31 Input/output library [input.output]

31.12 File systems [filesystems]

31.12.6 Class path [fs.class.path] Generic pathname format [fs.path.generic]

operating system dependent sequences of characters
implementation-defined sequences of characters
non-empty sequence of characters other than directory-separator characters
operating system dependent directory separator character
A filename is the name of a file.
The dot and dot-dot filenames, consisting solely of one and two period characters respectively, have special meaning.
The following characteristics of filenames are operating system dependent:
  • The permitted characters.
    [Example 1: 
    Some operating systems prohibit the ASCII control characters (0x00 – 0x1F) in filenames.
    — end example]
    [Note 1: 
    Wider portability can be achieved by limiting filename characters to the POSIX Portable Filename Character Set:
    A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
    a b c d e f g h i j k l m n o p q r s t u v w x y z
    0 1 2 3 4 5 6 7 8 9 . _ -
    — end note]
  • The maximum permitted length.
  • Filenames that are not permitted.
  • Filenames that have special meaning.
  • Case awareness and sensitivity during path resolution.
  • Special rules that may apply to file types other than regular files, such as directories.
Except in a root-name, multiple successive directory-separator characters are considered to be the same as one directory-separator character.
The dot filename is treated as a reference to the current directory.
The dot-dot filename is treated as a reference to the parent directory.
What the dot-dot filename refers to relative to root-directory is implementation-defined.
Specific filenames may have special meanings for a particular operating system.
A root-name identifies the starting location for pathname resolution ([fs.class.path]).
If there are no operating system dependent root-names, at least one implementation-defined root-name is required.
[Note 2: 
Many operating systems define a name beginning with two directory-separator characters as a root-name that identifies network or other resource locations.
Some operating systems define a single letter followed by a colon as a drive specifier — a root-name identifying a specific device such as a disk drive.
— end note]
If a root-name is otherwise ambiguous, the possibility with the longest sequence of characters is chosen.
[Note 3: 
On a POSIX-like operating system, it is impossible to have a root-name and a relative-path without an intervening root-directory element.
— end note]
Normalization of a generic format pathname means:
If the path is empty, stop.
Replace each slash character in the root-name with a preferred-separator.
[Note 4: 
The generic pathname grammar defines directory-separator as one or more slashes and preferred-separators.
— end note]
Remove each dot filename and any immediately following directory-separator.
As long as any appear, remove a non-dot-dot filename immediately followed by a directory-separator and a dot-dot filename, along with any immediately following directory-separator.
If there is a root-directory, remove all dot-dot filenames and any directory-separators immediately following them.
[Note 5: 
These dot-dot filenames attempt to refer to nonexistent parent directories.
— end note]
If the last filename is dot-dot, remove any trailing directory-separator.
If the path is empty, add a dot.
The result of normalization is a path in normal form, which is said to be normalized.