dbminit,  fetch,  store,  dbmclose  -  somewhat dbm-compatible database
   dbzfresh, dbzagain, dbzfetch, dbzstore - database routines
   dbzsync, dbzsize, dbzincore, dbzcancel, dbzdebug - database routines


   #include <dbz.h>

   char *base;

   datum key;

   store(key, value)
   datum key;
   datum value;


   dbzfresh(base, size, fieldsep, cmap, tagmask)
   char *base;
   long size;
   int fieldsep;
   int cmap;
   long tagmask;

   dbzagain(base, oldbase)
   char *base;
   char *oldbase;

   datum key;

   dbzstore(key, value)
   datum key;
   datum value;


   long nentries;





   These functions provide an indexing system for rapid random access to a
   text  file  (the  base file).  Subject to certain constraints, they are
   call-compatible  with  dbm(3),  although   they   also   provide   some
   extensions.   (Note  that  they are not file-compatible with dbm or any
   variant thereof.)

   In principle, dbz stores key-value pairs, where both key and value  are
   arbitrary  sequences  of bytes, specified to the functions by values of
   type datum, typedefed in the header file to be a structure with members
   dptr  (a value of type char * pointing to the bytes) and dsize (a value
   of type int indicating how long the byte sequence is).

   In practice, dbz is more restricted than dbm.  A dbz database  must  be
   an  index  into  a  base  file, with the database values being fseek(3)
   offsets into the base file.  Each such value must ``point to'' a  place
   in  the base file where the corresponding key sequence is found.  A key
   can be no longer than DBZMAXKEY (a constant defined in the header file)
   bytes.   No key can be an initial subsequence of another, which in most
   applications requires that keys be either bracketed  or  terminated  in
   some  way  (see  the  discussion of the fieldsep parameter of dbzfresh,
   below, for a fine point on terminators).

   Dbminit opens a database, an index into the base file base,  consisting
   of  files  base.dir  and  base.pag  which  must already exist.  (If the
   database  is  new,  they  should  be  zero-length  files.)   Subsequent
   accesses  go  to  that  database  until dbmclose is called to close the
   database.  The base file need not exist at the time of the dbminit, but
   it must exist before accesses are attempted.

   Fetch  searches  the  database  for  the  specified  key, returning the
   corresponding value if any.  Store stores the  key-value  pair  in  the
   database.   Store  will  fail  unless the database files are writeable.
   See below for a complication arising from case mapping.

   Dbzfresh is a variant of dbminit for creating a new database with  more
   control  over details.  Unlike for dbminit, the database files need not
   exist: they will be created if necessary, and truncated in any case.

   Dbzfresh's size parameter specifies the size of the  first  hash  table
   within  the  database, in key-value pairs.  Performance will be best if
   size is a prime number and the number of key-value pairs stored in  the
   database  does  not  exceed  about 2/3 of size.  (The dbzsize function,
   given the expected number of key-value pairs, will suggest  a  database
   size  that  meets  these criteria.)  Assuming that an fseek offset is 4
   bytes, the .pag file will be 4*size bytes (the .dir file  is  tiny  and
   roughly  constant  in size) until the number of key-value pairs exceeds
   about 80% of size.  (Nothing awful will happen if  the  database  grows
   beyond  100% of size, but accesses will slow down somewhat and the .pag
   file will grow somewhat.)

   Dbzfresh's fieldsep parameter specifies the field separator in the base
   file.  If this is not NUL (0), and the last character of a key argument
   is NUL, that NUL compares equal to either a NUL or a  fieldsep  in  the
   base  file.   This  permits use of NUL to terminate key strings without
   requiring that NULs appear  in  the  base  file.   The  fieldsep  of  a
   database created with dbminit is the horizontal-tab character.

   For  use in news systems, various forms of case mapping (e.g. uppercase
   to lowercase) in keys are available.  The cmap parameter to dbzfresh is
   a  single  character  specifying which of several mapping algorithms to
   use.  Available algorithms are:

          0      case-sensitive:  no case mapping

          B      same as 0

          NUL    same as 0

          =      case-insensitive:  uppercase and lowercase equivalent

          b      same as =

          C      RFC822 message-ID rules, case-sensitive before `@'  (with
                 certain exceptions) and case-insensitive after

          ?      whatever the local default is, normally C

   Mapping  algorithm  0  (no  mapping)  is  faster than the others and is
   overwhelmingly  the  correct  choice  for  most  applications.   Unless
   compatibility  constraints  interfere,  it is more efficient to pre-map
   the keys, storing mapped keys in the base file, than to have dbz do the
   mapping on every search.

   For  historical  reasons, fetch and store expect their key arguments to
   be pre-mapped, but expect unmapped keys in the base file.  Dbzfetch and
   dbzstore  do  the  same jobs but handle all case mapping internally, so
   the customer need not worry about it.

   Dbz stores only the database values in its files, relying on  reference
   to  the  base  file  to confirm a hit on a key.  References to the base
   file can be minimized, greatly speeding up searches, if a little bit of
   information  about  the  keys  can be stored in the dbz files.  This is
   ``free'' if there are some unused bits in an fseek offset, so that  the
   offset  can be tagged with some information about the key.  The tagmask
   parameter of dbzfresh allows specifying the location  of  unused  bits.
   Tagmask should be a mask with one group of contiguous 1 bits.  The bits
   in the mask should be unused (0) in most offsets.  The bit  immediately
   above  the  mask  (the  flag  bit) should be unused (0) in all offsets;
   (dbz)store will reject attempts to store a key-value pair in which  the
   value  has  the  flag  bit on.  Apart from this restriction, tagging is
   invisible to the user.  As a special case, a tagmask of  1  means  ``no
   tagging'',  for use with enormous base files or on systems with unusual
   offset representations.

   A size of 0 given to dbzfresh is synonymous with the local default; the
   normal default is suitable for tables of 90-100,000 key-value pairs.  A
   cmap of 0 (NUL) is synonymous with the character 0, signifying no  case
   mapping  (note  that  the  character  ?   specifies  the  local default
   mapping, normally C).  A tagmask of 0  is  synonymous  with  the  local
   default  tag  mask,  normally  0x7f000000  (specifying the top bit in a
   32-bit offset as the flag bit, and the next 7 bits as the  mask,  which
   is  suitable  for  base files up to circa 24MB).  Calling dbminit(name)
   with   the   database   files   empty   is   equivalent   to    calling

   When databases are regenerated periodically, as in news, it is simplest
   to pick the parameters for a new database based on the old  one.   This
   also  permits  some memory of past sizes of the old database, so that a
   new database  size  can  be  chosen  to  cover  expected  fluctuations.
   Dbzagain  is  a variant of dbminit for creating a new database as a new
   generation of an old database.  The database  files  for  oldbase  must
   exist.   Dbzagain is equivalent to calling dbzfresh with the same field
   separator, case mapping, and tag mask as the old database, and  a  size
   equal  to  the  result  of  applying  dbzsize  to the largest number of
   entries in the oldbase database and its previous 10 generations.

   When many accesses are being done by the same program, dbz is massively
   faster if its first hash table is in memory.  If an internal flag is 1,
   an attempt is made to read the table in when the  database  is  opened,
   and  dbmclose  writes it out to disk again (if it was read successfully
   and has been modified).  Dbzincore sets the  flag  to  newvalue  (which
   should  be 0 or 1) and returns the previous value; this does not affect
   the status of a database that has already been opened.  The default  is
   0.   The  attempt to read the table in may fail due to memory shortage;
   in this case dbz quietly falls back on its default behavior.  Stores to
   an  in-memory  database  are  not  (in general) written out to the file
   until dbmclose or dbzsync, so if robustness in the presence of  crashes
   or  concurrent accesses is crucial, in-memory databases should probably
   be avoided.

   Dbzsync causes all buffers etc. to be flushed out to the files.  It  is
   typically  used  as a precaution against crashes or concurrent accesses
   when a dbz-using process will be running for a  long  time.   It  is  a
   somewhat expensive operation, especially for an in-memory database.

   Dbzcancel  cancels  any pending writes from buffers.  This is typically
   useful only for in-core databases,  since  writes  are  otherwise  done
   immediately.   Its  main purpose is to let a child process, in the wake
   of a fork, do a dbmclose without writing its parent's data to disk.

   If dbz has been compiled with  debugging  facilities  available  (which
   makes  it  bigger  and  a  bit  slower), dbzdebug alters the value (and
   returns the previous value) of an internal flag which (when 1;  default
   is 0) causes verbose and cryptic debugging output on standard output.

   Concurrent  reading  of  databases  is  fairly  safe,  but  there is no
   (inter)locking, so concurrent updating is not.

   The database files include a record of the byte order of the  processor
   creating  the  database, and accesses by processors with different byte
   order will work, although they will be slightly slower.  Byte order  is
   preserved  by  dbzagain.   However,  agreement on the size and internal
   structure of an fseek offset is  necessary,  as  is  consensus  on  the
   character set.

   An  open  database occupies three stdio streams and their corresponding
   file descriptors; a fourth is needed for an in-memory database.  Memory
   consumption  is  negligible  (except  for stdio buffers) except for in-
   memory databases.


   dbz(1), dbm(3)


   Functions returning int values return 0 for success,  -1  for  failure.
   Functions  returning  datum values return a value with dptr set to NULL
   for failure.  Dbminit attempts to have errno set plausibly  on  return,
   but  otherwise  this  is not guaranteed.  An errno of EDOM from dbminit
   indicates that the database did not appear to be in dbz format.


   The  original  dbz  was  written  by   Jon   Zeeff   (zeeff@b-tech.ann-
   arbor.mi.us).   Later  contributions  by  David Butler and Mark Moraes.
   Extensive reworking, including this  documentation,  by  Henry  Spencer
   (henry@zoo.toronto.edu)  as  part  of  the  C  News  project.   Hashing
   function by Peter Honeyman.


   The dptr members of returned datum values point to static storage which
   is overwritten by later calls.

   Unlike  dbm,  dbz  will  misbehave  if  an  existing  key-value pair is
   `overwritten' by a new (dbz)store with  the  same  key.   The  user  is
   responsible  for  avoiding  this by using (dbz)fetch first to check for
   duplicates; an internal optimization remembers the result of the  first
   search so there is minimal overhead in this.

   Waiting  until after dbminit to bring the base file into existence will
   fail if chdir(2) has been used meanwhile.

   The RFC822 case mapper implements only a  first  approximation  to  the
   hideously-complex RFC822 case rules.

   The prime finder in dbzsize is not particularly quick.

   Should implement the dbm functions delete, firstkey, and nextkey.

   On  C  implementations  which trap integer overflow, dbz will refuse to
   (dbz)store an fseek offset equal to the greatest representable positive
   number, as this would cause overflow in the biased representation used.

   Dbzagain  perhaps ought to notice when many offsets in the old database
   were too big for tagging, and shrink the tag mask to match.

   Marking dbz's file descriptors close-on-exec would be a better approach
   to  the  problem  dbzcancel  tries  to address, but that's harder to do


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