8+3 Filenames

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# SQLite And 8+3 Filenames

The default configuration of SQLite assumes the underlying filesystem supports long filenames.

SQLite does not impose any naming requirements on database files. SQLite will happily work with a database file that has any filename extension or with no extension at all. When auxiliary files are needed for a rollback journal or a write-ahead log or for one of the other kinds of temporary disk files, then the name for the auxiliary file is normally constructed by appending a suffix onto the end of the database file name. For example, if the original database is call "`app.db`" then the rollback journal will be called "`app.db-journal`" and the write-ahead log will be called "`app.db-wal`". This approach to auxiliary file naming works great on systems that support long filenames. But on systems that impose 8+3 filename constraints, the auxiliary files do not fit the 8+3 format even though the original database file does.

## Changing Filesystems

The recommended fix for this problem is to select a different filesystem. These days, there is a huge selection of high-performance, reliable, patent-free filesystems that support long filenames. Where possible, it is recommended that embedded devices use one of these other filesystems. This will avoid compatibility issues and the danger of database corruption caused by inconsistent use of 8+3 filenames.

## Adjusting SQLite To Use 8+3 Filenames

Some devices are compelled to use an older filesystem with 8+3 filename restrictions for backwards compatibility, or due to other non-technical factors. In such situations, SQLite can be coerced into using auxiliary files that fit the 8+3 pattern as follows:

1. Compile the SQLite library with the either the compile-time options SQLITE_ENABLE_8_3_NAMES=1 or SQLITE_ENABLE_8_3_NAMES=2. Support for 8+3 filenames is not included in SQLite by default because it does introduce some overhead. The overhead is tiny, but even so, we do not want to burden the billions of SQLite applications that do not need 8+3 filename support.

2. If the SQLITE_ENABLE_8_3_NAMES=1 option is used, then SQLite is capable of using 8+3 filenames but that capabilities is disabled and must be enabled separately for each database connection by using using URI filenames when opening or ATTACH-ing the database files and include the "`8_3_names=1`" query parameter in the URI. If SQLite is compiled with SQLITE_ENABLE_8_3_NAMES=2 then 8+3 filenames are enabled by default and this step can be skipped.

3. Make sure that database filenames follow the 8+3 filename format and that they do not have an empty name or extension. In other words, the database filename must contain between 1 and 8 characters in the base name and between 1 and 3 characters in the extension. Blank extensions are not allowed.

When the steps above are used, SQLite will shorten filename extensions by only using the last 3 characters of the extension. Thus, for example, a file that would normally be called "`app.db-journal`" is shortened to just "`app.nal`". Similarly, "`app.db-wal`" will become "`app.wal`" and "`app.db-shm`" becomes "`app.shm`".

Note that it is very important that the database filename have some kind of extension. If there is no extension, then SQLite creates auxiliary filenames by appending to the base name of the file. Thus, a database named "`db01`" would have a rollback journal file named "`db01-journal`". And as this filename has no extension to shorten to 3 characters, it will be used as-is, and will violate 8+3 naming rules.

## Database Corruption Warning

If a database file is accessed using 8+3 naming rather than the default long filename, then it must be consistently accessed using 8+3 naming by every database connection every time it is opened, or else there is a risk of database corruption. The auxiliary rollback journal and write-ahead log files are essential to SQLite for being about to recover from a crash. If an application is using 8+3 names and crashes, then the information needed to safely recover from the crash is stored in files with the "`.nal`" or "`.wal`" extension. If the next application to open the database does not specify the "`8_3_names=1`" URI parameter, then SQLite will use the long filenames to try to locate the rollback journal or write-ahead log files. It will not find them, since they were saved using 8+3 names by the application that crashed, and hence the database will not be properly recovered and will likely go corrupt.

Using a database file with 8+3 filenames in some cases while in other cases using long filenames is equivalent to deleting a hot journal.

This page last modified on 2011-08-15 23:00:56 UTC