Mojo C++ Platform API

Mojo C++ Platform API

This document is a subset of the Mojo documentation.

Contents

• Overview
• Platform Handles
• Platform Channels
• Named Platform Channels

Overview

The Mojo C++ Platform API provides a lightweight set of abstractions around stable platform primitive APIs like UNIX domain sockets and Windows named pipes. This API is primarily useful in conjunction with Mojo Invitations to bootstrap Mojo IPC between two processes.

Platform Handles

The PlatformHandle type provides a move-only wrapper around an owned, platform-specific primitive handle types. The type of primitive it holds can be any of the following:

• Windows HANDLE (Windows only)
• Fuchsia zx_handle_t (Fuchsia only)
• Mach send right (OSX only)
• POSIX file descriptor (POSIX systems only)

See the header for more details.

Platform Channels

The PlatformChannel type abstracts a platform-specific IPC FIFO primitive primarily for use with the Mojo Invitations API. Constructing a PlatformChannel instance creates the underlying system primitive with two transferrable PlatformHandle instances, each thinly wrapped as a PlatformChannelEndpoint for additional type-safety. One endpoint is designated as local and the other remote, the intention being that the remote endpoint will be transferred to another process in the system.

See the header for more details. See the Invitations documentation for an example of using PlatformChannel with an invitation to bootstrap IPC between a process and one of its newly launched child processes.

Named Platform Channels

For cases where it is not feasible to transfer a PlatformHandle from one running process to another, the Platform API also provides NamedPlatformChannel, which abstracts a named system resource that can facilitate communication similarly to PlatformChannel.

A NamedPlatformChannel upon construction will begin listening on a platform-specific primitive (a named pipe server on Windows, a domain socket server on POSIX, etc.). The globally reachable name of the server (e.g. the socket path) can be specified at construction time via NamedPlatformChannel::Options::server_name, but if no name is given, a suitably random one is generated and used.

// In one process
mojo::NamedPlatformChannel::Options options;
mojo::NamedPlatformChannel named_channel(options);
OutgoingInvitation::Send(std::move(invitation),
                         named_channel.TakeServerEndpoint());
SendServerNameToRemoteProcessSomehow(named_channel.GetServerName());

// In the other process
void OnGotServerName(const mojo::NamedPlatformChannel::ServerName& name) {
  // Connect to the server.
  mojo::PlatformChannelEndpoint endpoint =
      mojo::NamedPlatformChannel::ConnectToServer(name);

  // Proceed normally with invitation acceptance.
  auto invitation = mojo::IncomingInvitation::Accept(std::move(endpoint));
  // ...
}

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