gdbm.texinfo

开源的数据库管理系统

\input texinfo	@c -*- Texinfo -*-
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@setfilename gdbm.info
@settitle gdbm
@dircategory Programming & development tools
@direntry
* GDBM: (gdbm).			The GNU database manager.
@end direntry
@c @setchapternewpage odd
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@iftex
@finalout
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@ifinfo
This file documents the GNU dbm utility.

Copyright (C) 1989-1999 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end ifinfo

@titlepage
@null
@sp 6
@center @titlefont{GNU dbm}
@sp 2
@center A Database Manager
@sp 2
@center by Philip A. Nelson and Jason Downs
@sp 4
@center Manual by Pierre Gaumond, Philip A. Nelson and Jason Downs
@sp 1
@center Edition 1.5
@sp 1
@center for GNU @code{dbm}, Version 1.8.3.
@page
@null
@vskip 0pt plus 1filll
Copyright @copyright{} 1993-99 Free Software Foundation, Inc.
@sp 2

This is Edition 1.5 of the @cite{GNU @code{dbm} Manual}, for @code{gdbm}
Version 1.8.3. @*
Last updated October 15, 2002

Published by the Free Software Foundation @*
675 Massachusetts Avenue, @*
Cambridge, MA 02139 USA @*

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Free Software Foundation.
@end titlepage
@page

@node Top, Copying, (dir), (dir)
@ifinfo
GNU @code{dbm} is a library of functions implementing a hashed database
on a disk file.  This manual documents GNU @code{dbm} Version 1.8.3
(@code{gdbm}).  The software was written by Philip A. Nelson. This
document was originally written by Pierre Gaumond from texts written by
Phil.
@end ifinfo


@menu
Introduction:

* Copying::                    Your rights.
* Intro::                      Introduction to GNU dbm.
* List::                       List of functions.

Functions:

* 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.
* Errors::                     Convert internal error codes into English.
* Options::                    Setting internal options.
* Locking::                    File locking.

Other topics:

* Variables::                  Two useful variables.
* Compatibility::              Compatibility with UNIX dbm and ndbm.
* Conversion::                 Converting dbm files to gdbm format.
* Bugs::                       Problems and bugs.
@end menu

@node Copying, Intro, Top, Top
@chapter Copying Conditions.
This library is @dfn{free}; this means that everyone is free to use
it and free to redistribute it on a free basis.  GNU @code{dbm} (@code{gdbm})
is not in the public domain; it is copyrighted and there
are restrictions on its distribution, but these restrictions are
designed to permit everything that a good cooperating citizen would want
to do.  What is not allowed is to try to prevent others from further
sharing any version of @code{gdbm} that they might get from
you.@refill

  Specifically, we want to make sure that you have the right to give
away copies @code{gdbm}, that you receive
source code or else can get it if you want it, that you can change these
functions or use pieces of them in new free programs, and that you know
you can do these things.@refill

  To make sure that everyone has such rights, we have to forbid you to
deprive anyone else of these rights.  For example, if you distribute
copies @code{gdbm}, you must give the recipients all
the rights that you have.  You must make sure that they, too, receive or
can get the source code.  And you must tell them their rights.@refill

  Also, for our own protection, we must make certain that everyone finds
out that there is no warranty for anything in the @code{gdbm} distribution.
If these functions are modified by someone else and passed on, we want
their recipients to know that what they have is not what we distributed,
so that any problems introduced by others will not reflect on our
reputation.@refill

@code{gdbm} is currently distributed under the terms of the GNU General
Public License, Version 2.  (@emph{NOT} under the GNU General Library
Public License.)  A copy the GNU General Public License is included with
the distribution of @code{gdbm}.

@node Intro, List, Copying, Top
@chapter Introduction to GNU @code{dbm}.

GNU @code{dbm} (@code{gdbm})is a library of database functions that use
extendible hashing and works similar to the standard UNIX @code{dbm}
functions.  These routines are provided to a programmer needing to
create and manipulate a hashed database. (@code{gdbm} is @emph{NOT} a
complete database package for an end user.)

The basic use of @code{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 basic unit
of data in @code{gdbm} is the structure:

@example
  typedef struct @{
             char *dptr;
             int  dsize;
          @} datum;
@end example

This structure allows for arbitrary sized keys and data items.

The key/data pairs are stored in a @code{gdbm} disk file, called a
@code{gdbm} database.  An application must open a @code{gdbm} database
to be able manipulate the keys and data contained in the database.
@code{gdbm} allows an application to have multiple databases open at the
same time.  When an application opens a @code{gdbm} database, it is
designated as a @code{reader} or a @code{writer}.  A @code{gdbm}
database opened by at most one writer at a time.  However, many readers
may open the database open simultaneously.  Readers and writers can not
open the @code{gdbm} database at the same time.

@node List, Open, Intro, Top
@chapter List of functions.

The following is a quick list of the functions contained in the @code{gdbm}
library. The include file @code{gdbm.h}, that can be included by the user,
contains a definition of these functions.

@example
#include <gdbm.h>

GDBM_FILE gdbm_open(name, block_size, flags, mode, fatal_func);
void gdbm_close(dbf);
int gdbm_store(dbf, key, content, flag);
datum gdbm_fetch(dbf, key);
int gdbm_delete(dbf, key);
datum gdbm_firstkey(dbf);
datum gdbm_nextkey(dbf, key);
int gdbm_reorganize(dbf);
void gdbm_sync(dbf);
int gdbm_exists(dbf, key);
char *gdbm_strerror(errno);
int gdbm_setopt(dbf, option, value, size);
int gdbm_fdesc(dbf);
@end example

The @code{gdbm.h} include file is often in the @file{/usr/local/include}
directory. (The actual location of @code{gdbm.h} depends on your local
installation of @code{gdbm}.)

@node Open, Close, List, Top
@chapter Opening the database.

Initialize @code{gdbm} system. 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 @code{gdbm} file is:

@example
GDBM_FILE dbf;

dbf = gdbm_open(name, block_size, flags, mode, fatal_func);
@end example

The parameters are:

@table @asis
@item char *name
The name of the file (the complete name, @code{gdbm} does not append any
characters to this name).
@item int 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 @code{block_size} is used.
@item int flags
If @code{flags} is set to 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 @code{flags} is
set to GDBM_WRITER, the user wants both read and write access to the database
and requires exclusive access. If @code{flags} is set to 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 @code{flags} is set to 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: GDBM_SYNC, which causes all database operations
to be synchronized to the disk, and GDBM_NOLOCK, which prevents the library
from performing any locking on the database file.  The option GDBM_FAST is
now obsolete, since @code{gdbm} defaults to no-sync mode.  Any error detected
will cause a return value of NULL and an appropriate value will be in
@code{gdbm_errno} (see Variables). If no errors occur, a pointer to the
@code{gdbm} file descriptor will be returned.
@item int mode
File mode (see chmod(2) and open(2) if the file is created).
@item void (*fatal_func) ()
A function for @code{gdbm} to call if it detects a fatal error. The only
parameter of this function is a string. If the value of NULL is provided,
@code{gdbm} will use a default function.
@end table

The return value, @code{dbf}, is the pointer needed by all other functions to
access that @code{gdbm} file. If the return is the NULL pointer,
@code{gdbm_open} was not successful. The errors can be found in
@code{gdbm_errno} for @code{gdbm} errors and in @code{errno} for file system
errors (for error codes, see @code{gdbm.h}).

In all of the following calls, the parameter @code{dbf} refers to the pointer
returned from @code{gdbm_open}.

@node Close, Store, Open, Top
@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. This is done by:

@example
gdbm_close(dbf);
@end example

The parameter is:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@end table

Closes the @code{gdbm} file and frees all memory associated with the file 
@code{dbf}. 

@node Store, Fetch, Close, Top
@chapter Inserting and replacing records in the database.

The function @code{gdbm_store} inserts or replaces records in the database.

@example
ret = gdbm_store(dbf, key, content, flag);
@end example

The parameters are:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@item datum key
The @code{key} data.
@item datum content
The data to be associated with the key.
@item int flag
Defines the action to take when the key is already in the database. The value
GDBM_REPLACE (defined in @code{gdbm.h}) asks that the old data be replaced by
the new @code{content}. The value GDBM_INSERT asks that an error be returned
and no action taken if the @code{key} already exists.
@end table

The values returned in @code{ret} are:

@table @asis
@item -1
The item was not stored in the database because the caller was not an
official writer or either @code{key} or @code{content} have a NULL dptr field.
Both @code{key} and @code{content} must have the dptr field be a non-NULL value.
Since a NULL dptr field is used by other functions to indicate an error, a NULL
field cannot be valid data.
@item +1
The item was not stored because the argument @code{flag} was GDBM_INSERT and
the @code{key} was already in the database.
@item 0
No error. @code{content} is keyed by @code{key}. The file on disk is updated
to reflect the structure of the new database before returning from this
function.
@end table

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

The size in @code{gdbm} is not restricted like @code{dbm} or @code{ndbm}. Your
data can be as large as you want.

@node Fetch, Delete, Store,Top
@chapter Searching for records in the database.

Looks up a given @code{key} and returns the information associated with that
key. The pointer in the structure that is  returned is a pointer to dynamically
allocated memory block. To search for some data:

@example
content = gdbm_fetch(dbf, key);
@end example

The parameters are:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@item datum key
The @code{key} data.
@end table

The datum returned in @code{content} is a pointer to the data found. If the
dptr is NULL, no data was found. If dptr is not NULL, then it points
to data allocated by malloc. @code{gdbm} does not automatically free this data.
The user must free this storage when done using it. This eliminates the
need to copy the result to save it for later use (you just save the pointer).

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

@example
ret = gdbm_exists(dbf, key);
@end example

The parameters are:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@item datum key
The @code{key} data.
@end table

Unlike @code{gdbm_fetch}, this routine does not allocate any memory, and
simply returns true or false, depending on whether the @code{key} exists,
or not.