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### 9.2 Native Filenames

In some circumstances, what is wanted is a Lisp pathname object which corresponds to a string produced by the Operating System. In this case, some of the default parsing rules are inappropriate: most filesystems do not have a native understanding of wild pathnames; such functionality is often provided by shells above the OS, often in mutually-incompatible ways.

To allow the user to deal with this, the following functions are provided: `parse-native-namestring` and `native-pathname` return the closest equivalent Lisp pathname to a given string (appropriate for the Operating System), while `native-namestring` converts a non-wild pathname designator to the equivalent native namestring, if possible. Some Lisp pathname concepts (such as the `:back` directory component) have no direct equivalents in most Operating Systems; the behaviour of `native-namestring` is unspecified if an inappropriate pathname designator is passed to it. Additionally, note that conversion from pathname to native filename and back to pathname should not be expected to preserve equivalence under `equal`.

— Function: sb-ext:parse-native-namestring thing &optional host defaults &key start end junk-allowed as-directory

> Convert `thing` into a pathname, using the native conventions appropriate for the pathname host `host`, or if not specified the host of `defaults`. If `thing` is a string, the parse is bounded by `start` and `end`, and error behaviour is controlled by `junk-allowed`, as with `parse-namestring`. For file systems whose native conventions allow directories to be indicated as files, if `as-directory` is true, return a pathname denoting `thing` as a directory.

— Function: sb-ext:native-pathname pathspec

> Convert `pathspec` (a pathname designator) into a pathname, assuming the operating system native pathname conventions.

— Function: sb-ext:native-namestring pathname &key as-file

> Construct the full native (name)string form of `pathname`. For file systems whose native conventions allow directories to be indicated as files, if `as-file` is true and the name, type, and version components of `pathname` are all `nil` or `:unspecific`, construct a string that names the directory according to the file system's syntax for files.

Because some file systems permit the names of directories to be expressed in multiple ways, it is occasionally necessary to parse a native file name “as a directory name” or to produce a native file name that names a directory “as a file”. For these cases, `parse-native-namestring` accepts the keyword argument `as-directory` to force a filename to parse as a directory, and `native-namestring` accepts the keyword argument `as-file` to force a pathname to unparse as a file. For example,

         ; On Unix, the directory "/tmp/" can be denoted by "/tmp/" or "/tmp".
         ; Under the default rules for native filenames, these parse and
         ; unparse differently.
         (defvar *p*)
         (setf *p* (parse-native-namestring "/tmp/")) => #P"/tmp/"
         (pathname-name *p*) => NIL
         (pathname-directory *p*) => (:ABSOLUTE "tmp")
         (native-namestring *p*) => "/tmp/"

         (setf *p* (parse-native-namestring "/tmp")) => #P"/tmp"
         (pathname-name *p*) => "tmp"
         (pathname-directory *p*) => (:ABSOLUTE)
         (native-namestring *p*) => "/tmp"

         ; A non-NIL AS-DIRECTORY argument to PARSE-NATIVE-NAMESTRING forces
         ; both the second string to parse the way the first does.
         (setf *p* (parse-native-namestring "/tmp"
                                            nil *default-pathname-defaults*
                                            :as-directory t)) => #P"/tmp/"
         (pathname-name *p*) => NIL
         (pathname-directory *p*) => (:ABSOLUTE "tmp")

         ; A non-NIL AS-FILE argument to NATIVE-NAMESTRING forces the pathname
         ; parsed from the first string to unparse as the second string.
         (setf *p* (parse-native-namestring "/tmp/")) => #P"/tmp/"
         (native-namestring *p* :as-file t) => "/tmp"