There are many kinds of file systems, varying widely both in their superficial syntactic details, and in their underlying power and structure. The facilities provided by Common Lisp for referring to and manipulating files has been chosen to be compatible with many kinds of file systems, while at the same time minimizing the program-visible differences between kinds of file systems.
In general, the syntax of namestrings involves the use of implementation-defined conventions, usually those customary for the file system in which the named file resides. The only exception is the syntax of a logical pathname namestring, which is defined in this specification; see Section 19.3.1 (Syntax of Logical Pathname Namestrings).
A conforming program must never unconditionally use a literal namestring other than a logical pathname namestring because Common Lisp does not define any namestring syntax other than that for logical pathnames that would be guaranteed to be portable. However, a conforming program can, if it is careful, successfully manipulate user-supplied data which contains or refers to non-portable namestrings.
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 Section 19.2.1 (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.
Parsing is the operation used to convert a namestring into a pathname. Except in the case of parsing logical pathname namestrings, this operation is implementation-dependent, because the format of namestrings is implementation-dependent.
A conforming implementation is free to accommodate other file system features in its pathname representation and provides a parser that can process such specifications in namestrings. Conforming programs must not depend on any such features, since those features will not be portable.