#### 19.1.2 Pathnames as Filenames

Pathnames are structured objects that can represent, in an implementation-independent way, the filenames that are used natively by an underlying file system.

In addition, pathnames can also represent certain partially composed filenames for which an underlying file system might not have a specific namestring representation.

A pathname need not correspond to any file that actually exists, and more than one pathname can refer to the same file. For example, the pathname with a version of `:newest` might refer to the same file as a pathname with the same components except a certain number as the version. Indeed, a pathname with version `:newest` might refer to different files as time passes, because the meaning of such a pathname depends on the state of the file system.

Some file systems naturally use a structural model for their filenames, while others do not. Within the Common Lisp pathname model, all filenames are seen as having a particular structure, even if that structure is not reflected in the underlying file system. The nature of the mapping between structure imposed by pathnames and the structure, if any, that is used by the underlying file system is implementation-defined.

Every pathname has six components: a host, a device, a directory, a name, a type, and a version. By naming files with pathnames, Common Lisp programs can work in essentially the same way even in file systems that seem superficially quite different. For a detailed description of these components, see Pathname Components.

The mapping of the pathname components into the concepts peculiar to each file system is implementation-defined. There exist conceivable pathnames for which there is no mapping to a syntactically valid filename in a particular implementation. An implementation may use various strategies in an attempt to find a mapping; for example, an implementation may quietly truncate filenames that exceed length limitations imposed by the underlying file system, or ignore certain pathname components for which the file system provides no support. If such a mapping cannot be found, an error of type file-error is signaled.

The time at which this mapping and associated error signaling occurs is implementation-dependent. Specifically, it may occur at the time the pathname is constructed, when coercing a pathname to a namestring, or when an attempt is made to open or otherwise access the file designated by the pathname.

Figure 19–1 lists some defined names that are applicable to pathnames.

      *default-pathname-defaults*  namestring          pathname-name
      directory-namestring         open                pathname-type
      enough-namestring            parse-namestring    pathname-version
      file-namestring              pathname            pathnamep
      file-string-length           pathname-device     translate-pathname
      host-namestring              pathname-directory  truename
      make-pathname                pathname-host       user-homedir-pathname
      merge-pathnames              pathname-match-p    wild-pathname-p

                         Figure 19–1: Pathname Operations