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 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.

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 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, 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.

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.

API

Members

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

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.

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

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.
• SecondaryNotification – Whether to trigger instanceStarted() even whenever secondary instances are started.

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.

---

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.

Additionally the library can recover from being killed with uncatchable signals 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 some of the examples distributed under the BSD license.