There can be multiple databases. Users can select which databases locate searches using an environment variable or command line option; see locate(1). 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(1).
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 signed 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 signed byte (+/-127), the byte has the value 0x80 (binary 10000000) and the actual count follows in a 2-byte word, with the high byte first (network byte order). This count can also be negative (the sign bit being in the first of the two bytes).
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.
In the future, the data within the locate database may not be sorted in any particular order. To obtain sorted results, pipe the output of locate through sort -f.
Starting with the second entry (if any) in the database, data is interpreted as for the GNU LOCATE02 format.
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.
Input to frcode: /usr/src /usr/src/cmd/aardvark.c /usr/src/cmd/armadillo.c /usr/tmp/zoo Length of the longest prefix of the preceding entry to share: 0 /usr/src 8 /cmd/aardvark.c 14 rmadillo.c 5 tmp/zooOutput from frcode, with trailing nulls changed to newlines and count bytes made printable:
0 LOCATE02 0 /usr/src 8 /cmd/aardvark.c 6 rmadillo.c -9 tmp/zoo (6 = 14 - 8, and -9 = 5 - 14)
The best way to report a bug is to use the form at http://savannah.gnu.org/bugs/?group=findutils. The reason for this is that you will then be able to track progress in fixing the problem. Other comments about locate(1) and about the findutils package in general can be sent to the bug-findutils mailing list. To join the list, send email to firstname.lastname@example.org.