal = Integer | Float | "true" | "false" | "default" | StringLiteral

Integer = IntConst | "+" IntConst | "-" IntConst
IntConst = IntConstDec | IntConstHex

Float = FloatConst | "+" FloatConst | "-" FloatConst

; The rules below are for tokens matched strictly according to the given regexes

Identifier = /[a-zA-Z_][0-9a-zA-Z_]*/
IntConstDec = /0|(1-9[0-9]*)/
IntConstHex = /0[xX][0-9a-fA-F]+/
OrdinalValue = /@(0|(1-9[0-9]*))/
FloatConst = ... # Imagine it's close enough to C-style float syntax.
StringLiteral = ... # Imagine it's close enough to C-style string literals, including escapes.
```

## Additional Documentation

[Mojom Message Format](https://docs.google.com/document/d/13pv9cFh5YKuBggDBQ1-AL8VReF-IYpFOFpRfvWFrwio/edit)
:    Describes the wire format used by Mojo bindings interfaces over message
     pipes.

[Input Format of Mojom Message Validation Tests](https://docs.google.com/document/d/1-y-2IYctyX2NPaLxJjpJfzVNWCC2SR2MJAD9MpIytHQ/edit)
:    Describes a text format used to facilitate bindings message validation
     tests.
