The filesystem used on OS X and iOS is HFS+, which stores filenames as a normalized UTF-16. The POSIX C interface (open(2), etc) takes UTF-8. If you try to make

The filesystem used on OS X and iOS is HFS+, which stores filenames as a normalized UTF-16. The POSIX C interface (open(2), etc) takes UTF-8. If you try to make a file with invalid UTF-8, the filesystem will silently munge it. For example, if your path has a trailing 0xc3 byte, that byte gets rewritten into the string "%c3". So you don't actually get arbitrary byte strings. I don't know what happens if you do something like mount an NFS share with filenames that aren't valid Unicode. I would guess a fair amount of stuff will break. Swift strings are Unicode, and Unicode is certainly capable of representing arbitrary byte streams, by just converting to and from any 8 bit fixed-width encoding (not UTF-8!). There are explicit conversion functions like -fileSystemRepresentation which handle the conversion between filesystem paths and Strings.