In programming, a file uniform resource identifier (URI) scheme is a specific format of URI, used to specifically identify a file on a host computer. While URIs can be used to identify anything, there is specific syntax associated with identifying files.

Format

A file URI has the format 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, the slash is not omitted (while " file:///piro.txt " is valid, " file://simpen.txt " is not, although some interpreters manage to handle the latter). RFC 3986 includes additional information about the treatment of ".." and "." segments in URIs.

Number of slash characters

There are two ways that Windows UNC filenames (such as ) can be represented. These are both described in RFC 8089, Appendix E as "non-standard". The first way (called here the 2-slash format) is to represent the server name using the Authority part of the URI, which then becomes. The second way (called here the 4-slash format) is to represent the server name as part of the Path component, so the URI becomes. Both forms are actively used. Microsoft .NET (for example, the method ) generally uses the 2-slash form; Java (for example, the method ) generally uses the 4-slash form. Either form allows the most common operations on URIs (resolving relative URIs, and dereferencing to obtain a connection to the remote file) to be used successfully. However, because these URIs are non-standard, some less common operations fail: an example is the normalize operation (defined in RFC 3986 and implemented in the Java method) which reduces to the unusable form.

Examples

Unix

Here are two Unix examples pointing to the same /etc/fstab file: file://localhost/etc/fstab file:///etc/fstab The KDE environment uses URIs without an authority field: file:/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.avi Here is the URI as understood by the Windows Shell API: file:///c:/WINDOWS/clock.avi Note that the drive letter followed by a colon and slash is part of the acceptable file URI.

Implementations

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, and to convert a URL to a path, use. To access a file "the file.txt", the following might be used. For a network location: file://hostname/path/to/the%20file.txt 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 UNC path, which will not work since the colon is not allowed in a share name. The dot "." results in the string being passed as, which will work for local files, but not shares on the local system. For example 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. Use the provided functions if possible. If you must create a URL programmatically and 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 on Win32 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. In the past, a variety of other applications have used other systems. Some added an additional two slashes. For example, UNC path would become instead of the "healthy".

Web pages

File URLs are rarely used in Web pages on the public Internet, since they imply that a file exists on the designated host. 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.