SingleApplication

This is a replacement of the QtSingleApplication for Qt5 and Qt6.

Keeps the Primary Instance of your Application and kills each subsequent instances. It can (if enabled) spawn secondary (non-related to the primary) instances and can send data to the primary instance from secondary instances.

Usage

The SingleApplication class inherits from whatever Q[Core|Gui]Application class you specify via the QAPPLICATION_CLASS macro (QCoreApplication is the default). Further usage is similar to the use of the Q[Core|Gui]Application classes.

You can use the library as if you use any other QCoreApplication derived class:

#include <QApplication>
#include <SingleApplication.h>

int main( int argc, char* argv[] )
{
    SingleApplication app( argc, argv );

    return app.exec();
}

To include the library files I would recommend that you add it as a git submodule to your project. Here is how:

git submodule add git@github.com:itay-grudev/SingleApplication.git singleapplication

Qmake:

Then include the singleapplication.pri file in your .pro project file.

include(singleapplication/singleapplication.pri)
DEFINES += QAPPLICATION_CLASS=QApplication

CMake:

Then include the subdirectory in your CMakeLists.txt project file.

set(QAPPLICATION_CLASS QApplication CACHE STRING "Inheritance class for SingleApplication")
add_subdirectory(src/third-party/singleapplication)
target_link_libraries(${PROJECT_NAME} SingleApplication::SingleApplication)

The library sets up a QLocalServer and a QSharedMemory block. The first instance of your Application is your Primary Instance. It would check if the shared memory block exists and if not it will start a QLocalServer and listen for connections. Each subsequent instance of your application would check if the shared memory block exists and if it does, it will connect to the QLocalServer to notify the primary instance that a new instance had been started, after which it would terminate with status code 0. In the Primary Instance SingleApplication would emit the instanceStarted() signal upon detecting that a new instance had been started.

The library uses stdlib to terminate the program with the exit() function.

Also don't forget to specify which QCoreApplication class your app is using if it is not QCoreApplication as in examples above.

The Instance Started signal

The SingleApplication class implements a instanceStarted() signal. You can bind to that signal to raise your application's window when a new instance had been started, for example.

// window is a QWindow instance
QObject::connect(
    &app,
    &SingleApplication::instanceStarted,
    &window,
    &QWindow::raise
);

Using SingleApplication::instance() is a neat way to get the SingleApplication instance for binding to it's signals anywhere in your program.

Note: On Windows the ability to bring the application windows to the foreground is restricted. See Windows specific implementations for a workaround and an example implementation.

Secondary Instances

If you want to be able to launch additional Secondary Instances (not related to your Primary Instance) you have to enable that with the third parameter of the SingleApplication constructor. The default is false meaning no Secondary Instances. Here is an example of how you would start a Secondary Instance send a message with the command line arguments to the primary instance and then shut down.

int main(int argc, char *argv[])
{
    SingleApplication app( argc, argv, true );

    if( app.isSecondary() ) {
        app.sendMessage(  app.arguments().join(' ')).toUtf8() );
        app.exit( 0 );
    }

    return app.exec();
}

Note: A secondary instance won't cause the emission of the instanceStarted() signal by default. See SingleApplication::Mode for more details.

You can check whether your instance is a primary or secondary with the following methods:

app.isPrimary();
// or
app.isSecondary();

Note: If your Primary Instance is terminated a newly launched instance will replace the Primary one even if the Secondary flag has been set.

Examples

There are three examples provided in this repository:

• Basic example that prevents a secondary instance from starting examples/basic
• An example of a graphical application raising it's parent window examples/calculator
• A console application sending the primary instance it's command line parameters examples/sending_arguments

API

Members

SingleApplication::SingleApplication( int &argc, char *argv[], bool allowSecondary = false, Options options = Mode::User, int timeout = 100, QString userData = QString() )

Depending on whether allowSecondary is set, this constructor may terminate your app if there is already a primary instance running. Additional Options can be specified to set whether the SingleApplication block should work user-wide or system-wide. Additionally the Mode::SecondaryNotification may be used to notify the primary instance whenever a secondary instance had been started (disabled by default). timeout specifies the maximum time in milliseconds to wait for blocking operations. Setting userData provides additional data that will isolate this instance from other instances that do not have the same (or any) user data set.

Note: argc and argv may be changed as Qt removes arguments that it recognizes.

Note: Mode::SecondaryNotification only works if set on both the primary and the secondary instance.

Note: Operating system can restrict the shared memory blocks to the same user, in which case the User/System modes will have no effect and the block will be user wide.

---

bool SingleApplication::sendMessage( QByteArray message, int timeout = 100 )

Sends message to the Primary Instance. Uses timeout as a the maximum timeout in milliseconds for blocking functions

---

bool SingleApplication::isPrimary()

Returns if the instance is the primary instance.

---

bool SingleApplication::isSecondary()

Returns if the instance is a secondary instance.

---

quint32 SingleApplication::instanceId()

Returns a unique identifier for the current instance.

---

qint64 SingleApplication::primaryPid()

Returns the process ID (PID) of the primary instance.

---

QString SingleApplication::primaryUser()

Returns the username the primary instance is running as.

---

QString SingleApplication::currentUser()

Returns the username the current instance is running as.

Signals

void SingleApplication::instanceStarted()

Triggered whenever a new instance had been started, except for secondary instances if the Mode::SecondaryNotification flag is not specified.

---

void SingleApplication::receivedMessage( quint32 instanceId, QByteArray message )

Triggered whenever there is a message received from a secondary instance.

---

Flags

enum SingleApplication::Mode

• Mode::User - The SingleApplication block should apply user wide. This adds user specific data to the key used for the shared memory and server name. This is the default functionality.
• Mode::System – The SingleApplication block applies system-wide.
• Mode::SecondaryNotification – Whether to trigger instanceStarted() even whenever secondary instances are started.
• Mode::ExcludeAppPath – Excludes the application path from the server name (and memory block) hash.
• Mode::ExcludeAppVersion – Excludes the application version from the server name (and memory block) hash.

Note: Mode::SecondaryNotification only works if set on both the primary and the secondary instance.

Note: Operating system can restrict the shared memory blocks to the same user, in which case the User/System modes will have no effect and the block will be user wide.

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Versioning

Each major version introduces either very significant changes or is not backwards compatible with the previous version. Minor versions only add additional features, bug fixes or performance improvements and are backwards compatible with the previous release. See CHANGELOG.md for more details.

Implementation

The library is implemented with a QSharedMemory block which is thread safe and guarantees a race condition will not occur. It also uses a QLocalSocket to notify the main process that a new instance had been spawned and thus invoke the instanceStarted() signal and for messaging the primary instance.

Additionally the library can recover from being forcefully killed on *nix systems and will reset the memory block given that there are no other instances running.

License

This library and it's supporting documentation are released under The MIT License (MIT) with the exception of the Qt calculator examples which is distributed under the BSD license.