19 Filenames  19.2 Pathnames 19.2.2 Interpreting Pathname Component Values 19.2.2.4 Restrictions on Examining Pathname Components
19.2.2.4.3 Restrictions on Examining a Pathname Directory Component
The directory might be a string,
:wild, :unspecific, or nil.
The directory can be a list of strings and symbols.
The car of the list is one of the symbols :absolute or
:relative, meaning:
- :absolute
A list whose car is the symbol :absolute represents
a directory path starting from the root directory. The list
(:absolute) represents the root directory. The list
(:absolute "foo" "bar" "baz") represents the directory called
"/foo/bar/baz" in Unix (except possibly for case).
- :relative
A list whose car is the symbol :relative represents
a directory path starting from a default directory.
The list (:relative) has the same meaning as nil and hence is not used.
The list (:relative "foo" "bar") represents the directory named "bar"
in the directory named "foo" in the default directory.
Each remaining element of the list is a string or a symbol.
Each string names a single level of directory structure.
The strings should contain only the directory names
themselves - no punctuation characters.
In place of a string, at any point in the list, symbols
can occur to indicate special file notations.
The next figure lists the symbols that have standard meanings.
Implementations are permitted to add additional objects
of any type that is disjoint from string
if necessary to represent features of their file systems that cannot be
represented with the standard strings and symbols.
Supplying any non-string, including any of the symbols listed below,
to a file system for which it does not make sense
signals an error of type file-error.
For example, Unix does not support :wild-inferiors in most implementations.
Special Markers In Directory Component
| Symbol |
Meaning |
|---|
| :wild |
Wildcard match of one level of directory structure |
| :wild-inferiors |
Wildcard match of any number of directory levels |
| :up |
Go upward in directory structure (semantic) |
| :back |
Go upward in directory structure (syntactic) |
The following notes apply to the previous figure:
- Invalid Combinations
Using :absolute or :wild-inferiors
immediately followed by :up or :back
signals an error of type file-error.
- Syntactic vs Semantic
"Syntactic" means that the action of :back
depends only on the pathname
and not on the contents of the file system.
"Semantic" means that the action of :up
depends on the contents of the file system;
to resolve a pathname containing
:up to a pathname whose directory component
contains only :absolute and
strings requires probing the file system.
:up differs from
:back only in file systems that support multiple
names for directories, perhaps via symbolic links. For example,
suppose that there is a directory
(:absolute "X" "Y" "Z")
linked to
(:absolute "A" "B" "C")
and there also exist directories
(:absolute "A" "B" "Q") and
(:absolute "X" "Y" "Q").
Then
(:absolute "X" "Y" "Z" :up "Q")
designates
(:absolute "A" "B" "Q")
while
(:absolute "X" "Y" "Z" :back "Q")
designates
(:absolute "X" "Y" "Q")
19.2.2.4.3.1 Directory Components in Non-Hierarchical File Systems
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