LOCATEDB(5L) UNIX Programmer's Manual LOCATEDB(5L)NAME
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 con-
tain 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(1L). The system administra-
tor 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 prede-
cessor. (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 data-
base 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 data-
base. 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.
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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 coin-
cidentally is the shell wildcard to match a single charac-
ter.
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)
SEE ALSOfind(1L), locate(1L), locatedb(5L), xargs(1L) Finding Files
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LOCATEDB(5L) UNIX Programmer's Manual LOCATEDB(5L)
(on-line in Info, or printed)
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