Mojo C++ System API

This document is a subset of the Mojo documentation.


The Mojo C++ System API provides a convenient set of helper classes and functions for working with Mojo primitives. Unlike the low-level C API (upon which this is built) this library takes advantage of C++ language features and common STL and //base types to provide a slightly more idiomatic interface to the Mojo system layer, making it generally easier to use.

This document provides a brief guide to API usage with example code snippets. For a detailed API references please consult the headers in //mojo/public/cpp/system.

Note that all API symbols referenced in this document are implicitly in the top-level mojo namespace.

Scoped, Typed Handles

All types of Mojo handles in the C API are simply opaque, integral MojoHandle values. The C++ API has more strongly typed wrappers defined for different handle types: MessagePipeHandle, SharedBufferHandle, DataPipeConsumerHandle, DataPipeProducerHandle, and WatcherHandle.

Each of these also has a corresponding, move-only, scoped type for safer usage: ScopedMessagePipeHandle, ScopedSharedBufferHandle, and so on. When a scoped handle type is destroyed, its handle is automatically closed via MojoClose. When working with raw handles you should always prefer to use one of the scoped types for ownership.

Similar to std::unique_ptr, scoped handle types expose a get() method to get at the underlying unscoped handle type as well as the -> operator to dereference the scoper and make calls directly on the underlying handle type.

Message Pipes

There are two ways to create a new message pipe using the C++ API. You may construct a MessagePipe object:

mojo::MessagePipe pipe;

// NOTE: Because pipes are bi-directional there is no implicit semantic
// difference between |handle0| or |handle1| here. They're just two ends of a
// pipe. The choice to treat one as a "client" and one as a "server" is entirely
// a the API user's decision.
mojo::ScopedMessagePipeHandle client = std::move(pipe.handle0);
mojo::ScopedMessagePipeHandle server = std::move(pipe.handle1);

or you may call CreateMessagePipe:

mojo::ScopedMessagePipeHandle client;
mojo::ScopedMessagePipeHandle server;
mojo::CreateMessagePipe(nullptr, &client, &server);

There are also some helper functions for constructing message objects and reading/writing them on pipes using the library's more strongly-typed C++ handles:

mojo::ScopedMessageHandle message;
mojo::AllocMessage(6, nullptr, 0, MOJO_ALLOC_MESSAGE_FLAG_NONE, &message);

void *buffer;
mojo::GetMessageBuffer(message.get(), &buffer);

const std::string kMessage = "hello";
std::copy(kMessage.begin(), kMessage.end(), static_cast<char*>(buffer));

mojo::WriteMessageNew(client.get(), std::move(message),

// Some time later...

mojo::ScopedMessageHandle received_message;
uint32_t num_bytes;
mojo::ReadMessageNew(server.get(), &received_message, &num_bytes, nullptr,
                     nullptr, MOJO_READ_MESSAGE_FLAG_NONE);

See message_pipe.h for detailed C++ message pipe API documentation.

Data Pipes

Similar to Message Pipes, the C++ library has some simple helpers for more strongly-typed data pipe usage:

mojo::DataPipe pipe;
mojo::ScopedDataPipeProducerHandle producer = std::move(pipe.producer);
mojo::ScopedDataPipeConsumerHandle consumer = std::move(pipe.consumer);

// Or alternatively:
mojo::ScopedDataPipeProducerHandle producer;
mojo::ScopedDataPipeConsumerHandle consumer;
mojo::CreateDataPipe(null, &producer, &consumer);

C++ helpers which correspond directly to the Data Pipe C API for immediate and two-phase I/O are provided as well. For example:

uint32_t num_bytes = 7;
producer.WriteData("hihihi", &num_bytes, MOJO_WRITE_DATA_FLAG_NONE);

// Some time later...

char buffer[64];
uint32_t num_bytes = 64;
consumer.ReadData(buffer, &num_bytes, MOJO_READ_DATA_FLAG_NONE);

See data_pipe.h for detailed C++ data pipe API documentation.

Shared Buffers

A new shared buffers can be allocated like so:

mojo::ScopedSharedBufferHandle buffer =

This new handle can be cloned arbitrarily many times by using the underlying handle's Clone method:

mojo::ScopedSharedBufferHandle another_handle = buffer->Clone();
mojo::ScopedSharedBufferHandle read_only_handle =

And finally the library also provides a scoper for mapping the shared buffer's memory:

mojo::ScopedSharedBufferMapping mapping = buffer->Map(64);
static_cast<int*>(mapping.get()) = 42;

mojo::ScopedSharedBufferMapping another_mapping = buffer->MapAtOffset(64, 4);
static_cast<int*>(mapping.get()) = 43;

When mapping and another_mapping are destroyed, they automatically unmap their respective memory regions.

See buffer.h for detailed C++ shared buffer API documentation.

Native Platform Handles (File Descriptors, Windows Handles, etc.)

The C++ library provides several helpers for wrapping system handle types. These are specifically useful when working with a few //base types, namely base::PlatformFile and base::SharedMemoryHandle. See platform_handle.h for detailed C++ platform handle API documentation.

Signals & Watchers

For an introduction to the concepts of handle signals and watchers, check out the C API's documentation on Signals & Watchers.

Querying Signals

Any C++ handle type's last known signaling state can be queried by calling the QuerySignalsState method on the handle:

mojo::MessagePipe message_pipe;
mojo::DataPipe data_pipe;
mojo::HandleSignalsState a = message_pipe.handle0->QuerySignalsState();
mojo::HandleSignalsState b = data_pipe.consumer->QuerySignalsState();

The HandleSignalsState is a thin wrapper interface around the C API's MojoHandleSignalsState structure with convenient accessors for testing the signal bitmasks. Whereas when using the C API you might write:

struct MojoHandleSignalsState state;
MojoQueryHandleSignalsState(handle0, &state);
if (state.satisfied_signals & MOJO_HANDLE_SIGNAL_READABLE) {
  // ...

the C++ API equivalent would be:

if (message_pipe.handle0->QuerySignalsState().readable()) {
  // ...

Watching Handles

The mojo::SimpleWatcher class serves as a convenient helper for using the low-level watcher API to watch a handle for signaling state changes. A SimpleWatcher is bound to a single sequence and always dispatches its notifications on a base::SequencedTaskRunner.

SimpleWatcher has two possible modes of operation, selected at construction time by the mojo::SimpleWatcher::ArmingPolicy enum:

  • MANUAL mode requires the user to manually call Arm and/or ArmOrNotify before any notifications will fire regarding the state of the watched handle. Every time the notification callback is run, the SimpleWatcher must be rearmed again before the next one can fire. See Arming a Watcher and the documentation in SimpleWatcher's header.

  • AUTOMATIC mode ensures that the SimpleWatcher always either is armed or has a pending notification task queued for execution.

AUTOMATIC mode is more convenient but can result in redundant notification tasks, especially if the provided callback does not make a strong effort to return the watched handle to an uninteresting signaling state (by e.g., reading all its available messages when notified of readability.)

Example usage:

class PipeReader {
  PipeReader(mojo::ScopedMessagePipeHandle pipe)
      : pipe_(std::move(pipe)),
        watcher_(mojo::SimpleWatcher::ArmingPolicy::AUTOMATIC) {
    // NOTE: base::Unretained is safe because the callback can never be run
    // after SimpleWatcher destruction.
    watcher_.Watch(pipe_.get(), MOJO_HANDLE_SIGNAL_READABLE,
                   base::Bind(&PipeReader::OnReadable, base::Unretained(this)));

  ~PipeReader() {}

  void OnReadable(MojoResult result) {
    while (result == MOJO_RESULT_OK) {
      mojo::ScopedMessageHandle message;
      uint32_t num_bytes;
      result = mojo::ReadMessageNew(pipe_.get(), &message, &num_bytes, nullptr,
                                    nullptr, MOJO_READ_MESSAGE_FLAG_NONE);
      DCHECK_EQ(result, MOJO_RESULT_OK);

  mojo::ScopedMessagePipeHandle pipe_;
  mojo::SimpleWatcher watcher_;
  std::vector<mojo::ScopedMessageHandle> messages_;

mojo::MessagePipe pipe;
PipeReader reader(std::move(pipe.handle0));

// Written messages will asynchronously end up in |reader.messages_|.

Synchronous Waiting

The C++ System API defines some utilities to block a calling sequence while waiting for one or more handles to change signaling state in an interesting way. These threads combine usage of the low-level Watcher API with common synchronization primitives (namely base::WaitableEvent.)

While these API features should be used sparingly, they are sometimes necessary.

See the documentation in wait.h and wait_set.h for a more detailed API reference.

Waiting On a Single Handle

The mojo::Wait function simply blocks the calling sequence until a given signal mask is either partially satisfied or fully unsatisfiable on a given handle.

mojo::MessagePipe pipe;
mojo::WriteMessageRaw(pipe.handle0.get(), "hey", 3, nullptr, nullptr,
MojoResult result = mojo::Wait(pipe.handle1.get(), MOJO_HANDLE_SIGNAL_READABLE);

// Guaranteed to succeed because we know |handle1| is readable now.
mojo::ScopedMessageHandle message;
uint32_t num_bytes;
mojo::ReadMessageNew(pipe.handle1.get(), &num_bytes, nullptr, nullptr,

mojo::Wait is most typically useful in limited testing scenarios.

Waiting On Multiple Handles

mojo::WaitMany provides a simple API to wait on multiple handles simultaneously, returning when any handle's given signal mask is either partially satisfied or fully unsatisfiable.

mojo::MessagePipe a, b;
GoDoSomethingWithPipes(std:move(a.handle1), std::move(b.handle1));

mojo::MessagePipeHandle handles[2] = {a.handle0.get(), b.handle0.get()};
MojoHandleSignals signals[2] = {MOJO_HANDLE_SIGNAL_READABLE,
size_t ready_index;
MojoResult result = mojo::WaitMany(handles, signals, 2, &ready_index);
if (ready_index == 0) {
  // a.handle0 was ready.
} else {
  // b.handle0 was ready.

Similar to mojo::Wait, mojo::WaitMany is primarily useful in testing. When waiting on multiple handles in production code, you should almost always instead use a more efficient and more flexible mojo::WaitSet as described in the next section.

Waiting On Handles and Events Simultaneously

Typically when waiting on one or more handles to signal, the set of handles and conditions being waited upon do not change much between consecutive blocking waits. It's also often useful to be able to interrupt the blocking operation as efficiently as possible.

mojo::WaitSet is designed with these conditions in mind. A WaitSet maintains a persistent set of (not-owned) Mojo handles and base::WaitableEvents, which may be explicitly added to or removed from the set at any time.

The WaitSet may be waited upon repeatedly, each time blocking the calling sequence until either one of the handles attains an interesting signaling state or one of the events is signaled. For example let's suppose we want to wait up to 5 seconds for either one of two handles to become readable:

base::WaitableEvent timeout_event(
mojo::MessagePipe a, b;

GoDoStuffWithPipes(std::move(a.handle1), std::move(b.handle1));

mojo::WaitSet wait_set;
wait_set.AddHandle(a.handle0.get(), MOJO_HANDLE_SIGNAL_READABLE);
wait_set.AddHandle(b.handle0.get(), MOJO_HANDLE_SIGNAL_READABLE);

// Ensure the Wait() lasts no more than 5 seconds.
    base::Bind([](base::WaitableEvent* e) { e->Signal(); }, &timeout_event);

base::WaitableEvent* ready_event = nullptr;
size_t num_ready_handles = 1;
mojo::Handle ready_handle;
MojoResult ready_result;
wait_set.Wait(&ready_event, &num_ready_handles, &ready_handle, &ready_result);

// The apex of thread-safety.

if (ready_event) {
  // The event signaled...

if (num_ready_handles > 0) {
  // At least one of the handles signaled...
  // NOTE: This and the above condition are not mutually exclusive. If handle
  // signaling races with timeout, both things might be true.