SingleApplication

This is a replacement of the QSingleApplication for Qt5.

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.

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 then 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 it that a new instance had been started, after which it would terminate with status code 0. 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.

You can use the library as if you use any other QCoreApplication 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 and include it's contents with a .pri file. Here is how:

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

Then include the singleapplication.pri file in your .pro project file. Also don't forget to specify which QCoreApplication class your app is using if it is not QCoreApplication.

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

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.

// 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.

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.

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.

Versioning

The current library versions is 3.0a.

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.

To handle an issue on *nix systems, where the operating system owns the shared memory block and if the program crashes the memory remains untouched, the library binds to the following signals and closes the program with error code = 128 + signum where signum is the number representation of the signal listed below. Handling the signal is required in order to safely delete the QSharedMemory block. Each of these signals are potentially lethal and will results in process termination.

• SIGHUP - 1, Hangup.
• SIGINT - 2, Terminal interrupt signal
• SIGQUIT - 3, Terminal quit signal.
• SIGILL - 4, Illegal instruction.
• SIGABRT - 6, Process abort signal.
• SIGBUS - 7, Access to an undefined portion of a memory object.
• SIGFPE - 8, Erroneous arithmetic operation (such as division by zero).
• SIGSEGV - 11, Invalid memory reference.
• SIGSYS - 12, Bad system call.
• SIGPIPE - 13, Write on a pipe with no one to read it.
• SIGALRM - 14, Alarm clock.
• SIGTERM - 15, Termination signal.
• SIGXCPU - 24, CPU time limit exceeded.
• SIGXFSZ - 25, File size limit exceeded.

License

This library and it's supporting documentation are released under The MIT License (MIT).