Ubuntu Manpage: dbminit, fetch, store, dbmclose - somewhat dbm-compatible database routines

NAME

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

SYNOPSIS

       #include <dbz.h>

       dbminit(base)
       char *base;

       datum
       fetch(key)
       datum key;

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

       dbmclose()

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

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

       datum
       dbzfetch(key)
       datum key;

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

       dbzsync()

       long
       dbzsize(nentries)
       long nentries;

       dbzincore(newvalue)

       dbzcancel()

       dbzdebug(newvalue)

DESCRIPTION

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
dbzfresh(name,0,'\t','?',0).

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.

DIAGNOSTICS

       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.

HISTORY

       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.

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