Unique number between 0 to 255 across the processes

Is there any way to generate a unique number between 0 to 255. This number must be unique across the processes. So If my application has multiple instances each instance will be given a unique number between 0 to 255. Of course, maximum number of instances my application can have are 256.

As suggested by Brad, Using a file is always as an option. But I am wondering if there are any windows API to do so?

Is this something I can do in C++?