SingleApplication

This is a replacement of the QSingleApplication for Qt5.

Keeps the Primary Instance of your Application and kills each subsequent instances.

Usage

The SingleApplication class inherits from whatever Q[Core|Gui]Application class you specify in the *.pro file. Further usage is similar to use one 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 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 would terminate the new instance with status code 0. The Primary Instance, SingleApplication would emmit 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:

// project.pro
DEFINES += QAPPLICATION_CLASS=QApplication # or whatever app class you want

// main.cpp
#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();
}

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.

Note that since SingleApplication extends the QApplication class you can do the following:

// Truly raise your window.
QObject::connect(&app, &SingleApplication::showUp, [&]
{
    window.show();
    window.raise();
    window.activateWindow();
}); // 'window' is your QWindow instance

Using QApplication::instance() is a neat way to get the SingleApplication instance at any place in your program.

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).