Restore an application that is running while using QtSingleApplication

[Eldritch]

Here is one (it also mentions QtSingleApplication. ;-)

Often, the best solutions are platform-specific. You can use named pipes, a TCP or UPD connection, a simple file (these are all portable). In MSWin, the standard trick is a named mutex (not available on other platforms). I haven't looked at the code for QtSingleApplication, but it suggests the use of some form of pipe or socket just from its method list.

To the O.P.'s question. When the new instance checks for a previous one, the previous one will receive the text you send it. When it receives that message, it can activate its main window. It may not make the app jump to front (there's already a seperate thread about this), but it will restore its window if minimized, etc.

[wysota]

n MSWin, the standard trick is a named mutex (not available on other platforms).

On u*ix you can use system semaphores or message queues for that. Based on the fact that MacOS is also a u*ix, it should be available there as well. On modern Linuxes, it would be wise to just use dbus

[ber_44]

One thing that bothers me now is that when the user exits with Alt+F4 or through the Task Manager, then the mutex/textfile/etc. will never get deleted, so it will be impossible to restart the program.

[elcuco]

so... there is no gpl code available for doing this?

[wysota]

One thing that bothers me now is that when the user exits with Alt+F4 or through the Task Manager, then the mutex/textfile/etc. will never get deleted, so it will be impossible to restart the program.

You should be able to catch the signal using C function signal() and clean up the mutex before exiting the program.