Provided by: locate_4.2.32-1ubuntu2_i386 bug


       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(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 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:
       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)


       find(1), locate(1), locatedb(5), xargs(1)  Finding  Files  (on-line  in
       Info, or printed)


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