Inter-process Communication (IPC)
Inter-process Communication (IPC)
OverviewChromium has a multi-process architecture which means that we have a lot of processes communicating with each other. Our main inter-process communication primitive is the named pipe. On Linux & OS X, we use a socketpair(). A named pipe is allocated for each renderer process for communication with the browser process. The pipes are used in asynchronous mode to ensure that neither end is blocked waiting for the other. For advice on how to write safe IPC endpoints, please see Security Tips for IPC. IPC in the browserWithin the browser, communication with the renderers is done in a separate I/O thread. Messages to and from the views then have to be proxied over to the main thread using a IPC in the rendererEach renderer also has a thread that manages communication (in this case, the main thread), with the rendering and most processing happening on another thread (see the diagram in multi-process architecture). Most messages are sent from the browser to the WebKit thread through the main renderer thread and vice-versa. This extra thread is to support synchronous renderer-to-browser messages (see "Synchronous messages" below). MessagesTypes of messagesWe have two primary types of messages: "routed" and "control." Control messages are handled by the class that created the pipe. Sometimes that class will allow others to received message by having a MessageRouter object that other listeners can register with and received "routed" messages sent with their unique (per pipe) id. For example, when rendering, control messages are not specific to a given view and will be handled by the Routed messages have historically been used to get messages to a specific RenderViewHost. However, technically any class can receive routed messages by using RenderProcessHost::GetNextRoutingID and registering itself with RenderProcessHost::AddRoute. Currently both RenderViewHost and RenderFrameHost instances have their own routing IDs. Independent of the message type is whether the message is sent from the browser to the renderer, or from the renderer to the browser. Messages related to a document's frame sent from the browser to the renderer are called Plugins also have separate processes. Like the render messages, there are The same organization applies for other groups of messages exchanged between the browser and the renderer, as for View and ViewHost labeled messages exchanged between RenderViewHost and RenderView, defined in view_messages.h. Declaring messagesSpecial macros are used to declare messages. To declare a routed message from the renderer to the browser (e.g. a FrameHost message specific to a frame) that contains a URL and an integer as an argument, write: IPC_MESSAGE_ROUTED2(FrameHostMsg_MyMessage, GURL, int) To declare a control message from the browser to the renderer (e.g. a Frame message not specific to a frame) that contains no parameters, write: IPC_MESSAGE_CONTROL0(FrameMsg_MyMessage) Pickling valuesParameters are serialized and de-serialized to message bodies using the Sometimes, a message has too many values to be reasonably put in a message. In this case, we define a separate structure to hold the values. For example, for the Sending messagesYou send messages through "channels" (see below). In the browser, the Messages are sent by pointer and will be deleted by the IPC layer after they are dispatched. Therefore, once you can find the appropriate Send(new ViewMsg_StopFinding(routing_id_)); Notice that you must specify the routing ID in order for the message to be routed to the correct View/ViewHost on the receiving end. Both the Handling messagesMessages are handled by implementing the MyClass::OnMessageReceived(const IPC::Message& message) { You can also use Other macros:
IPC_MESSAGE_FORWARD(ViewHostMsg_MyMessage, some_object_pointer, SomeObject::OnMyMessage)
IPC_MESSAGE_HANDLER_GENERIC(ViewHostMsg_MyMessage, printf("Hello, world, I got the message.")) Security considerationsSecurity bugs in IPC can have nasty consequences (file theft, sandbox escapes, remote code execution). Check out our security for IPC document for tips on how to avoid common pitfalls. Channels
Channels are not thread safe. We often want to send messages using a channel on another thread. For example, when the UI thread wants to send a message, it must go through the I/O thread. For this, we use a Synchronous messagesSome messages should be synchronous from the renderer's perspective. This happens mostly when there is a WebKit call to us that is supposed to return something, but that we must do in the browser. Examples of this type of messages are spell-checking and getting the cookies for JavaScript. Synchronous browser-to-renderer IPC is disallowed to prevent blocking the user-interface on a potentially flaky renderer. Danger: Do not handle any synchronous messages in the UI thread! You must handle them only in the I/O thread. Otherwise, the application might deadlock because plug-ins require synchronous painting from the UI thread, and these will be blocked when the renderer is waiting for synchronous messages from the browser. Declaring synchronous messagesSynchronous messages are declared using the IPC_SYNC_MESSAGE_CONTROL2_1(SomeMessage, // Message name Likewise, you can also have messages that are routed to the view in which case you would replace "control" with "routed" to get Issuing synchronous messagesWhen the WebKit thread issues a synchronous IPC request, the request object (derived from While the WebKit thread is waiting for the synchronous reply, the main thread is still receiving messages from the browser process. These messages will be added to the queue of the WebKit thread for processing when it wakes up. When the synchronous message reply is received, the thread will be un-blocked. Note that this means that the synchronous message reply can be processed out-of-order. Synchronous messages are sent the same way normal messages are, with output parameters being given to the constructor. For example: const GURL input_param("http://www.google.com/"); Handling synchronous messagesSynchronous messages and asynchronous messages use the same IPC_MESSAGE_HANDLER(MyMessage, OnMyMessage) to the void RenderProcessHost::OnMyMessage(GURL input_param, std::string* result) { Converting message type to a message nameIf you get a crash and you have the message type you can convert this to a message name. The message type will be 32-bit value, the high 16-bits are the class and the low 16-bits are the id. The class is based on the enums in ipc/ipc_message_start.h, the id is based on the line number in the file that defines the message. This means that you need to get the exact revision of Chromium in order to accurately get the message name. Example of this in 554011 was 0x1c0098 at Chromium revision ad0950c1ac32ef02b0b0133ebac2a0fa4771cf20. That's class 0x1c which is line 40 which matches ChildProcessMsgStart. ChildProcessMsgStart messages are in content/common/child_process_messages.h and the IPC will be on line 0x98 or line 152 which is ChildProcessHostMsg_ChildHistogramData. This technique is particularly useful if you are dealing with crashes caused by content::RenderProcessHostImpl::OnBadMessageReceived |
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