LOCATEDB(5L)LOCATEDB(5L)NAMElocatedb - 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(1L). The system adminis-
trator 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-insensi-
tively, 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 pre-
ceding 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 contain-
ing 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 wild-
card 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 new-
lines 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 (on-line in Info, or printed)
LOCATEDB(5L)
