This folder contains list of symbols that should be excluded from the builtin libraries for Darwin. There are two reasons symbols are excluded: (1) They aren't supported on Darwin (2) They are contained within the OS on the minimum supported target The builtin libraries must contain all symbols not provided by the lowest supported target OS. Meaning if minimum deployment target is iOS 6, all builtins not included in the ios6-<arch>.txt files need to be included. The one catch is that this is per-architecture. Since iOS 6 doesn't support arm64, when supporting iOS 6, the minimum deployment target for arm64 binaries is iOS 7.