The file URI scheme is a URI scheme specified in RFC 1630 and RFC 1738, typically used to retrieve files from within one's own computer.
Format
A file URL takes the form of
file://host/path
where host is the fully qualified domain name of the system on which the path is accessible, and path is a hierarchical directory path of the form directory/directory/.../name. If host is omitted, it is taken to be "localhost", the machine from which the URL is being interpreted. Note that when omitting host you do not omit the slash ("file:///foo.txt" is okay, while "file://foo.txt" is not, although some interpreters manage to handle the latter).
Meaning of slash character
The slash character (/), depending on its position, is used in different meanings in a file URL.
- The // after the file: is part of the general syntax of hierarchical URLs. (The double slash // should always appear in a file URL according to the specification, but in practice many Web browsers allow you to omit it, in some cases at least.)
- The single slash between host and path is part of the syntax of file URLs.
- And the slashes in path separate directory names in a hierarchical system of directories and subdirectories. In this usage, the slash is a general, system-independent way of separating the parts, and in a particular host system it might be used as such in a pathname (as in Unix systems).
Examples
Unix
Here are two Unix examples pointing to the same /etc/fstab file:
file://localhost/etc/fstabfile:///etc /fstab
Windows
Here are some examples which may be accepted by some applications on Windows systems, referring to the same, local file c:\WINDOWS\clock.avi
file://localhost/c|/WINDOWS/clock.avi file:///c|/WINDOWS/clock.avifile://lo calhost/c:/WINDOWS/clock.avi
Here is the correct URI as understood by the Windows Shell API:
file:///c:/WINDOWS/clock.avi
For a local file, the last is the most obvious and human-readable, and also the canonical one understood by the operating system [1].
Things to consider
Windows
On Microsoft Windows systems, the normal colon (:) after a device letter has sometimes been replaced by a vertical bar (|) in file URLs. This reflected the original URL syntax, which made the colon a reserved character in a path part.
Since Internet Explorer 4, file URIs have been standardized on Windows, and should follow the following scheme. This applies to all applications which use URLMON or SHLWAPI for parsing, fetching or binding to URIs. To convert a path to a URL, use UrlCreateFromPath, and to convert a URL to a path, use PathCreateFromUrl. [2]
To access a file "the file.txt", the following might be used.
For a network location:
file://hostname/path/to/the%20file.tx t
Or for a local file, the hostname is omitted, but the slash is not (note the third slash):
file:///c:/path/to/the%20file.txt
This is not the same as providing the string "localhost" or the dot "." in place of the hostname. The string "localhost" will attempt to access the file as \localhost\c:\path\to\the file.txt, which will not work since the colon is not allowed in a share name. The dot "." results in the string being passed as \.\c:\path\to\the file.txt, which will work for local files, but not shares on the local system. For example file://./sharename/path/to/the%20file .txt will not work, because it will result in sharename being interpreted as part of the DOSDEVICES namespace, not as a network share.
The following outline roughly describes the requirements.
- The colon should be used, and should not be replaced with a vertical bar.
- Forward slashes should be used to delimit paths. note that while DOS requires backslashes, the Win32 API will accept either a forward or back slash.
- Characters such as the hash (#) or question mark (?) which are part of the filename should be percent-encoded.
- Characters which are not allowed in URIs, but which are allowed in filenames, must also be percent-encoded. For example, any of "{}`^ " and all control characters. In the example above, the space in the filename is encoded as %20.
- Characters which are allowed in both URIs and filenames must NOT be percent-encoded.
- Must not use legacy ACP encodings. (ACP code pages are specified by DOS CHCP or Windows Control Panel language setting.)
- Unicode characters outside of the ASCII range must be UTF-8 encoded, and those UTF-8 encodings must be percent-encoded.
Use the provided functions if you can. If you must create a URL programmatically and you cannot access SHLWAPI.dll (for example from script, or another programming environment where the equivalent functions are not available) the above outline will help.
Legacy URLs
To aid the installed base of legacy applications, the PathCreateFromUrl recognizes certain URLs which do not meet these criteria, and treats them uniformly. These are called "legacy" file URLs as opposed to "healthy" file URLs. [3]
In the past, a variety of other applications have used other systems. Some added an additional two slashes. For example, \remotehost\share\dir\file.txt, would become file:////remotehost/share/dir/file.tx t instead of the "healthy" file://remotehost/share/dir/file.txt.
Web pages
File URLs are rarely used in Web pages on the Internet, since they make the assumption that such a file exists on the client's computer. The host specifier can be used to retrieve a file from an external source, although no specific file-retrieval protocol is specified; and using it should result in a message that informs the user that no mechanism to access that machine is available.
Mozilla
Mozilla browsers refuse to follow file URLs on a page that it has fetched with the HTTP protocol (i.e. the page's own URL is an http URL). When you click on such a link, nothing happens. The purpose is security: to prevent a remote page from executing a program on the visitor's computer. File links work on Mozilla on pages that are local files on the user's disk. It is not a surprise that Firefox 3.5.3 does nothing when requesting a file on a remote host:
file:////remotehost/share/dir/file.txt
However, it successfully opens:
file://localhost///remotehost/share/dir/file.txt
or its equivalent:
file://///remotehost/share/dir/file.txt
as it is referred to as a file on the local host.
Mozilla browsers can be configured to override this security restriction as detailed in Mozillazine's "Links to Local Pages Don't Work".
Mozilla browsers also treat file URLs similarly to the Gopher protocol in the way a directory is represented textually (i.e. the source) and graphically.
Internet Explorer
Internet Explorer browsers, prior to version 7, will attempt to access file: URLs even if they reside on pages fetched over HTTP.[citation needed] [1] [2] [3] [4] [5] [6]
Other
The original Web browser, WorldWideWeb, provided editing of resources in file space [4]. Amaya still has this ability.
References
External links
