locatedb - front-compressed file name database
This manual page documents the format of file name databases for the
GNU version of
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
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
program periodically, typically nightly; see
GNU LOCATE02 database format
This is the default format of databases produced by
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 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',
checks for to ensure that the database file has the
correct format; it ignores the entry in doing the search.
Databases cannot 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
slocate database format
program uses a database format similar to, but not quite the same as,
The first byte of the database specifies its
If the security level is 0,
will read, match and print filenames on the basis of the information
in the database only. However, if the security level byte is 1,
omits entries from its output if the invoking user is unable to access
them. The second byte of the database is zero. The second byte is
followed by the first database entry. The first entry in the database
is not preceded by any differential count or dummy entry. Instead
the differential count for the first item is assumed to be zero.
Starting with the second entry (if any) in the database, data is
interpreted as for the GNU LOCATE02 format.
Old Locate Database format
There is also an old database format, used by Unix
programs and earlier releases of the GNU ones. updatedb
programs called bigram
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)
(on-line in Info, or printed)
The best way to report a bug is to use the form at
The reason for this is that you will then be able to track progress in
fixing the problem. Other comments about locate(1)
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