Mojom Interface Definition Language (IDL)

This document is a subset of the Mojo documentation.


Mojom is the IDL for Mojo interfaces. Given a .mojom file, the bindings generator 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 {

This defines a single interface named Frobinator in a module 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/


mojom("mojom") {
  sources = [

and then build this target:

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

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


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.

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.

boolBoolean type (true or false.)
int8, uint8Signed or unsigned 8-bit integer.
int16, uint16Signed or unsigned 16-bit integer.
int32, uint32Signed or unsigned 32-bit integer.
int64, uint64Signed or unsigned 64-bit integer.
float, double32- or 64-bit floating point number.
stringUTF-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.
handleGeneric 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
pending_associated_receiver<InterfaceType>A pending associated receiver. See Associated Interfaces
T?An optional (nullable) value. Primitive numeric types (integers, floats, booleans, and enums) used to be non-nullable, but they are now nullable. (see


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 {

Generated C++ bindings will define a class interface MoneyGenerator in the business::stuff namespace, while Java bindings will define an interface MoneyGenerator in the 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.


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

struct StringPair {
  string first;
  string second;

Struct fields may be comprised of any of the types listed above in the Primitive Types section.

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

struct Request {
  int32 id = -1;
  string details;

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

struct StringPair {
  string first;
  string second;

enum AnEnum {

interface SampleInterface {

struct AllTheThings {
  // All the primitive numeric types may be nullable.
  bool boolean_value;
  bool? maybe_a_bool;
  int8 signed_8bit_value = 42;
  int8? maybe_signed_8bit_value = 42;
  uint8? maybe_unsigned_8bit_value;
  int16? maybe_signed_16bit_value;
  uint16? maybe_unsigned_16bit_value;
  int32? maybe_signed_32bit_value;
  uint32? maybe_unsigned_32bit_value;
  int64? maybe_signed_64bit_value;
  uint64? maybe_unsigned_64bit_value;
  float? maybe_float_value_32bit;
  double? maybe_float_value_64bit;
  AnEnum? maybe_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 unless it's a primitive numeric type.
  array<AllTheThings?> more_maybe_things;
  // array<int32?> no_primitive_in_array; This doesn't work.

  // 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. Please note that nullable primitive numeric types
  // cannot be the key or value. Maps themselves may 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;
  // map<string?, int32?>; This doesn't work.

  // 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.


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 fields. For example:

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.

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,

struct Employee {
  enum Type {

  Type type;
  // ...

C++ constant-style enum value names are preferred as specified in the Google C++ Style Guide.

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.


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 {

  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.


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;

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.
  CallElevator(int floor);

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


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.

  // 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) is also a valid request or response argument type. The type notation is the same for both.


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 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 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 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 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={}) 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

  • [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 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.

  • [UnlimitedSize] The UnlimitedSize attribute is used to tag methods that are expected to have large payload size exceeding Mojo's predefined threshold. Without this tag, those methods would trigger a DumpWithoutCrashing call. Instead of using UnlimitedSize, consider refactoring to avoid such message contents, for example by batching calls or leveraging shared memory where feasible.

  • [EstimateSize]: The EstimateSize attribute can be used to tag methods with large payload sizes that tend to cause frequent reallocations during serialization. This attribute instructs Mojo to track the history of recent allocation sizes for the method. With this information, Mojo can make better decisions about subsequent allocations, rather than gradually expanding the serialization buffer. Since the tracking adds a small amount of runtime overhead, use the EstimateSize tag selectively – only for frequently-called methods with large payloads that may trigger many allocations.

  • [DispatchDebugAlias]: The DispatchDebugAlias attribute can be used on an interface to opt into having every dispatched message retain an aliased copy of the message ID on the stack for the duration of the dispatch. This can aid in crash debugging if other factors such as inlining or code folding end up obscuring the message information. This generates extra code, so it is not the default behavior.

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:

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, Java Connection Errors, or JavaScript 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.) 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 declared in Mojom are automatically validated against the range of legal values. For example if a Mojom declares the enum:

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: It's possible to avoid this type of validation error by explicitly marking an enum as Extensible if you anticipate your enum being exchanged between two different versions of the binding interface. See 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 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 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.



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++ uses versioned mojo to send IPC to the Android container. Likewise, the Lacros 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 to indicate from which version a struct field is introduced. Assume you have the following struct:

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:

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

NOTE: Mojo object or handle types added with a MinVersion MUST be optional (nullable). On the other hand, primitive numeric types (including enums) added with a MinVersion are allowed to be either nullable or non-nullable.

See Primitive Types for details on nullable values.

See Ensuring Backward Compatible Behavior for more details on choosing between nullable and non-nullable primitive numeric types.

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: 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:

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

Conversion between Different Versions

When a struct of version X is passed to a destination using version Y:

  • If X is older than Y, then all fields newer than version X are populated automatically: null for nullable types, and 0/false for primitive numeric types, including enums. See Ensuring Backward Compatible Behavior for more details on choosing between nullable and non-nullable primitive numeric types.
  • If X is newer than Y, then all fields newer than version Y are truncated.

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.

// 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);

When you pass the parameter list of a request or response method to a destination using a different version of an interface, the conversion rules of versioned structs also apply. Unrecognized fields from a newer version are silently discarded; missing fields from an older version are populated automatically with null/0/false.

NOTE: 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:

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

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

  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 and Java 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:

enum Department {

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

enum Department {
  [MinVersion=1] kResearch,
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 for details.

Renaming versioned structs

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

module asdf.mojom;

// Old version:
struct OldStruct {

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

Ensuring Backward Compatible Behavior

In addition to following versioning rules to ensure an interface is syntactically backward compatible, it is important to also ensure it is semantically backward compatible. When a client uses version X of a mojom definition to communicate with a service using a different version Y:

  • If X is newer than Y, the client will receive downgraded service as if it initiates the communication with version Y. If silently downgraded service is not desirable or not achievable (e.g., calling a method that doesn't exist at the service side), the client is responsible for querying service side version and act accordingly.
  • If X is older than Y, the service is responsible for behaving in the same way as an older service running version X, or report an error if the interface itself supports such error reporting.

Choosing between Nullable and Non-nullable Primitive Numeric Types

Primitive numeric types, including enums, are allowed to be either nullable or non-nullable when extending structs or method parameter lists. There are several tradeoffs to consider when choosing between the two:

  • Nullable numeric primitives: they can offer more semantic safety for new fields because it is more obvious that such fields are optional, and whether their values are set.
  • Non-nullable numeric primitives: The caveat is that they can be used only if auto-populated 0/false doesn't break backward compatibility. (See example below.) When they are used properly, however, there are some benefits: they are slightly more efficient (although that is usually negligible). And they can avoid additional null checks if value 0/false already represents the invalid state.
NOTE: A non-nullable enum‘s automatically populated value is distinct from the value used when an extensible enum is deserialised with an enumerator value that is not defined in the current enum definition (the enum’s [Default] enumerator value, if one exists).

If the consequences of auto-populated 0/false have not been thoroughly and carefully considered, prefer nullable numeric primitives.

Consider an example where a non-nullable numeric primitive breaks backward compatibility:

// Supports a third operand with non-nullable int32 in version 1.
Multiply(int32 operand1, int32 operand2, [MinVersion=1] int32 operand3)
    => (int64 result);

In this case, it is wrong to use non-nullable int32 for operand3, because when a client using version 0 calls a service implementing version 1, operand3 is automatically populated with value 0, the result will always be 0!

Consider an example where a non-nullable numeric primitive results in more intuitive code:

// Awesome encoding is only available from version >= 1.
CompressFile(string filename, [MinVersion=1] bool uses_awesome_encoding);

In the example above, using non-nullable bool for uses_awesome_encoding makes sense. Because when a client uses version 0 definition to call CompressFile() with a service implementing version 1, uses_awesome_encoding is automatically populated with false, which matches the version 0 behavior naturally and preserves backward compatibility.

As a comparison, if uses_awesome_encoding is defined as bool?, it is mapped to std::optional<bool>. The service needs to add additional null checks:

// Verbose and less intuitive code:
if (uses_awesome_encoding.value_or(false)) { ... }
// or:
if (uses_awesome_encoding && *uses_awesome_encoding) { ... }

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: 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_* 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 : Describes the wire format used by Mojo bindings interfaces over message pipes.

Input Format of Mojom Message Validation Tests : Describes a text format used to facilitate bindings message validation tests.