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+SingleApplication
+=================
+
+This is a replacement of the QtSingleApplication 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:
+
+```cpp
+#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:
+
+```bash
+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`.
+
+```qmake
+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.
+
+```cpp
+// 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](Windows.md)
+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.
+
+```cpp
+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:
+
+```cpp
+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
+
+```cpp
+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.*
+
+---
+
+```cpp
+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
+
+---
+
+```cpp
+bool SingleApplication::isPrimary()
+```
+
+Returns if the instance is the primary instance.
+
+---
+
+```cpp
+bool SingleApplication::isSecondary()
+```
+Returns if the instance is a secondary instance.
+
+---
+
+```cpp
+quint32 SingleApplication::instanceId()
+```
+
+Returns a unique identifier for the current instance.
+
+---
+
+```cpp
+qint64 SingleApplication::primaryPid()
+```
+
+Returns the process ID (PID) of the primary instance.
+
+### Signals
+
+```cpp
+void SingleApplication::instanceStarted()
+```
+
+Triggered whenever a new instance had been started, except for secondary
+instances if the `Mode::SecondaryNotification` flag is not specified.
+
+---
+
+```cpp
+void SingleApplication::receivedMessage( quint32 instanceId, QByteArray message )
+```
+
+Triggered whenever there is a message received from a secondary instance.
+
+---
+
+### Flags
+
+```cpp
+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.*
+
+---
+
+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`](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.