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I have a problem with this function :
When I run my app on the simulator 7, I have an ID. I quit the application and I run, I have another ID.
I think that I don't have this problem with the simulator 6.1.
Since I installed XCode 5, I have this problem.
Normally, I must have always the same ID unless I remove the application and I reinstalls.
This is a known issue (or should I say Apple design decision).
Apple documents several reasons where `IdentifierForVendor` might change . While I don't agree that it's clear  (from Apple documentation) for the simulator value can change (beside that it can change when installing from Xcode) it is not something we have any control over.
Note that the fact that it happens with Xcode 5 is _likely_ related to the change that Apple made to compute the value. There's a bit of details in the iOS7 release notes .
I suggest you to use an approach similar to  in your simulator builds, i.e. hard code a value in your simulator builds.