Darwin targets support the CFString (or __CFString__) in the format
attribute context. Declarations made with such attribution are parsed for correct syntax
and format argument types. However, parsing of the format string itself is currently undefined
and is not carried out by this version of the compiler.
Additionally, CFStringRefs (defined by the CoreFoundation headers) may
also be used as format arguments. Note that the relevant headers are only likely to be
available on Darwin (OSX) installations. On such installations, the XCode and system
documentation provide descriptions of CFString, CFStringRefs and
associated functions.