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There can be multiple databases. Users can select which databases locate searches using an environment variable or command line option; see locate(1L). The system administrator can choose the file name of the default database, the frequency with which the databases are updated, and the directories for which they contain entries. Normally, file name databases are updated by running the updatedb program periodically, typically nightly; see updatedb(1L).
updatedb runs a program called frcode to compress the list of file names using front-compression, which reduces the database size by a factor of 4 to 5. Front-compression (also known as incremental encoding) works as follows.
The database entries are a sorted list (case-insensitively, for users' convenience). Since the list is sorted, each entry is likely to share a prefix (initial string) with the previous entry. Each database entry begins with an offset-differential count byte, which is the additional number of characters of prefix of the preceding entry to use beyond the number that the preceding entry is using of its predecessor. (The counts can be negative.) Following the count is a null-terminated ASCII remainder the part of the name that follows the shared prefix.
If the offset-differential count is larger than can be stored in a byte (+/-127), the byte has the value 0x80 and the count follows in a 2-byte word, with the high byte first (network byte order).
Every database begins with a dummy entry for a file called `LOCATE02', which locate checks for to ensure that the database file has the correct format; it ignores the entry in doing the search.
Databases can not be concatenated together, even if the first (dummy) entry is trimmed from all but the first database. This is because the offset-differential count in the first entry of the second and following databases will be wrong.
There is also an old database format, used by Unix locate and find programs and earlier releases of the GNU ones. updatedb runs programs called bigram and code to produce old-format databases. The old format differs from the above description in the following ways. Instead of each entry starting with an offset-differential count byte and ending with a null, byte values from 0 through 28 indicate offset-differential counts from -14 through 14. The byte value indicating that a long offset-differential count follows is 0x1e (30), not 0x80. The long counts are stored in host byte order, which is not necessarily network byte order, and host integer word size, which is usually 4 bytes. They also represent a count 14 less than their value. The database lines have no termination byte; the start of the next line is indicated by its first byte having a value <= 30.
In addition, instead of starting with a dummy entry, the old database format starts with a 256 byte table containing the 128 most common bigrams in the file list. A bigram is a pair of adjacent bytes. Bytes in the database that have the high bit set are indexes (with the high bit cleared) into the bigram table. The bigram and offset-differential count coding makes these databases 20-25% smaller than the new format, but makes them not 8-bit clean. Any byte in a file name that is in the ranges used for the special codes is replaced in the database by a question mark, which not coincidentally is the shell wildcard to match a single character.
Output from frcode, with trailing nulls changed to newlines and count bytes made printable:
Input to frcode:
Length of the longest prefix of the preceding entry to share:
(6 = 14 - 8, and -9 = 5 - 14)
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