path: root/utils/ipc/mojo/public/tools/bindings/README.md

# 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 maping 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 {
  YES,
  NO
};

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.YES;

  // 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 {
  SALES = 0,
  DEV,
};

struct Employee {
  enum Type {
    FULL_TIME,
    PART_TIME,
  };

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

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 {
    FULL_TIME,
    PART_TIME,
  };

  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).

### 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 that
  will be used if an `Extensible` enumeration does not deserialize to a known
  value on the receiver side, i.e. the sender is using a newer version of the
  enum. This allows unknown values to be mapped to a well-defined value that can
  be appropriately handled.

* **`[Extensible]`**:
  The `Extensible` attribute may be specified for any enum definition. 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.

  Note: in the future, an `Extensible` enumeration will require that a `Default`
  enumerator value also be specified.

* **`[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.

* **`[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`.

* **`[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.

## 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 {
  TRUE = 0,
  FALSE = 1,
  FILE_NOT_FOUND = 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. Specifically, all parts of Chrome are updated
atomically today and there is not yet any possibility of any two Chrome
processes communicating with two different versions of any given Mojom
interface.
***

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). 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 or *N*-method interface, the set of explicitly
    assigned ordinal values must be limited to the range *[0, N-1]*. Interfaces
    should include placeholder methods to fill the ordinal positions of removed
    methods (for example "Unused_Message_7@7()" or "RemovedMessage@42()", 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.

    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 {
  AddEmployee(Employee employee) => (bool success);
  QueryEmployee(uint64 id) => (Employee? employee);
};

// New version:
interface HumanResourceDatabase {
  AddEmployee(Employee employee) => (bool success);

  QueryEmployee(uint64 id, [MinVersion=1] bool retrieve_finger_print)
      => (Employee? employee,
          [MinVersion=1] array<uint8>? finger_print);

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

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. However, if the method
call itself 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 {
  SALES,
  DEV,
};
```

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

``` cpp
[Extensible]
enum Department {
  SALES,
  DEV,
  [MinVersion=1] RESEARCH,
};
```

*** 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 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 "(" ParamterList ")" 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

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.