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553 lines
21 KiB
553 lines
21 KiB
DCOP: Desktop COmmunications Protocol
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Preston Brown <pbrown@kde.org>
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October 14, 1999
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Revised and extended by Matthias Ettrich <ettrich@kde.org>
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Mar 29, 2000
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Extended with DCOP Signals by Waldo Bastian <bastian@kde.org>
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Feb 19, 2001
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Motivation and Background:
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--------------------------
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The motivation behind building a protocol like DCOP is simple. For
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the past year, we have been attempting to enable interprocess
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communication between KDE applications. KDE already has an extremely
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simple IPC mechanism called KWMcom, which is (was!) used for communicating
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between the panel and the window manager for instance. It is about as
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simple as it gets, passing messages via X Atoms. For this reason it
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is limited in the size and complexity of the data that can be passed
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(X atoms must be small to remain efficient) and it also makes it so
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that X is required. CORBA was thought to be a more effective IPC/RPC
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solution. However, after a year of attempting to make heavy use of
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CORBA in KDE, we have realized that it is a bit slow and memory
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intensive for simple use. It also has no authentication available.
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What we really needed was an extremely simple protocol with basic
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authorization, along the lines of MIT-MAGIC-COOKIE, as used by X. It
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would not be able to do NEARLY what CORBA was able to do, but for the
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simple tasks required it would be sufficient. Some examples of such
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tasks might be an application sending a message to the panel saying,
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"I have started, stop displaying the 'application starting' wait
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state," or having a new application that starts query to see if any
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other applications of the same name are running. If they are, simply
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call a function on the remote application to create a new window,
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rather than starting a new process.
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Implementation:
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---------------
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DCOP is a simple IPC/RPC mechanism built to operate over sockets.
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Either unix domain sockets or tcp/ip sockets are supported. DCOP is
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built on top of the Inter Client Exchange (ICE) protocol, which comes
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standard as a part of X11R6 and later. It also depends on Qt, but
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beyond that it does not require any other libraries. Because of this,
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it is extremely lightweight, enabling it to be linked into all KDE
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applications with low overhead.
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Model:
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------
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The model is simple. Each application using DCOP is a client. They
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communicate to each other through a DCOP server, which functions like
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a traffic director, dispatching messages/calls to the proper
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destinations. All clients are peers of each other.
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Two types of actions are possible with DCOP: "send and forget"
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messages, which do not block, and "calls," which block waiting for
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some data to be returned.
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Any data that will be sent is serialized (marshalled, for you CORBA
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types) using the built-in QDataStream operators available in all of
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the Qt classes. This is fast and easy. In fact it's so little work
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that you can easily write the marshalling code by hand. In addition,
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there's a simple IDL-like compiler available (dcopidl and dcopidl2cpp)
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that generates stubs and skeletons for you. Using the dcopidl compiler
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has the additional benefit of type safety.
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This HOWTO describes the manual method first and covers the dcopidl
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compiler later.
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Establishing the Connection:
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----------------------------
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TDEApplication has gained a method called "TDEApplication::dcopClient()"
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which returns a pointer to a DCOPClient instance. The first time this
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method is called, the client class will be created. DCOPClients have
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unique identifiers attached to them which are based on what
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TDEApplication::name() returns. In fact, if there is only a single
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instance of the program running, the appId will be equal to
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TDEApplication::name().
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To actually enable DCOP communication to begin, you must use
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DCOPClient::attach(). This will attempt to attach to the DCOP server.
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If no server is found or there is any other type of error, attach()
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will return false. TDEApplication will catch a dcop signal and display an
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appropriate error message box in that case.
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After connecting with the server via DCOPClient::attach(), you need to
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register this appId with the server so it knows about you. Otherwise,
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you are communicating anonymously. Use the
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DCOPClient::registerAs(const QCString &name) to do so. In the simple
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case:
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/*
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* returns the appId that is actually registered, which _may_ be
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* different from what you passed
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*/
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appId = client->registerAs(kApp->name());
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If you never retrieve the DCOPClient pointer from TDEApplication, the
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object will not be created and thus there will be no memory overhead.
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You may also detach from the server by calling DCOPClient::detach().
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If you wish to attach again you will need to re-register as well. If
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you only wish to change the ID under which you are registered, simply
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call DCOPClient::registerAs() with the new name.
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KUniqueApplication automatically registers itself to DCOP. If you
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are using KUniqueApplication you should not attach or register
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yourself, this is already done. The appId is by definition
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equal to kapp->name(). You can retrieve the registered DCOP client
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by calling kapp->dcopClient().
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Sending Data to a Remote Application:
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-------------------------------------
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To actually communicate, you have one of two choices. You may either
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call the "send" or the "call" method. Both methods require three
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identification parameters: an application identifier, a remote object,
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a remote function. Sending is asynchronous (i.e. it returns immediately)
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and may or may not result in your own application being sent a message at
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some point in the future. Then "send" requires one and "call" requires
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two data parameters.
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The remote object must be specified as an object hierarchy. That is,
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if the toplevel object is called "fooObject" and has the child
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"barObject", you would reference this object as "fooObject/barObject".
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Functions must be described by a full function signature. If the
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remote function is called "doIt", and it takes an int, it would be
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described as "doIt(int)". Please note that the return type is not
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specified here, as it is not part of the function signature (or at
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least the C++ understanding of a function signature). You will get
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the return type of a function back as an extra parameter to
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DCOPClient::call(). See the section on call() for more details.
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In order to actually get the data to the remote client, it must be
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"serialized" via a QDataStream operating on a QByteArray. This is how
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the data parameter is "built". A few examples will make clear how this
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works.
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Say you want to call "doIt" as described above, and not block (or wait
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for a response). You will not receive the return value of the remotely
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called function, but you will not hang while the RPC is processed either.
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The return value of send() indicates whether DCOP communication succeeded
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or not.
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QByteArray data;
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QDataStream arg(data, IO_WriteOnly);
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arg << 5;
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if (!client->send("someAppId", "fooObject/barObject", "doIt(int)",
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data))
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tqDebug("there was some error using DCOP.");
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OK, now let's say we wanted to get the data back from the remotely
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called function. You have to execute a call() instead of a send().
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The returned value will then be available in the data parameter "reply".
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The actual return value of call() is still whether or not DCOP
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communication was successful.
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QByteArray data, replyData;
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QCString replyType;
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QDataStream arg(data, IO_WriteOnly);
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arg << 5;
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if (!client->call("someAppId", "fooObject/barObject", "doIt(int)",
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data, replyType, replyData))
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tqDebug("there was some error using DCOP.");
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else {
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QDataStream reply(replyData, IO_ReadOnly);
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if (replyType == "QString") {
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QString result;
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reply >> result;
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print("the result is: %s",result.latin1());
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} else
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tqDebug("doIt returned an unexpected type of reply!");
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}
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N.B.: You cannot call() a method belonging to an application which has
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registered with an unique numeric id appended to its textual name (see
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dcopclient.h for more info). In this case, DCOP would not know which
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application it should connect with to call the method. This is not an issue
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with send(), as you can broadcast to all applications that have registered
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with appname-<numeric_id> by using a wildcard (e.g. 'konsole-*'), which
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will send your signal to all applications called 'konsole'.
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Receiving Data via DCOP:
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------------------------
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Currently the only real way to receive data from DCOP is to multiply
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inherit from the normal class that you are inheriting (usually some
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sort of QWidget subclass or QObject) as well as the DCOPObject class.
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DCOPObject provides one very important method: DCOPObject::process().
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This is a pure virtual method that you must implement in order to
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process DCOP messages that you receive. It takes a function
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signature, QByteArray of parameters, and a reference to a QByteArray
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for the reply data that you must fill in.
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Think of DCOPObject::process() as a sort of dispatch agent. In the
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future, there will probably be a precompiler for your sources to write
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this method for you. However, until that point you need to examine
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the incoming function signature and take action accordingly. Here is
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an example implementation.
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bool BarObject::process(const QCString &fun, const QByteArray &data,
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QCString &replyType, QByteArray &replyData)
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{
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if (fun == "doIt(int)") {
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QDataStream arg(data, IO_ReadOnly);
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int i; // parameter
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arg >> i;
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QString result = self->doIt (i);
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QDataStream reply(replyData, IO_WriteOnly);
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reply << result;
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replyType = "QString";
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return true;
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} else {
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tqDebug("unknown function call to BarObject::process()");
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return false;
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}
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}
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Receiving Calls and processing them:
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------------------------------------
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If your applications is able to process incoming function calls
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right away the above code is all you need. When your application
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needs to do more complex tasks you might want to do the processing
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out of 'process' function call and send the result back later when
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it becomes available.
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For this you can ask your DCOPClient for a transactionId. You can
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then return from the 'process' function and when the result is
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available finish the transaction. In the mean time your application
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can receive incoming DCOP function calls from other clients.
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Such code could like this:
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bool BarObject::process(const QCString &fun, const QByteArray &data,
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QCString &, QByteArray &)
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{
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if (fun == "doIt(int)") {
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QDataStream arg(data, IO_ReadOnly);
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int i; // parameter
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arg >> i;
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QString result = self->doIt(i);
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DCOPClientTransaction *myTransaction;
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myTransaction = kapp->dcopClient()->beginTransaction();
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// start processing...
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// Calls slotProcessingDone when finished.
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startProcessing( myTransaction, i);
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return true;
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} else {
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tqDebug("unknown function call to BarObject::process()");
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return false;
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}
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}
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slotProcessingDone(DCOPClientTransaction *myTransaction, const QString &result)
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{
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QCString replyType = "QString";
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QByteArray replyData;
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QDataStream reply(replyData, IO_WriteOnly);
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reply << result;
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kapp->dcopClient()->endTransaction( myTransaction, replyType, replyData );
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}
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DCOP Signals
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------------
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Sometimes a component wants to send notifications via DCOP to other
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components but does not know which components will be interested in these
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notifications. One could use a broadcast in such a case but this is a very
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crude method. For a more sophisticated method DCOP signals have been invented.
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DCOP signals are very similair to Qt signals, there are some differences
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though. A DCOP signal can be connected to a DCOP function. Whenever the DCOP
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signal gets emitted, the DCOP functions to which the signal is connected are
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being called. DCOP signals are, just like Qt signals, one way. They do not
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provide a return value.
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A DCOP signal originates from a DCOP Object/DCOP Client combination (sender).
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It can be connected to a function of another DCOP Object/DCOP Client
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combination (receiver).
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There are two major differences between connections of Qt signals and
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connections of DCOP signals. In DCOP, unlike Qt, a signal connections can
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have an anonymous sender and, unlike Qt, a DCOP signal connection can be
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non-volatile.
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With DCOP one can connect a signal without specifying the sending DCOP Object
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or DCOP Client. In that case signals from any DCOP Object and/or DCOP Client
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will be delivered. This allows the specification of certain events without
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tying oneself to a certain object that implementes the events.
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Another DCOP feature are so called non-volatile connections. With Qt signal
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connections, the connection gets deleted when either sender or receiver of
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the signal gets deleted. A volatile DCOP signal connection will behave the
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same. However, a non-volatile DCOP signal connection will not get deleted
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when the sending object gets deleted. Once a new object gets created with
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the same name as the original sending object, the connection will be restored.
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There is no difference between the two when the receiving object gets deleted,
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in that case the signal connection will always be deleted.
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A receiver can create a non-volatile connection while the sender doesn't (yet)
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exist. An anonymous DCOP connection should always be non-volatile.
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The following example shows how TDELauncher emits a signal whenever it notices
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that an application that was started via TDELauncher terminates.
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QByteArray params;
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QDataStream stream(params, IO_WriteOnly);
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stream << pid;
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kapp->dcopClient()->emitDCOPSignal("clientDied(pid_t)", params);
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The task manager of the TDE panel connects to this signal. It uses an
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anonymous connection (it doesn't require that the signal is being emitted
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by TDELauncher) that is non-volatile:
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connectDCOPSignal(0, 0, "clientDied(pid_t)", "clientDied(pid_t)", false);
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It connects the clientDied(pid_t) signal to its own clientDied(pid_t) DCOP
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function. In this case the signal and the function to call have the same name.
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This isn't needed as long as the arguments of both signal and receiving function
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match. The receiving function may ignore one or more of the trailing arguments
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of the signal. E.g. it is allowed to connect the clientDied(pid_t) signal to
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a clientDied(void) DCOP function.
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Using the dcopidl compiler
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---------------------
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dcopidl makes setting up a DCOP server easy. Instead of having to implement
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the process() method and unmarshalling (retrieving from QByteArray) parameters
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manually, you can let dcopidl create the necessary code on your behalf.
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This also allows you to describe the interface for your class in a
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single, separate header file.
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Writing an IDL file is very similar to writing a normal C++ header. An
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exception is the keyword 'ASYNC'. It indicates that a call to this
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function shall be processed asynchronously. For the C++ compiler, it
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expands to 'void'.
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Example:
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#ifndef MY_INTERFACE_H
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#define MY_INTERFACE_H
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#include <dcopobject.h>
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class MyInterface : virtual public DCOPObject
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{
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K_DCOP
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k_dcop:
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virtual ASYNC myAsynchronousMethod(QString someParameter) = 0;
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virtual QRect mySynchronousMethod() = 0;
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};
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#endif
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As you can see, you're essentially declaring an abstract base class, which
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virtually inherits from DCOPObject.
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If you're using the standard KDE build scripts, then you can simply
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add this file (which you would call MyInterface.h) to your sources
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directory. Then you edit your Makefile.am, adding 'MyInterface.skel'
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to your SOURCES list and MyInterface.h to include_HEADERS.
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The build scripts will use dcopidl to parse MyInterface.h, converting
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it to an XML description in MyInterface.kidl. Next, a file called
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MyInterface_skel.cpp will automatically be created, compiled and
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linked with your binary.
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The next thing you have to do is to choose which of your classes will
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implement the interface described in MyInterface.h. Alter the inheritance
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of this class such that it virtually inherits from MyInterface. Then
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add declarations to your class interface similar to those on MyInterface.h,
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but virtual, not pure virtual.
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Example:
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class MyClass: public QObject, virtual public MyInterface
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{
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Q_OBJECT
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public:
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MyClass();
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~MyClass();
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ASYNC myAsynchronousMethod(QString someParameter);
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QRect mySynchronousMethod();
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};
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Note: (Qt issue) Remember that if you are inheriting from QObject, you must
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place it first in the list of inherited classes.
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In the implementation of your class' ctor, you must explicitly initialize
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those classes from which you are inheriting from. This is, of course, good
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practice, but it is essential here as you need to tell DCOPObject the name of
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the interface which your are implementing.
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Example:
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MyClass::MyClass()
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: QObject(),
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DCOPObject("MyInterface")
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{
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// whatever...
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}
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Now you can simply implement the methods you have declared in your interface,
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exactly the same as you would normally.
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Example:
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void MyClass::myAsynchronousMethod(QString someParameter)
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{
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tqDebug("myAsyncMethod called with param `" + someParameter + "'");
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}
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It is not necessary (though very clean) to define an interface as an
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abstract class of its own, like we did in the example above. We could
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just as well have defined a k_dcop section directly within MyClass:
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class MyClass: public QObject, virtual public DCOPObject
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{
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Q_OBJECT
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K_DCOP
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public:
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MyClass();
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~MyClass();
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k_dcop:
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ASYNC myAsynchronousMethod(QString someParameter);
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QRect mySynchronousMethod();
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};
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In addition to skeletons, dcopidl2cpp also generate stubs. Those make
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it easy to call a DCOP interface without doing the marshalling
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manually. To use a stub, add MyInterface.stub to the SOURCES list of
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your Makefile.am. The stub class will then be called MyInterface_stub.
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Conclusion:
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-----------
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Hopefully this document will get you well on your way into the world
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of inter-process communication with KDE! Please direct all comments
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and/or suggestions to Preston Brown <pbrown@kde.org> and Matthias
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Ettrich <ettrich@kde.org>.
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Inter-user communication
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------------------------
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Sometimes it might be interesting to use DCOP between processes
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belonging to different users, e.g. a frontend process running
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with the user's id, and a backend process running as root.
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For this you can use tdesu with the --nonewdcop option. tdesu will
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then forward the address of the DCOP server as well as the authentication
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information to the new user.
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*WARNING*: This gives the user that you su to, full access to your session!
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If you su to root this will not be a problem, but it may be a problem if
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you su to another user.
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By default, KDE applications (e.g. the ones that run as root) that connect
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to the dcopserver of another user will not accept any incoming DCOP calls.
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You can override this with DCOPClient::setAcceptCalls() after you have
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carefully reviewed that your DCOPClient does not provide objects/functions
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that could be abused for privilege escalation.
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Example: tdesu --nonewdcop -u root -c kcmroot
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This will, after tdesu got the root password, execute kcmroot as root,
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talking to the user's dcop server.
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Performance Tests:
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------------------
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A few back-of-the-napkin tests folks:
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Code:
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#include <tdeapplication.h>
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int main(int argc, char **argv)
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{
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TDEApplication *app;
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app = new TDEApplication(argc, argv, "testit");
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return app->exec();
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}
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Compiled with:
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g++ -O2 -o testit testit.cpp -I$TQTDIR/include -L$TQTDIR/lib -ltdecore
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on Linux yields the following memory use statistics:
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VmSize: 8076 kB
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VmLck: 0 kB
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VmRSS: 4532 kB
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VmData: 208 kB
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VmStk: 20 kB
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VmExe: 4 kB
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VmLib: 6588 kB
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If I create the TDEApplication's DCOPClient, and call attach() and
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registerAs(), it changes to this:
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VmSize: 8080 kB
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VmLck: 0 kB
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VmRSS: 4624 kB
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VmData: 208 kB
|
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VmStk: 20 kB
|
|
VmExe: 4 kB
|
|
VmLib: 6588 kB
|
|
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Basically it appears that using DCOP causes 100k more memory to be
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resident, but no more data or stack. So this will be shared between all
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processes, right? 100k to enable DCOP in all apps doesn't seem bad at
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all. :)
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OK now for some timings. Just creating a TDEApplication and then exiting
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(i.e. removing the call to TDEApplication::exec) takes this much time:
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0.28user 0.02system 0:00.32elapsed 92%CPU (0avgtext+0avgdata 0maxresident)k
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0inputs+0outputs (1084major+62minor)pagefaults 0swaps
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I.e. about 1/3 of a second on my PII-233. Now, if we create our DCOP
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object and attach to the server, it takes this long:
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0.27user 0.03system 0:00.34elapsed 87%CPU (0avgtext+0avgdata 0maxresident)k
|
|
0inputs+0outputs (1107major+65minor)pagefaults 0swaps
|
|
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|
I.e. about 1/3 of a second. Basically DCOPClient creation and attaching
|
|
gets lost in the statistical variation ("noise"). I was getting times
|
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between .32 and .48 over several runs for both of the example programs, so
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obviously system load is more relevant than the extra two calls to
|
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DCOPClient::attach and DCOPClient::registerAs, as well as the actual
|
|
DCOPClient constructor time.
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|