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 a certain number of secondary instances (with the --secondary command line argument).

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 uses your Organization Name and Application Name to set 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 on it. 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 showUp() signal upon detecting that a new instance had been started.

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

Here is an example usage of the library:

In your main you need to set the the applicationName and organizationName of the QCoreApplication class like so:

#include <QApplication>
#include "singleapplication.h"

int main( int argc, char* argv[] )
{
    QApplication::setApplicationName("{Your App Name}");
    QApplication::setOrganizationName("{Your Organization Name}");

    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

And include the singleapplication.pri file in your .pro project file:

include(singleapplication/singleapplication.pri)

The Show Up signal

The SingleApplication class implements a showUp() 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::showUp, window, &QWindow::raise );

Using QCoreApplication::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 0 meaning no Secondary Instances. Here is an example allowing spawning up to 2 Secondary Instances.

SingleApplication app( argc, argv, 2 );

After which just call your program with the --secondary argument to launch a secondary instance.

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 upon launch of a new one it will replace it as Primary even if the --secondary argument has been set.

P.S. If you think this behavior could be improved create an issue and explain why.

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 showUp() 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).