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diff --git a/dcop/HOWTO b/dcop/HOWTO new file mode 100644 index 000000000..8609021c0 --- /dev/null +++ b/dcop/HOWTO @@ -0,0 +1,552 @@ + 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 +practice, 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. + +For this you can use kdesu with the --nonewdcop option. kdesu will +then forward the address of the DCOP server as well as the authentication +information to the new user. + +*WARNING*: This gives the user that you su to, full access to your session! +If you su to root this will not be a problem, but it may be a problem if +you su to another user. + +By default, KDE applications (e.g. the ones that run as root) that connect +to the dcopserver of another user will not accept any incoming DCOP calls. +You can override this with DCOPClient::setAcceptCalls() after you have +carefully reviewed that your DCOPClient does not provide objects/functions +that could be abused for privilege escalation. + + +Example: kdesu --nonewdcop -u root -c kcmroot + +This will, after kdesu got the root password, execute kcmroot as root, +talking to the user's dcop server. + + +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. + |