\input texinfo @c -*-texinfo-*-
@comment %**start of header
@setfilename mit-scheme-gdbm.info
@include version.texi
@set SCMVERS 11.0.90
@settitle MIT/GNU Scheme GDBM Plugin Manual
@comment %**end of header

@copying
This manual documents MIT/GNU Scheme GDBM @value{VERSION}.

Copyright @copyright{} 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993,
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    2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015,
    2016, 2017, 2018, 2019, 2020 Massachusetts Institute of Technology
Copyright @copyright{} 1993-99 Free Software Foundation, Inc.

@quotation
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@ignore
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Permission is granted to copy and distribute modified versions of this
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Permission is granted to copy and distribute translations of this manual
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@end copying

@dircategory Programming Languages
@direntry
* MIT/GNU Scheme GDBM: (mit-scheme-gdbm).
                                GNU database manager plugin
@end direntry

@titlepage
@title MIT/GNU Scheme GDBM Plugin Manual
@subtitle a GNU database manager plugin (version @value{VERSION})
@subtitle for MIT/GNU Scheme version @value{SCMVERS}
@subtitle @value{UPDATED}
@author by Matt Birkholz
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@ifnottex
@node Top
@top GDBM Plugin Manual

@insertcopying
@end ifnottex

@menu
* Introduction::

Functions:

* List::		       The exported bindings.
* Open::                       Opening the database.
* Close::                      Closing the database.
* Store::                      Inserting and replacing records in the database.
* Fetch::                      Searching records in the database.
* Delete::                     Removing records from the database.
* Sequential::                 Sequential access to records.
* Reorganization::             Database reorganization.
* Sync::                       Insure all writes to disk have competed.
* Options::                    Setting internal options.
@end menu


@node Introduction
@chapter Introduction to GNU dbm.

This plugin is a dynamically loadable wrapper for the GNU dbm
(DataBase Management) C library.  This manual is a derivative of
Edition 1.5 of the @cite{GNU dbm Manual}, for library version
1.8.3, last updated October 15, 2002.

GNU dbm (gdbm) is a library of database functions that
use extendible hashing; it works similarly to the standard UNIX dbm
functions.
The basic use of gdbm is to store key/data pairs in a data file.
Each key must be unique and each key is paired with only one data item.
The keys can not be directly accessed in sorted order.

The key/data pairs are stored in a gdbm disk file, called a gdbm
database.  A program must connect to a gdbm database to be able
manipulate the keys and data contained in it.  Gdbm allows Scheme to
connect to multiple databases at the same time.  When Scheme connects
to a gdbm database, the connection is designated as a @dfn{reader} or
a @dfn{writer}.  A gdbm database may be connected to at most one
writer at a time.  However, many readers may connect to the database
simultaneously.  Readers and writers may not connect to the database
at the same time.  (Note that these restrictions are not enforced by
the library nor the wrapper.)

Each connection is encapsulated in a Scheme @code{gdbf} structure
which should be used by one Scheme thread at a time.  A mutex is used
to block any thread attempting to access the database while an
operation is in progress.  No file locks are used by gdbm or the
Scheme wrapper to ensure exclusive access by a Scheme writer.

@node List
@chapter The exported bindings.

The following is a quick list of the procedures provided by the plugin.

@example
	  gdbm-open
	  gdbm-close
	  gdbm-store
	  gdbm-fetch
	  gdbm-delete
	  gdbm-firstkey
	  gdbm-nextkey
	  gdbm-reorganize
	  gdbm-sync
	  gdbm-exists?
	  gdbm-setopt
@end example

Neither @code{gdbm_errno} nor @code{gdbm_strerror} are exposed because
the plugin automatically tests and calls them to detect errors and
convert codes into strings.  @code{gdbm_fdesc} is also not exposed,
treated as an implementation detail the plugin should probably hide,
used by tricky code that cooperates with multiple file locking
libraries.

There is one global variable, @code{gdbm-version}, which is
initialized from the library's @code{gdbm_version} string.

And several constants:
@example
	  gdbm_cachesize
          gdbm_fast
          gdbm_insert
          gdbm_newdb
          gdbm_reader
          gdbm_replace
          gdbm_wrcreat
          gdbm_writer
@end example

You can load these bindings into your global environment with the
following expresson.
@smallexample
    (load-option 'gdbm)
@end smallexample

And you can include these bindings in your package description
(@file{.pkg}) file with the following expression.
@smallexample
    (global-definitions gdbm/)
@end smallexample


@node Open
@chapter Opening the database.

Connect to the file.  If the file has a size of zero bytes, a file
initialization procedure is performed, setting up the initial structure in the
file.

The procedure for opening a gdbm file is:

@deffn Procedure gdbm-open name block-size flags mode
The parameters are:

@table @var
@item name
The name of the file (the complete name, gdbm does not append any
characters to this name).
@item block-size
It is used during initialization to determine the size of various constructs. It
is the size of a single transfer from disk to memory. This parameter is ignored
if the file has been previously initialized. The minimum size is 512.
If the value is less than 512, the file system blocksize is used, otherwise the
value of @var{block-size} is used.
@item flags
If @var{flags} is @code{gdbm_reader}, the user wants to just read the
database and any call to @code{gdbm-store} or @code{gdbm-delete} will fail.
Many readers can access the database at the same time. If @var{flags} is
@code{gdbm_writer}, the user wants both read and write access to the database
and requires exclusive access. If @var{flags} is @code{gdbm_wrcreat}, the
user wants both read and write access to the database and if the database does
not exist, create a new one. If @var{flags} is @code{gdbm_newdb}, the
user want a new database created, regardless of whether one existed, and wants
read and write access to the new database.  The following may also be logically
or'd into the database flags: @code{gdbm_sync}, which causes all database operations
to be synchronized to the disk, and @code{gdbm_nolock}, which prevents the library
from performing any locking on the database file.  @code{gdbm_fast} is
now obsolete, since gdbm defaults to no-sync mode.
@item mode
File mode (see chmod(2) and open(2) if the file is created).
@end table

The return value is the object needed by all other procedures to
access that gdbm file.
@end deffn


@node Close
@chapter Closing the database.

It is important that every file opened is also closed. This is needed to
update the reader/writer count on the file.  Scheme will do this
automatically if an open gdbm object is garbage collected, but you can
close the file immediately with the @code{gdbm-close} procedure.

@deffn Procedure gdbm-close dbf
The parameter is:

@table @var
@item dbf
The object returned by @code{gdbm-open}.
@end table

Closes the gdbm file and frees all memory associated with @var{dbf}.
@end deffn


@node Store
@chapter Inserting and replacing records in the database.

The procedure @code{gdbm-store} inserts or replaces records in the database.

@deffn Procedure gdbm-store dbf key content flag
The parameters are:

@table @var
@item dbf
The object returned by @code{gdbm-open}.
@item key
A non-empty string, converted to utf-8 bytes for lookup in the database.
@item content
Another non-empty string, the content to be stored in the database file, also
converted to utf-8.
@item flag
The action to take when @var{key} is already in the database. The value
of @code{gdbm_replace} indicates that the old content should be replaced
by @var{content}. The value of @code{gdbm_insert} indicates that
@code{#f} should be returned and no action taken if @var{key} already
exists.
@end table

The values returned are:

@table @code
@item #t
Success. @var{content} is keyed by @var{key}. The file on disk is updated
to reflect the structure of the new database before returning from this
procedure.
@item #f
The item was not stored because @var{flag} was @code{gdbm_insert} and
@var{key} was already in the database.
@end table

An error is signaled if the caller is not a writer.

If you store content for a key that is already in the database,
gdbm replaces the old content with the new content if called with
@code{gdbm_replace}. You do not get two content items for the same key and you do
not get an error from @code{gdbm-store}.

The size in gdbm is not restricted like dbm or ndbm. Your
content can be as large as you want.
@end deffn


@node Fetch
@chapter Searching for records in the database.

Read content associated with a key.

@deffn Procedure gdbm-fetch dbf key
The parameters are:

@table @var
@item dbf
The object returned by @code{gdbm-open}.
@item key
A non-empty string, converted to utf-8 bytes for lookup in the database.
@end table

The return value is a string created from the utf-8 bytes found in the
database, or @code{#f} if no content was found.
@end deffn

You may also search for a particular key without retrieving it, using:

@deffn Procedure gdbm-exists? dbf key
The parameters are:

@table @var
@item dbf
The pointer returned by @code{gdbm-open}.
@item key
A non-empty string, converted to utf-8 bytes for lookup in the database.
@end table

Unlike @code{gdbm-fetch} this procedure does not read any content and
simply returns true or false depending on whether @var{key} exists.
@end deffn


@node Delete
@chapter Removing records from the database.

To remove some content from the database:

@deffn Procedure gdbm-delete dbg key
The parameters are:

@table @var
@item dbf
The object returned by @code{gdbm-open}.
@item key
A non-empty string, converted to utf-8 bytes for lookup in the database.
@end table

The return value is @code{#f} if the item is not present or the requester is a reader.
The return value is @code{#t} if there was a successful delete.

The keyed content and the key are removed from the database. The file
on disk is updated to reflect the structure of the new database before
returning from this procedure.
@end deffn


@node Sequential
@chapter Sequential access to records.

The next two functions allow for accessing all content in a database
@var{dbf}.  This access is not key sequential, but it is guaranteed to
visit every key in the database once.  (The order has to do with the
hash values.)

@deffn Procedure gdbm-firstkey dbf
Starts the visit of all keys in the database @var{dbf}.
Returns the first key to visit, converting its utf-8 bytes to a string.
If there are no keys, returns @code{#f}.
@end deffn

@deffn Procedure gdbm-nextkey dbf key
Returns the key to visit after @var{key}, converting its utf-8 bytes
to a string.
If there are no more keys, returns @code{#f}.
@end deffn

These functions were intended to visit the database in read-only algorithms,
for instance, to validate the database or similar operations.

Visiting keys traverses a hash table which writers may re-arrange.
The original key order is @emph{not} guaranteed to
remain unchanged in all instances. It is possible that some key will not be
visited if the database is changed while traversing the table.


@node Reorganization
@chapter Database reorganization.

The following procedure should be used very seldom.

@deffn Procedure gdbm-reorganize dbf
If you have made a lot of deletions and would like to shrink the space
used by the gdbm file, this function will reorganize the database.
Gdbm will not shorten a gdbm file (will not reuse deleted space)
until this procedure is called.

The reorganization requires creating a new file and inserting all the elements
in the old file @code{dbf} into the new file. The new file is then renamed to
the same name as the old file and @code{dbf} is updated to contain all the
correct information about the new file.
@end deffn


@node Sync
@chapter Database Synchronization

Unless you opened your database with the @code{gdbm_sync} flag, gdbm does not
wait for writes to be flushed to the disk.  This allows
faster writing of databases at the risk of having a corrupted database if
Scheme terminates in an abnormal fashion.  The following function
allows the programmer to flush all changes to disk.

@deffn Procedure gdbm-sync dbf
This would usually be called after a complete set of changes have been
made to the database and before some long waiting time.
@code{Gdbm-close} always flushes any changes to disk.
@end deffn


@node Options
@chapter Setting options.

Gdbm supports the ability to set certain options on an already
open database.

@deffn Procedure gdbm-setopt dbf option value
The parameters are:

@table @var
@item dbf
The pointer returned by @code{gdbm-open}.
@item option
The option to be set, the value of @code{gdbm_cachesize} or
@code{gdbm_syncmode}.
@item value
The value to be set, an integer.
@end table

If @var{option} is @code{gdbm_cachesize} the size of the internal
bucket cache is set to the given integer.  This option may only be set
once on a database, and is set to 100 by default when the database is
first accessed.

If @var{option} is @code{gdbm_syncmode} file system synchronization is
turned on or off.  By default it is off.  @var{Value} should @code{1}
to turn it on, or @code{0} to turn it off.
@end deffn

The obsolete and experimental options @code{GDBM_FASTMODE},
@code{GDBM_CENTFREE} and @code{GDBM_COALESCEBLKS} are not supported by
this plugin.

@bye
