Detecting when gdb is attached

["Gruenhagen, Andreas" <andreas dot gruenhagen at sap dot com>]

Dear gdb community,
I'm developing an application which cannot be started from within the gdb. Therefore, I have the start the application and attach the gdb afterwards. In some cases it would be nice to have the application wait in a spin loop until the debugger is attached. Is there any way of accomplishing such a thing?  
I certainly can program the spin loop myself, attach the debugger, and then manually change a variable from within the debugger to end the loop. But, obviously, this is not an elegant solution (lots of manual work, code has to be changed and it has to be recompiled,and eventually, I might forget to delete the spin loop after debugging).

I thought of having some sort of action which is executed when the gdb is attached and detached, this could be for example used to switch a static flag in the debugee.
Is something like this possible ?

Best regards
Andreas