Beginning with version 3.7.7, the SQLite database file argument to the sqlite3_open(), sqlite3_open16(), and sqlite3_open_v2() interfaces and to the ATTACH command can be specified either as an ordinary filename or as a Uniform Resource Identifier or URI. The advantage of using a URI filename is that query parameters on the URI can be used to control details of the newly created database connection. For example, an alternative VFS can be specified using a "vfs=" query parameter. Or the database can be opened read-only by using "mode=ro" as a query parameter.
In order to maintain full backwards compatibility for legacy applications, the URI filename capability is disabled by default. URI filenames can be enabled or disabled using the SQLITE_USE_URI=1 or SQLITE_USE_URI=0 compile-time options. The compile-time setting for URI filenames can be changed at start-time using the sqlite3_config(SQLITE_CONFIG_URI,1) or sqlite3_config(SQLITE_CONFIG_URI,0) configuration calls. Regardless of the compile-time or start-time settings, URI filenames can be enabled for individual database connections by including the SQLITE_OPEN_URI bit in the set of bits passed as the F parameter to sqlite3_open_v2(N,P,F,V).
If URI filenames are recognized when the database connection is originally opened, then URI filenames will also be recognized on ATTACH statements. Similarly, if URI filenames are not recognized when the database connection is first opened, they will not be recognized by ATTACH.
Since SQLite always interprets any filename that does not begin with "file:" as an ordinary filename regardless of the URI setting, and because it is very unusual to have an actual file begin with "file:", it is safe for most applications to enable URI processing even if URI filenames are not currently being used.
According to RFC 3986, a URI consists of a scheme, an authority, a path, a query string, and a fragment. The scheme is always required. One of either the authority or the path is also always required. The query string and fragment are optional.
SQLite uses the "file:" URI syntax to identify database files. SQLite strives to interpret file: URIs in exactly the same way as popular web-browsers such as Firefox, Chrome, Safari, Internet Explorer, and Opera, and command-line programs such as Windows "start" and the Mac OS-X "open" command. A succinct summary of the URI parsing rules follows:
Zero or more escape sequences of the form "%HH" (where H represents any hexadecimal digit) can occur in the path, query string, or fragment.
A filename that is not a well-formed URI is interpreted as an ordinary filename.
URIs are processed as UTF8 text. The filename argument sqlite3_open16() is converted from UTF16 native byte order into UTF8 prior to processing.
The path component of the URI specifies the disk file that is the SQLite database to be opened. If the path component is omitted, then the database is stored in a temporary file that will be automatically deleted when the database connection closes. If the authority section is present, then the path is always an absolute pathname. If the authority section is omitted, then the path is an absolute pathname if it begins with the "/" character (ASCII code 0x2f) and is a relative pathname otherwise. On windows, if the absolute path begins with "/X:/" where X is any single ASCII alphabetic character ("a" through "z" or "A" through "Z") then the "X:" is understood to be the drive letter of the volume containing the file, not the toplevel directory.
An ordinary filename can usually be converted into an equivalent URI by the steps shown below. The one exception is that a relative windows pathname with a drive letter cannot be converted directly into a URI; it must be changed into an absolute pathname first.
A URI filename can optionally be followed by a query string. The query string consists of text following the first "?" character but excluding the optional fragment that begins with "#". The query string is divided into key/value pairs. We usually refer to these key/value pairs as "query parameters". Key/value pairs are separated by a single "&" character. The key comes first and is separated from the value by a single "=" character. Both key and value may contain %HH escape sequences.
The text of query parameters is appended to the filename argument of the xOpen method of the VFS. Any %HH escape sequences in the query parameters are resolved prior to being appended to the xOpen filename. A single zero-byte separates the xOpen filename argument from the key of the first query parameters, each key and value, and each subsequent key from the prior value. The list of query parameters appended to the xOpen filename is terminated by a single zero-length key. Note that the value of a query parameter can be an empty string.
Some query parameters are interpreted by the SQLite core and used to modify the characteristics of the new connection. All query parameters are always passed through into the xOpen method of the VFS even if they are previously read and interpreted by the SQLite core.
The following query parameters are recognized by SQLite as of version 3.8.0. Other query parameters might be added to this set in future releases.
The vfs query parameter causes the database connection to be opened using the VFS called NAME. The open attempt fails if NAME is not the name of a VFS that is built into SQLite or that has been previously registered using sqlite3_vfs_register().
The mode query parameter determines if the new database is opened read-only, read-write, read-write and created if it does not exist, or that the database is a pure in-memory database that never interacts with disk, respectively.
The cache query parameter determines if the new database is opened using shared cache mode or with a private cache.
The psow query parameter overrides the powersafe overwrite property of the database file being opened. The psow query parameter works with the default windows and unix VFSes but might be a no-op for other proprietary or non-standard VFSes.
The nolock query parameter is a boolean that disables all calls to the xLock, xUnlock, and xCheckReservedLock methods of the VFS when true. The nolock query parameter might be used, for example, when trying to access a file on a filesystem that does not support file locking. Caution: If two or more database connections try to interact with the same SQLite database and one or more of those connections has enabled "nolock", then database corruption can result. The "nolock" query parameter should only be used if the application can guarantee that writes to the database are serialized.
The immutable query parameter is a boolean that signals to SQLite that the underlying database file is held on read-only media and cannot be modified, even by another process with elevated privileges. SQLite always opens immutable database files read-only and it skips all file locking and change detection on immutable database files. If these query parameter (or the SQLITE_IOCAP_IMMUTABLE bit in xDeviceCharacteristics) asserts that a database file is immutable and that file changes anyhow, then SQLite might return incorrect query results and/or SQLITE_CORRUPT errors.