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