How do I open a new document in running application without opening a new instance of the application?

Question

I have a situation that has been partially covered by other answers at SO, but I cannot find a complete answer. In short, we are trying to use URL's for our specific data types that when double clicked will open up our application and load those data sets into that app. We have this part working.

(for example, an URL might look like this: resource://shaders/basic_shader.hlsl)

What we would like to do is to prevent new instances of the application from opening when a new URL is double clicked. For example, let's say we have a URL that opens up a shader in our shader editor. When clicking this resource URL, it will open our shader editor. When a new shader URL is clicked, we'd like to be able to open up the shader in the currently running application and have it open up the new shader in a new tab in our editor.

We can easily detect if another instance of our application is running. The problem that we don't know how to easily solve is how to tell the currently running application to open up this new file for editing. This behavior is very much like the Apple Finder.

In unix, you could emulate this behavior by having your application open some named pipe and then new apps could check if this pipe is active and then send the document data down the pipe. Is there a more standard windows way of achieving this behavior?

We need a C/C++ solution. Thanks.

Answer 1

You can't avoid the program associated with the url to be executed.

The "windows" solutions would be to send a message (via DDE in the old days but maybe there is something more "modern" now) to the previously running application with the url then quit ...

Answer 2

Named pipe is the best way. First instance of your application opens the pipe and listens to it (use PIPE_ACCESS_INBOUND as dwOpenMode and the same code will also allow you to detect running instances). All subsequent instances check that they are not alone, send command line argument to the pipe and shut down.

Answer 3

You can acquire a named mutex upon startup and enforce it that way.

• http://www.google.com/search?hl=en&q=named+mutex+single+instance
• CreateMutex on MSDN

Answer 4

Create a named mutex when application launches as David Grant said, then before displaying the UI for the second URL, check for this mutex, if it is already created then just quit by passing the new URL to the first launched application (Have interface in the application to set the URL and tell to redirect programatically)

Answer 5

I got this working pretty well for my C++ MFC application by following Joseph Newcomer's tutorial here. He uses a named mutex that is checked on startup and a message sent to the already-running application with the new resource to be opened.