如何在iOS 6和iOS 7中获得UDID

identifierForvendor：

An alphanumeric string that uniquely identifies a device to the app’s
vendor. (read-only)

The value of this property is the same for apps that come from the
same vendor running on the same device. A different value is returned
for apps on the same device that come from different vendors,and for
apps on different devices regardless of vendor.

advertisingIdentifier：

An alphanumeric string unique to each device,used only for serving
advertisements. (read-only)

Unlike the identifierForvendor property of UIDevice,the same value is
returned to all vendors. This identifier may change—for example,if
the user erases the device—so you should not cache it.

This帖子解释得很好.

另请参阅Apple的identifierForVendor和advertisingIdentifier文档.