locatedb - front-compressed file name database
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
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.
Input to frcode:
Length of the longest prefix of the preceding entry to share:
Output from frcode, with trailing nulls changed to newlines and count
bytes made printable:
(6 = 14 - 8, and -9 = 5 - 14)
find(1L), locate(1L), locatedb(5L), xargs(1L) Finding Files (on-line in
Info, or printed)
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