From Linux Man Pages
locatedb - front-compressed file name database
DESCRIPTION
This manual page documents the format of file name databases for the GNU version of locate. The file name
databases contain lists of files that were in particular directory trees when the databases were last updated.
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).
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 fol-
lows.
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 con-
taining 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.
EXAMPLE
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/zoo
Output 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)
RELATED
find(1), locate(1), locatedb(5), xargs(1) Finding Files (on-line in Info, or printed)
BUGS
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 bug-findutils-request@gnu.org.
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