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@settitle gdbm manual

@ifinfo
@dircategory Programming & development tools
@direntry
* GDBM: (gdbm).                 The GNU database manager.
@end direntry
@end ifinfo

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@c Use @kwindex for keywords
@defcodeindex kw
@syncodeindex kw cp
@c Use @flindex for files
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@c Merge all indices into a single one
@syncodeindex fn cp
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@iftex
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@copying
This file documents the GNU dbm utility.

Copyright @copyright{} 1989-1999, 2007, 2008, 2009-2011 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``The GNU Database
Manager,'' and with the Back-Cover Texts as in (a) below.  A copy of the
license is included in the section entitled ``GNU Free Documentation
License.''

(a) The FSF's Back-Cover Text is: ``You have the freedom to
copy and modify this GNU manual.  Buying copies from the FSF
supports it in developing GNU and promoting software freedom.''
@end copying

@titlepage
@sp 6
@center @titlefont{GNU dbm}
@sp 2
@center A Database Manager
@sp 2
@center by Philip A.  Nelson, Jason Downs and Sergey Poznyakoff
@sp 4
@center Manual by Pierre Gaumond, Philip A.  Nelson, Jason Downs
@center and Sergey Poznyakoff
@sp 1
@center Edition @value{EDITION}
@sp 1
@center for GNU @code{dbm}, Version @value{VERSION}
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1993-1999, 2007-2011 Free Software Foundation, Inc.
@sp 2

This is Edition @value{EDITION} of the @cite{GNU @code{dbm} Manual},
for @code{gdbm} Version @value{VERSION}.  @*
Last updated @value{UPDATED}

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

@ifnothtml
@page
@summarycontents
@page
@end ifnothtml
@contents

@ifnottex
@node Top
@top The GNU database manager.

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

@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.
* Flat files::                 Export and import to Flat file format.
* Errors::                     Convert internal error codes into English.
* Options::                    Setting internal options.
* Locking::                    File locking.

Programs

* testgdbm::                   Test and modify a GDBM database. 
* gdbmexport::                 Export a database into a portable format.  

Other topics:

* Error codes::                Error codes returned by @code{gdbm} calls.
* Variables::                  Two useful variables.
* Compatibility::              Compatibility with UNIX dbm and ndbm.
* Bugs::                       Problems and bugs.
* Resources::                  Additional resources,

* GNU Free Documentation License::      Document license.
* Index::                       Index
@end menu

@node Copying
@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 3.  (@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
@chapter Introduction to GNU @code{dbm}.

GNU @code{dbm} (@code{gdbm}) is a library of database functions that use
extensible 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 can be opened by at most one writer at a time.  However, many readers
may open the database simultaneously.  Readers and writers can not
open the @code{gdbm} database at the same time.

@node List
@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
@chapter Opening the database.

@cindex opening the database
@cindex database, opening or creating
@deftypefn {gdbm interface} GDBM_FILE gdbm_open (const char *@var{name}, int @var{block_size}, @
  int @var{flags}, int @var{mode}, void (*fatal_func)(const char *))
Initializes @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 arguments are:

@table @var
@item name
The name of the file (the complete name, @code{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 block size is used, otherwise the value of
@var{block_size} is used.
@item flags
@kwindex GDBM_READER
@kwindex GDBM_WRITER
@kwindex GDBM_WRCREAT
@kwindex GDBM_NEWDB
If @code{flags} is set to @samp{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 @samp{GDBM_WRITER}, the user wants both read and write access
to the database and requires exclusive access.  If @code{flags} is set
to @samp{GDBM_WRCREAT}, the user wants both read and write access to
the database and wants it created if it does not already exist.  If
@code{flags} is set to @samp{GDBM_NEWDB}, the user want a new database
created, regardless of whether one existed, and wants read and write
access to the new database.

@kwindex GDBM_SYNC
@kwindex GDBM_NOLOCK
@kwindex GDBM_NOMMAP
The following may also be logically or'd into the database flags:
@samp{GDBM_SYNC}, which causes all database operations to be
synchronized to the disk, @samp{GDBM_NOLOCK}, which prevents the library 
from performing any locking on the database file, and @samp{GDBM_NOMMAP},
which disables the memory mapping mechanism.  The option @samp{GDBM_FAST} is
now obsolete, since @code{gdbm} defaults to no-sync mode.

@kwindex GDBM_CLOEXEC
@cindex close-on-exec
If the host @samp{open} call
@ifhtml
(@uref{http://www.manpagez.com/man/2/open, open(2)})
@end ifhtml
@ifnothtml
(@pxref{open,,,open(2),open(2) man page})
@end ifnothtml
supports the @samp{O_CLOEXEC} flag, the @samp{GDBM_CLOEXEC} can be
or'd into the flags, to enable the close-on-exec flag for the
database file descriptor.
@item mode
File mode (see
@ifhtml
@uref{http://www.manpagez.com/man/2/chmod},
@end ifhtml
@ifnothtml
@ref{chmod,,change permissions of a file,chmod(2),
chmod(2) man page},
@end ifnothtml
and
@ifhtml
@uref{http://www.manpagez.com/man/2/open}),
@end ifhtml
@ifnothtml
@pxref{open,,open a file,open(2), open(2) man page}),
@end ifnothtml
which is used if the file is created).
@item 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 @samp{NULL} is
provided, @code{gdbm} will use a default function.
@end table

The return value, is the pointer needed by all other functions to
access that @code{gdbm} file.  If the return is the @samp{NULL} pointer,
@code{gdbm_open} was not successful.  The errors can be found in
@code{gdbm_errno} variable (@pxref{Variables, gdbm_errno}).  Available
error codes are discussed in @ref{Error codes}.

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

@node Close
@chapter Closing the database.
@cindex closing database
@cindex database, closing

It is important that every file opened is also closed.  This is needed to
update the reader/writer count on the file:

@deftypefn {gdbm interface} void gdbm_close (GDBM_FILE @var{dbf})
This function closes the @code{gdbm} file and frees all memory
associated with it.  The parameter is:

@table @var
@item dbf
The pointer returned by @code{gdbm_open}.
@end table
@end deftypefn

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

@deftypefn {gdbm interface} int gdbm_store (GDBM_FILE @var{dbf}, datum @var{key}, @
           datum @var{content}, int @var{flag})
The function @code{gdbm_store} inserts or replaces records in the database.

The parameters are:

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

This function can return the following values:

@table @asis
@item -1
The item was not stored in the database because the caller was not an
official writer or either @var{key} or @var{content} have a
@samp{NULL} @samp{dptr} field.

Both @var{key} and @var{content} must have the @samp{dptr} field be a
non-@samp{NULL} value.  Since a @samp{NULL} @samp{dptr} field is used by
other functions to indicate an error, it cannot be valid data.
@item +1
The item was not stored because the argument @var{flag} was
@samp{GDBM_INSERT} and the @var{key} was already in the database.
@item 0
No error.  The value of @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 function.
@end table
@end deftypefn

If you store data for a @var{key} that is already in the data base,
@code{gdbm} replaces the old data with the new data if called with
@samp{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
@chapter Searching for records in the database.
@cindex fetching records
@cindex looking up records
@cindex record, fetching

@deftypefn {gdbm interface} datum gdbm_fetch (GDBM_FILE @var{dbf}, datum @var{key})
Looks up a given @var{key} and returns the information associated with it.
The @samp{dptr} field in the structure that is returned points to a
memory block allocated by @code{malloc}.  It is the caller's
responsibility to free it when no longer needed.

If the @samp{dptr} is @samp{NULL}, no data was found.

The parameters are:

@table @var
@item dbf
The pointer returned by @code{gdbm_open}.
@item key
The search key.
@end table
@end deftypefn

An example of using this function:

@example
content = gdbm_fetch (dbf, key);
if (content.dptr == NULL)
  @{
    fprintf(stderr, "key not found\n");
  @}
else
  @{
    /* do something with content.dptr */
  @}
@end example

@cindex records, testing existence
You may also search for a particular key without retrieving it:

@deftypefn {gdbm interface} int gdbm_exists (GDBM_FILE @var{dbf}, datum @var{key})
Returns @samp{true} (@samp{1}) if the @var{key} exists in @var{dbf}
and @samp{false} (@samp{0}) otherwise.

The parameters are:

@table @var
@item dbf
The pointer returned by @code{gdbm_open}.
@item key
The search key.
@end table
@end deftypefn

@node Delete
@chapter Removing records from the database.
@cindex deleting records
@cindex record, deleting

To remove some data from the database, use the @code{gdbm_delete}
function.

@deftypefn {gdbm interface} int gdbm_delete (GDBM_FILE @var{dbf}, datum @var{key})
Deletes the data associated with the given @var{key}, if it exists in
the database @var{dbf}.  The file on disk is updated to reflect the
structure of the new database before returning from this function.

The parameters are:

@table @var
@item dbf
The pointer returned by @code{gdbm_open}.
@item datum key
The search key.
@end table

The function returns @samp{-1} if the item is not present or the
requester is a reader.  The return of @samp{0} marks a successful delete.
@end deftypefn

@node Sequential
@chapter Sequential access to records.
@cindex sequential access
@cindex iterating over records
@cindex records, iterating over

The next two functions allow for accessing all items in the database.  This
access is not @code{key} sequential, but it is guaranteed to visit every
@code{key} in the database once.  The order has to do with the hash values.
@code{gdbm_firstkey} starts the visit of all keys in the database.
@code{gdbm_nextkey} finds and reads the next entry in the hash structure for
@code{dbf}.

@deftypefn {gdbm interface} datum gdbm_firstkey (GDBM_FILE @var{dbf})
Initiate sequential access to the database @var{dbf}.  The returned
value is the first key accessed in the database.  If the @samp{dptr}
field in the returned datum is @samp{NULL}, the database contains no
data.

Otherwise, @samp{dptr} points to a memory block obtained from
@code{malloc}, which holds the key value.  The caller is responsible
for freeing this memory block when no longer needed.
@end deftypefn

@deftypefn {gdbm interface} datum gdbm_nextkey (GDBM_FILE @var{dbf}, datum @var{prev})
This function continues the iteration over the keys in @var{dbf},
initiated by @code{gdbm_firstkey}.  The parameter @var{prev} holds the
value returned from a previous call to @code{gdbm_nextkey} or
@code{gdbm_firstkey}.

The function returns next key from the database.  If the @samp{dptr}
field in the returned datum is @samp{NULL}, all keys in the database
has been visited.

Otherwise, @samp{dptr} points to a memory block obtained from
@code{malloc}, which holds the key value.  The caller is responsible
for freeing this memory block when no longer needed.
@end deftypefn

@cindex iteration loop
These functions were intended to visit the database in read-only algorithms,
for instance, to validate the database or similar operations.  The
usual algorithm for sequential access is:

@example
@group
   key = gdbm_firstkey (dbf);
   while (key.dptr)
     @{
        datum nextkey;

        /* do something with the key */
        ...

        /* Obtain the next key */
        nextkey = gdbm_nextkey (dbf, key);
        /* Reclaim the memory used by the key */
        free (key.dptr);
        /* Use nextkey in the next iteration. */
        key = nextkey;
     @}
@end group
@end example

@cindex iteration and @code{gdbm_delete}
@cindex deletion in iteration loops
@cindex @code{gdbm_delete} and sequential access
Care should be taken when the @code{gdbm_delete} function is used in
such a loop.  File visiting is based on a @dfn{hash table}.  The
@code{gdbm_delete} function re-arranges the hash table to make sure
that any collisions in the table do not leave some item
@dfn{un-findable}.  The original key order is @emph{not} guaranteed to
remain unchanged in all instances.  So it is possible that some key
will not be visited if a loop like the following is executed:

@example
@group
   key = gdbm_firstkey (dbf);
   while (key.dptr)
     @{
        datum nextkey;
        if (some condition)
          @{
             gdbm_delete (dbf, key);
          @}
         nextkey = gdbm_nextkey (dbf, key);
         free (key.dptr);
         key = nextkey;
      @}
@end group
@end example

@node Reorganization
@chapter Database reorganization.
@cindex database reorganization
@cindex reorganization, database

The following function should be used very seldom.

@deftypefn {gdbm interface} int gdbm_reorganize (GDBM_FILE @var{dbf})
Reorganizes the database.

The parameter is:

@table @var
@item dbf
The pointer returned by @code{gdbm_open}.
@end table
@end deftypefn

If you have had a lot of deletions and would like to shrink the space
used by the @code{gdbm} file, this function will reorganize the database.
This results, in particular, in shortening the length of a @code{gdbm}
file by removing the space occupied by deleted records.

This reorganization requires creating a new file and inserting all the elements
in the old file @var{dbf} into the new file.  The new file is then renamed to
the same name as the old file and @var{dbf} is updated to contain all the
correct information about the new file.  If an error is detected, the return
value is negative.  The value zero is returned after a successful
reorganization.

@node Sync
@chapter Database Synchronization
@cindex database synchronization
@cindex synchronization, database

@kwindex GDBM_SYNC
Unless your database was opened with the @samp{GDBM_SYNC} flag,
@code{gdbm} does not wait for writes to be flushed to the disk before
continuing.  This allows for faster writing of databases at the risk
of having a corrupted database if the application terminates in an
abnormal fashion.  The following function allows the programmer to
make sure the disk version of the database has been completely updated
with all changes to the current time.

@deftypefn {gdbm interface} void gdbm_sync (GDBM_FILE @var{dbf})
Synchronizes the changes in @var{dbf} with its disk file.  The
parameter is a pointer returned by @code{gdbm_open}.

This function would usually be called after a complete set of changes
have been made to the database and before some long waiting time.
The @code{gdbm_close} function automatically calls the equivalent of
@code{gdbm_sync} so no call is needed if the database is to be closed
immediately after the set of changes have been made.
@end deftypefn

@node Flat files
@chapter Export and Import
@cindex Flat file format
@cindex export
@cindex import
@code{Gdbm} databases can be converted into a portable @dfn{flat
format}.  This format can be used, for example, to migrate between
the different versions of @code{gdbm} databases.  Generally speaking,
flat files are safe to send over the network, and can be used to
recreate the database on another machine.  The recreated database is
guaranteed to be a byte-to-byte equivalent of the database from which
the flat file was created.  This does not necessarily mean, however,
that this file can be used in the same way as the original one.  For
example, if the original database contained non-@acronym{ASCII} data
(e.g.  @acronym{C} structures, integers etc.), the recreated database
can be of any use only if the target machine has the same integer
size and byte ordering as the source one and if its @acronym{C}
compiler uses the same packing conventions as the one which generated
@acronym{C} which populated the original database.  In general, such
binary databases are not portable between machines, unless you follow
some stringent rules on what data is written to them and how it is
interpreted.

@deftypefn {gdbm interface} int gdbm_export (GDBM_FILE @var{dbf}, @
           const char *@var{exportfile}, int @var{flag}, int @var{mode})
Create a flat file from the @code{gdbm} database.  The parameters are:

@table @var
@item dbf
A pointer to the source database, returned by a call to
@code{gdbm_open}.  The database must be open in @samp{GDBM_WRITER} mode.

@item exportfile
The name of the output file.

@item flag
@kwindex GDBM_WRCREAT
@kwindex GDBM_NEWDB
How to create the output file.  If @var{flag} is @samp{GDBM_WRCREAT},
the file will be created if it does not exist already.  Otherwise, if
it is @samp{GDBM_NEWDB}, it will be created if it does not exist, and
truncated otherwise.

@item mode
The permissions to use when creating the output file.
See @ifhtml
@uref{http://www.manpagez.com/man/2/open},
@end ifhtml
@ifnothtml
@ref{open,,open a file,open(2), open(2) man page},
@end ifnothtml
for a detailed discussion.
@end table
@end deftypefn

@deftypefn {gdbm interface} int gdbm_import (GDBM_FILE @var{dbf}, @
           const char *@var{importfile}, int @var{flag})
Populates the database from an existing flat file.

@table @var
@item dbf
A pointer to the source database, returned by a call to
@code{gdbm_open}.  The database must be open in @samp{GDBM_WRITER} mode.

@item importfile
The name of the input flat file.  The file must exist.

@item flag
The @var{flag} argument to be passed to @code{gdbm_store} function
when adding new records.  @xref{Store}, for a description of its
effect.
@end table
@end deftypefn

See also @ref{gdbmexport}, @ref{testgdbm export}, and
@ref{testgdbm import}.

@node Errors
@chapter Error strings.
@cindex error strings

To convert a @code{gdbm} error code into English text, use this
routine:

@deftypefn {gdbm interface} {const char *} gdbm_strerror (gdbm_error @var{errno})
Converts @var{errno} (which is an integer value) into a human-readable
descriptive text.  Returns a pointer to a static string.  The caller
must not alter or free the returned pointer.

The @var{errno} argument is usually the value of the global variable
@code{gdbm_errno}.  @xref{Variables, gdbm_errno}.
@end deftypefn

@node Options
@chapter Setting options
@cindex database options
@cindex options, database

@code{Gdbm} supports the ability to set certain options on an already
open database.

@deftypefn {gdbm interface} int gdbm_setopt (GDBM_FILE @var{dbf}, int @var{option}, @
           void *@var{value}, int @var{size})
Sets an option on the database or returns the value of an option.

The parameters are:

@table @var
@item dbf
The pointer returned by @code{gdbm_open}.
@item option
The option to be set or retreived.
@item value
A pointer to the value to which @var{option} will be set or where to
place the option value (depending on the option).
@item size
The length of the data pointed to by @var{value}.
@end table
@end deftypefn

The valid options are:

@table @asis
@kwindex GDBM_CACHESIZE
@kwindex GDBM_SETCACHESIZE
@item GDBM_SETCACHESIZE
@itemx GDBM_CACHESIZE
Set the size of the internal bucket cache.  This option may only be
set once on each GDBM_FILE descriptor, and is set automatically to 100
upon the first access to the database.  The @var{value} should point
to a @code{size_t} holding the desired cache size.

The @samp{GDBM_CACHESIZE} option is provided for compatibility with
earlier versions.

@kwindex GDBM_GETCACHESIZE
@item GDBM_GETCACHESIZE
Return the size of the internal bucket cache.  The @var{value} should
point to a @code{size_t} variable, where the size will be stored.

@kwindex GDBM_GETFLAGS
@item GDBM_GETFLAGS
Return the flags describing the state of the database.  The @var{value} should
point to a @code{int} variable where to store the flags.  The return
is the same as the flags used when opening the database (@pxref{Open,
gdbm_open}), except that it reflects the current state (which may have
been altered by another calls to @code{gdbm_setopt}.

@kwindex GDBM_FASTMODE
@item GDBM_FASTMODE
Enable or disable the @dfn{fast writes mode}, i.e. writes without
subsequent synchronization.  The @var{value} should point
to an integer: @samp{TRUE} to enable fast mode, and @samp{FALSE} to
disable it.

This option is retained for compatibility with previous versions of
@code{gdbm}.  Its effect is the reverse of @code{GDBM_SETSYNCMODE}
(see below).

@kwindex GDBM_SETSYNCMODE
@kwindex GDBM_SYNCMODE
@item GDBM_SETSYNCMODE
@itemx GDBM_SYNCMODE
Turn on or off file system synchronization operations.  This
setting defaults to off.  The @var{value} should point
to an integer: @samp{TRUE} to turn synchronization on, and @samp{FALSE} to
turn it off.

Note, that this option is a reverse of @code{GDBM_FASTMODE},
i.e. calling @code{GDBM_SETSYNCMODE} with @samp{TRUE} has the same effect
as calling @code{GDBM_FASTMODE} with @samp{FALSE}.

The @samp{GDBM_SYNCMODE} option is provided for compatibility with
earlier versions.

@kwindex GDBM_GETSYNCMODE
@item GDBM_GETSYNCMODE
Return the current synchronization status.  The @var{value} should
point to an @code{int} where the status will be stored.

@kwindex GDBM_SETCENTFREE
@kwindex GDBM_CENTFREE
@item GDBM_SETCENTFREE
@itemx GDBM_CENTFREE
@emph{NOTICE: This feature is still under study.}

Set central free block pool to either on or off.  The default is off,
which is how previous versions of @code{gdbm} handled free blocks.  If
set, this option causes all subsequent free blocks to be placed in the
@emph{global} pool, allowing (in theory) more file space to be reused
more quickly.  The @var{value} should point to an integer: @samp{TRUE} to
turn central block pool on, and @samp{FALSE} to turn it off.

The @samp{GDBM_CENTFREE} option is provided for compatibility with
earlier versions.

@kwindex GDBM_SETCOALESCEBLKS
@kwindex GDBM_COALESCEBLKS
@item GDBM_SETCOALESCEBLKS
@itemx GDBM_COALESCEBLKS
@emph{NOTICE: This feature is still under study.}

Set free block merging to either on or off.  The default is off, which
is how previous versions of @code{gdbm} handled free blocks.  If set,
this option causes adjacent free blocks to be merged.  This can become
a @acronym{CPU} expensive process with time, though, especially if
used in conjunction with GDBM_CENTFREE.  The @var{value} should point
to an integer: @samp{TRUE} to turn free block merging on, and @samp{FALSE} to
turn it off.

@kwindex GDBM_GETCOALESCEBLKS
@item GDBM_GETCOALESCEBLKS
Return the current status of free block merging.  The @var{value} should
point to an @code{int} where the status will be stored.

@kwindex GDBM_SETMAXMAPSIZE
@item GDBM_SETMAXMAPSIZE
Sets maximum size of a memory mapped region.  The @var{value} should
point to a value of type @code{size_t}, @code{unsigned long} or
@code{unsigned}.  The actual value is rounded to the nearest page
boundary (the page size is obtained from
@code{sysconf(_SC_PAGESIZE)}).

@kwindex GDBM_GETMAXMAPSIZE
@item GDBM_GETMAXMAPSIZE
Return the maximum size of a memory mapped region.  The @var{value} should
point to a value of type @code{size_t} where to return the data.

@kwindex GDBM_SETMMAP
@item GDBM_SETMMAP
Enable or disable memory mapping mode.  The @var{value} should point
to an integer: @samp{TRUE} to enable memory mapping or @samp{FALSE} to
disable it.

@kwindex GDBM_GETMMAP
@item GDBM_GETMMAP
Check whether memory mapping is enabled.  The @var{value} should point
to an integer where to return the status.

@kwindex GDBM_GETDBNAME
@item GDBM_GETDBNAME
Return the name of the database disk file.  The @var{value} should
point to a variable of type @code{char**}.  A pointer to the newly
allocated copy of the file name will be placed there.  The caller is
responsible for freeing this memory when no longer needed.  For
example:

@example
char *name;

if (gdbm_setopt (dbf, GDBM_GETDBNAME, &name, sizeof (name)))
  @{
     fprintf (stderr, "gdbm_setopt failed: %s\n",
	      gdbm_strerror (gdbm_errno));
  @}
else
  @{
    printf ("database name: %s\n", name);
    free (name);
  @}
@end example

@end table

The return value will be @samp{-1} upon failure, or @samp{0} upon
success.  The global variable @code{gdbm_errno} will be set upon failure.

For instance, to set a database to use a cache of 10, after opening it
with @code{gdbm_open}, but prior to accessing it in any way, the following
code could be used:

@example
@group
int value = 10;
ret = gdbm_setopt (dbf, GDBM_CACHESIZE, &value, sizeof (int));
@end group
@end example

@node Locking
@chapter File Locking.
@cindex locking

@kwindex GDBM_NOLOCK
With locking disabled (if @code{gdbm_open} was called with @samp{GDBM_NOLOCK}),
the user may want to perform their own file locking on the database file
in order to prevent multiple writers operating on the same file
simultaneously.

In order to support this, the @code{gdbm_fdesc} routine is provided.

@deftypefn {gdbm interface} int gdbm_fdesc (GDBM_FILE @var{dbf})
Returns the file descriptor of the database @var{dbf}.  This value
can be used as an argument to @code{flock}, @code{lockf} or similar
calls.
@end deftypefn

@node testgdbm
@chapter Test and modify a GDBM database.
@cindex testgdbm

The @command{testgdbm} utility allows you to view and modify an
existing @acronym{GDBM} database or to create a new one.

@cindex default database, @command{testgdbm} 
@cindex @option{-g}, @command{testgdbm} option
@flindex junk.gdbm
When invoked without options, it tries to open a database file called
@file{junk.gdbm}, located in the current working directory.  You can
change this default using the @option{-g} command line option.  This
option takes a single argument, specifying the file name to open, e.g.:

@example
$ testgdbm -g file.db
@end example

@cindex read-only mode, @command{testgdbm}
@cindex @option{-r}, @command{testgdbm} option
The database will be opened in read-write mode, unless the @option{-r}
option is specified, in which case it will be opened only for reading.

@cindex creating a database, @command{testgdbm}
@cindex @option{-n}, @command{testgdbm} option
If the database does not exist, @command{testgdbm} will create it.
There is a special option @option{-n}, which instructs the utility to
create a new database.  If it is used and if the database already
exists, it will be deleted, so use it sparingly.

@menu
* invocation::
* shell::
@end menu

@node invocation
@section testgdbm invocation
@cindex command line options, @command{testgdbm}

The following table summarizes all @command{testgdbm} command line
options:

@table @option
@item -b @var{size}
Set block size.
@item -c @var{size}
Set cache size.
@item -g @var{file}
Operate on @var{file} instead of the default @file{junk.gdbm}.
@item -h
Print a concise help summary.
@item -n
Create the database.
@item -r
Open the database in read-only mode.
@item -s
Synchronize to the disk after each write.
@item -v
Print program version and licensing information and exit.
@end table

@node shell
@section testgdbm interactive mode
@cindex interactive mode, @command{testgdbm}

After successful startup, @command{testgdbm} starts a loop, in which
it reads commands from the user, executes them and prints the results
on the standard output.  If the standard input is attached to a console,
@command{testgdbm} runs in interactive mode, which is indicated by its
@dfn{prompt}:

@example
testgdbm> _
@end example

The utility finishes when it reads the @samp{quit} command (see below) or
detects end-of-file on its standard input, whichever occurs first.

A @command{testgdbm} command consists of a @dfn{command verb},
optionally followed by one or two @dfn{arguments}, separated by any
amount of white space.  A command verb can be entered either in full
or in an abbreviated form, as long as that abbreviation does not match
any other verb.  For example, @samp{co} can be used instead of
@samp{count} and @samp{ca} instead of @samp{cache}.  Furthermore,
many command verbs also have single-letter forms, called @dfn{command
letters}.

An argument is any sequence of non-whitespace characters.  Notice,
that currently there is no way to enter arguments containing white
space.  This limitation will be removed in future releases.

Each command takes at most two @dfn{formal parameters}, which can be
optional or mandatory.  If the number of actual arguments is less than the
number of mandatory parameters, @command{testgdbm} will prompt you to
supply missing arguments.  For example, the @samp{store} command takes two
mandatory parameters, so if you invoked it with no arguments, you
would be prompted twice to supply the necessary data, as shown in 
example below:

@example
testgdbm> @kbd{store}
key> @kbd{three}
data> @kbd{3}
@end example

However, such prompting is possible only in interactive mode.  In
non-interactive mode (e.g. when running a script), all arguments must
be supplied with each command, otherwise @command{testgdbm} will report an
error and exit immediately.

@anchor{pager}
@cindex pager, @command{testgdbm}
@cindex @env{PAGER}
Some commands produce excessive amounts of output.  To help you follow
it, @command{testgdbm} uses a pager utility to display such
output.  The name of the pager utility is taken from the environment
variable @env{PAGER}.  The pager is invoked only in interactive mode
and only if the estimated number of output lines is greater then the
number of lines on your screen.  

@anchor{nul-termination}
Many of the @command{testgdbm} commands operate on database key and
data values.  The utility assumes that both keys and data are
@acronym{ASCII} strings, either nul-terminated or not.  By default,
it is assumed that strings are nul-terminated.  You can change this
by using @code{z} (@code{key-zero}, for keys) and @code{Z}
(@code{data-zero}, for data) commands.

The following table summarizes all available commands:

@deffn {command verb} count
@deffnx {command abbrev} co
@deffnx {command letter} c
Print the number of entries in the database.
@end deffn

@deffn {command verb} delete @var{key}
@deffnx {command abbrev} de @var{key}
@deffnx {command letter} d @var{key}
Delete entry with a given @var{key}
@end deffn

@anchor{testgdbm export}
@deffn {command verb} export @var{file-name} [truncate]
@deffnx {command abbrev} e @var{file-name} [truncate]
Export the database to the flat file @var{file-name}.  @xref{Flat files},
for a description of the flat file format and its purposes.  This
command will not overwrite an existing file, unless the word
@samp{truncate} is given as its second argument.

See also @ref{gdbmexport}.
@end deffn

@deffn {command verb} fetch @var{key}
@deffnx {command abbrev} fe @var{key}
@deffnx {command letter} f @var{key}
Fetch and display a record with the given @var{key}.
@end deffn

@anchor{testgdbm import}
@deffn {command verb} import @var{file-name} [replace]
@deffnx {command abbrev} i @var{file-name} [replace]
Import data from a flat dump file @var{file-name}
(@pxref{Flat files}).  If the word @samp{replace} is given
as the second argument, any records with the same keys as the already
existing ones will replace them.
@end deffn

@deffn {command verb} list
@deffnx {command abbrev} l
List the contents of the database (@pxref{pager}).
@end deffn

@deffn {command verb} next [@var{key}]
@deffnx {command abbrev} n [@var{key}]
Sequential access: fetch and display a next record.  If @var{key} is
given, a record following one with this key will be fetched.
Otherwise, the key supplied by the latest @code{1}, @code{2} or
@var{n} command will be used.

See also @code{first}, below.

@xref{Sequential}, for more information on sequential access.
@end deffn

@deffn {command verb} quit
@deffnx {command abbrev} q
Close the database and quit the utility.
@end deffn

@deffn {command verb} store @var{key} @var{data}
@deffnx {command abbrev} sto @var{key} @var{data}
@deffnx {command letter} s @var{key} @var{data}
Store the @var{data} with @var{key} in the database.  If @var{key}
already exists, its data will be replaced.
@end deffn

@deffn {command verb} first
@deffnx {command abbrev} fi 
@deffnx {command letter} 1
Fetch and display the first record in the database.  Subsequent
records can be fetched using @code{next} command (see above). 
@xref{Sequential}, for more information on sequential access.
@end deffn

@deffn {command verb} read @var{file} [replace]
@deffnx {command abbrev} rea @var{file} [replace] 
@deffnx {command letter} < @var{file} [replace] 
Read entries from @var{file} and store them in the database.  If the
word @samp{replace} is given as the second argument, any existing
records with matching keys will be replaced.
@end deffn

@deffn {command verb} reorganize
@deffnx {command abbrev} reo 
@deffnx {command letter} r
Reorganize the database (@pxref{Reorganization}).
@end deffn

@deffn {command verb} key-zero
@deffnx {command abbrev} k
@deffnx {command letter} z
Toggle key nul-termination.  Use @code{status} to inspect the current
state.  @xref{nul-termination}.
@end deffn

@deffn {command verb} avail
@deffnx {command abbrev} a
@deffnx {command letter} A
Print the @dfn{avail list}.
@end deffn

@deffn {command verb} bucket
@deffnx {command abbrev} b
@deffnx {command letter} B
Print the bucket number @var{num}.
@end deffn

@deffn {command verb} current
@deffnx {command abbrev} cu
@deffnx {command letter} C
Print the current bucket.
@end deffn

@deffn {command verb} dir
@deffnx {command abbrev} di
@deffnx {command letter} D
Print hash directory.
@end deffn

@deffn {command verb} header
@deffnx {command abbrev} hea
@deffnx {command letter} F
Print file header.
@end deffn

@deffn {command verb} hash @var{key}
@deffnx {command abbrev} ha @var{key}
@deffnx {command letter} H @var{key}
Compute and display the hash value for the given @var{key}.
@end deffn

@deffn {command verb} cache
@deffnx {command abbrev} ca
@deffnx {command letter} K
Print the bucket cache.
@end deffn

@deffn {command verb} status
@deffnx {command abbrev} sta
@deffnx {command letter} S
Print current program status.  The following example shows the
information displayed:

@example
Database file: junk.gdbm
Zero terminated keys: yes
Zero terminated data: yes
@end example
@end deffn

@deffn {command verb} version
@deffnx {command abbrev} v
Print the version of @command{gdbm}.
@end deffn

@deffn {command verb} data-zero
@deffnx {command abbrev} da 
@deffnx {command letter} Z
Toggle data nul-termination.  Use @code{status} to examine the current
status.

@xref{nul-termination}.
@end deffn

@deffn {command verb} help
@deffnx {command abbrev} hel
@deffnx {command letter} ?
Print a concise command summary, showing each command letter and verb
with its parameters and a short description of what it does.  Optional
arguments are enclosed in square brackets.
@end deffn

@node gdbmexport
@chapter Export a database into a portable format.
@pindex gdbmexport

The @command{gdbmexport} utility converts the database into a portable
@dfn{flat format}.  Files in this format can be used to populate
databases using the @code{gdbm_import} function (@pxref{Flat files,
gdbm_import}) or the @code{i} command of @command{testgdbm} utility
(@pxref{testgdbm import}).  In many cases files in this format are suitable for
sending over the network to populate the database on another machine.
The only exception to this are databases whose records contain
non-@acronym{ASCII} data (e.g.  @acronym{C} structures, integers
etc.).  For such databases you will be better off by writing a
specialized utility to convert them to an architecture-independent
format.

If @command{gdbmexport} is linked with @file{libgdbm}
version 1.8.3, it can be used to convert databases from old to new
format.

The utility takes two mandatory arguments: the name of the database
file to convert and the output file name, e.g.:

@example
$ gdbmexport junk.gdbm junk.flat
@end example

In addition two options are understood:

@table @option
@item -h
Display short usage summary and exit.

@item -v
Display program version and licensing information, and exit.
@end table

@node Variables
@chapter Useful global variables.

The following global variables and constants are available:

@deftypevar gdbm_error gdbm_errno
This variable contains error code from the last failed @code{gdbm}
call.  @xref{Error codes}, for a list of available error codes and
their descriptions.

Use @code{gdbm_strerror} (@pxref{Errors}) to convert it to a
descriptive text.
@end deftypevar

@deftypevar {const char *} gdbm_errlist[]
This variable is an array of error descriptions, which is used by
@code{gdbm_strerror} to convert error codes to human-readable text
(@pxref{Errors}).  You can access it directly, if you wish so.  It
contains @code{_GDBM_MAX_ERRNO + 1} elements and can be directly
indexed by the error code to obtain a corresponding descriptive
text.
@end deftypevar

@defvr {Constant} _GDBM_MIN_ERRNO
The minimum error code used by @code{gdbm}.
@end defvr

@defvr {Constant} _GDBM_MAX_ERRNO
The maximum error code used by @code{gdbm}.
@end defvr

@cindex version number
@deftypevar {const char *} gdbm_version
A string containing the version information.
@end deftypevar

@deftypevar {int const} gdbm_version_number[3]
This variable contains the @code{gdbm} version numbers:

@multitable @columnfractions 0.4 0.5
@headitem Index @tab Meaning
@item 0         @tab Major number
@item 1         @tab Minor number
@item 2         @tab Patchlevel number
@end multitable

Additionally, the following constants are defined in the @file{gdbm.h}
file:

@table @asis
@kwindex GDBM_VERSION_MAJOR
@item GDBM_VERSION_MAJOR
Major number.

@kwindex GDBM_VERSION_MINOR
@item GDBM_VERSION_MINOR
Minor number.

@kwindex GDBM_VERSION_PATCH
@item GDBM_VERSION_PATCH
Patchlevel number.
@end table

These can be used to verify whether the header file matches the library.
@end deftypevar

To compare two split-out version numbers, use the following function:

@deftypefn {gdbm interface} int gdbm_version_cmp (int const @var{a}[3], @
                                                  int const @var{b}[3])
Compare two version numbers.  Return @samp{-1} if @var{a} is less than
@var{b}, @samp{1} if @var{a} is greater than @var{b} and @samp{0} if
they are equal.

Comparison is done from left to right, so that:

@example
a = @{ 1, 8, 3 @};
b = @{ 1, 8, 3 @};
gdbm_version_cmp (a, b) @result{} 0

a = @{ 1, 8, 3 @};
b = @{ 1, 8, 2 @};
gdbm_version_cmp (a, b) @result{} 1

a = @{ 1, 8, 3 @};
b = @{ 1, 9. 0 @};
gdbm_version_cmp (a, b) @result{} -1
@end example
@end deftypefn

@node Error codes
@chapter Error codes
@cindex error codes

This chapter summarizes the error codes which can be set by the
functions in @code{gdbm} library.

@table @asis
@kwindex GDBM_NO_ERROR
@item GDBM_NO_ERROR
No error occurred.

@kwindex GDBM_MALLOC_ERROR
@item GDBM_MALLOC_ERROR
Memory allocation failed.  Not enough memory.

@kwindex GDBM_BLOCK_SIZE_ERROR
@item GDBM_BLOCK_SIZE_ERROR
This error is set by the @code{gdbm_open} function (@pxref{Open}), if
the value of its @var{block_size} argument is incorrect.

@kwindex GDBM_FILE_OPEN_ERROR
@item GDBM_FILE_OPEN_ERROR
The library was not able to open a disk file.  This can be set by
@code{gdbm_open} (@pxref{Open}), @code{gdbm_export} and
@code{gdbm_import} functions (@pxref{Flat files}).

Inspect the value of the system @code{errno} variable to get more
detailed diagnostics.

@kwindex GDBM_FILE_WRITE_ERROR
@item GDBM_FILE_WRITE_ERROR
Writing to a disk file failed.  This can be set by
@code{gdbm_open} (@pxref{Open}), @code{gdbm_export} and
@code{gdbm_import} functions.

Inspect the value of the system @code{errno} variable to get more
detailed diagnostics.

@kwindex GDBM_FILE_SEEK_ERROR
@item GDBM_FILE_SEEK_ERROR
Positioning in a disk file failed.  This can be set by
@code{gdbm_open} (@pxref{Open}) function.

Inspect the value of the system @code{errno} variable to get a more
detailed diagnostics.

@kwindex GDBM_FILE_READ_ERROR
@item GDBM_FILE_READ_ERROR
Reading from a disk file failed.  This can be set by
@code{gdbm_open} (@pxref{Open}), @code{gdbm_export} and
@code{gdbm_import} functions.

Inspect the value of the system @code{errno} variable to get a more
detailed diagnostics.

@kwindex GDBM_BAD_MAGIC_NUMBER
@item GDBM_BAD_MAGIC_NUMBER
The file given as argument to @code{gdbm_open} function is not a valid
@code{gdbm} file: it has a wrong magic number.

@kwindex GDBM_EMPTY_DATABASE
@item GDBM_EMPTY_DATABASE
The file given as argument to @code{gdbm_open} function is not a valid
@code{gdbm} file: it has zero length.

@kwindex GDBM_CANT_BE_READER
@item GDBM_CANT_BE_READER
This error code is set by the @code{gdbm_open} function if it is not
able to lock file when called in @samp{GDBM_READER} mode (@pxref{Open,
GDBM_READER}).

@kwindex GDBM_CANT_BE_WRITER
@item GDBM_CANT_BE_WRITER       
This error code is set by the @code{gdbm_open} function if it is not
able to lock file when called in writer mode (@pxref{Open}).

@kwindex GDBM_READER_CANT_DELETE
@item GDBM_READER_CANT_DELETE
Set by the @code{gdbm_delete} (@pxref{Delete}) if it attempted to
operate on a database that is open in read-only mode (@pxref{Open,
GDBM_READER}).

@kwindex GDBM_READER_CANT_STORE
@item GDBM_READER_CANT_STORE    
Set by the @code{gdbm_store} (@pxref{Store}) if it attempted to
operate on a database that is open in read-only mode (@pxref{Open,
GDBM_READER}).

@kwindex GDBM_READER_CANT_REORGANIZE
@item GDBM_READER_CANT_REORGANIZE       
Set by the @code{gdbm_reorganize} (@pxref{Reorganization}) if it attempted to
operate on a database that is open in read-only mode (@pxref{Open,
GDBM_READER}).

@kwindex GDBM_UNKNOWN_UPDATE
@item GDBM_UNKNOWN_UPDATE
Currently unused.  Reserved for future uses.

@kwindex GDBM_ITEM_NOT_FOUND
@item GDBM_ITEM_NOT_FOUND
Requested item was not found.  This error is set by @code{gdbm_delete}
(@pxref{Delete}) and @code{gdbm_fetch} (@pxref{Fetch}) when the requested
@var{key} value is not found in the database.

@kwindex GDBM_REORGANIZE_FAILED
@item GDBM_REORGANIZE_FAILED
The @code{gdbm_reorganize} function is not
able to create a temporary database.  @xref{Reorganization}.

@kwindex GDBM_CANNOT_REPLACE
@item GDBM_CANNOT_REPLACE
Cannot replace existing item.  This error is set by the
@code{gdbm_store} if the requested @var{key} value is found in the
database and the @var{flag} parameter is not @samp{GDBM_REPLACE}.
@xref{Store}, for a detailed discussion.

@kwindex GDBM_ILLEGAL_DATA
@item GDBM_ILLEGAL_DATA
Either @var{key} or @var{content} parameter was wrong in a call to
to @code{gdbm_store} (@pxref{Store}).

@kwindex GDBM_OPT_ALREADY_SET
@item GDBM_OPT_ALREADY_SET      
Requested option can be set only once and was already set.  This error
is returned by the @code{gdbm_setopt} function.  @xref{Options,
GDBM_CACHESIZE}.

@kwindex GDBM_OPT_ILLEGAL
@item GDBM_OPT_ILLEGAL
The @var{option} argument is not valid or the @var{value} argument
points to an invalid value in a call to @code{gdbm_setopt} function.
@xref{Options}.

@kwindex GDBM_BYTE_SWAPPED
@item GDBM_BYTE_SWAPPED
The @code{gdbm_open} function (@pxref{Open}) attempts to open a
database which is created on a machine with different byte ordering.

@kwindex GDBM_BAD_FILE_OFFSET
@item GDBM_BAD_FILE_OFFSET
The @code{gdbm_open} function (@pxref{Open}) sets this error code if
the file it tries to open has a wrong magic number.

@kwindex GDBM_BAD_OPEN_FLAGS
@item GDBM_BAD_OPEN_FLAGS
Set by the @code{gdbm_export} function if supplied an invalid
@var{flags} argument.  @xref{Flat files}.

@kwindex GDBM_FILE_STAT_ERROR
@item GDBM_FILE_STAT_ERROR
Getting information about a disk file failed.  The system @code{errno}
will give more details about the error.

This error can be set by the following functions: @code{gdbm_open},
@code{gdbm_reorganize}.

@kwindex GDBM_FILE_EOF
@item GDBM_FILE_EOF
End of file was encountered where more data was expected to be
present.  This error can occur when fetching data from the database
and usually means that the database is truncated or otherwise corrupted.

This error can be set by any GDBM function that does I/O.  Some of
these functions are: @code{gdbm_delete}, @code{gdbm_exists},
@code{gdbm_fetch}, @code{gdbm_export}, @code{gdbm_import},
@code{gdbm_reorganize}, @code{gdbm_firstkey}, @code{gdbm_nextkey},
@code{gdbm_store}. 
@end table

@node Compatibility
@chapter Compatibility with standard @code{dbm} and @code{ndbm}.

@cindex compatibility layer
@code{Gdbm} includes a compatibility layer, which provides traditional
@samp{ndbm} and older @samp{dbm} functions.  The layer is compiled and
installed if the @option{--enable-libgdbm-compat} option is used when
configuring the package.

@findex ndbm.h
@findex dbm.h
@findex libgdbm_compat
The compatibility layer consists of two header files: @file{ndbm.h}
and @file{dbm.h} and the @file{libgdbm_compat} library.

Older programs using @code{ndbm} or @code{dbm} interfaces can
use @file{libgdbm_compat} without any changes.  To link a program with
the compatibility library, add the following two options to the
@command{cc} invocation: @option{-lgdbm -lgdbm_compat}.  The @option{-L}
option may also be required, depending on where @code{gdbm} is
installed, e.g.:

@example
cc ... -L/usr/local/lib -lgdbm -lgdbm_compat
@end example

@cindex @samp{dir} file
@cindex @samp{pag} file
Databases created and manipulated by the compatibility interfaces
consist of two different files: @file{@var{file}.dir} and
@file{@var{file}.pag}.  This is required by the @acronym{POSIX}
specification and corresponds to the traditional usage.  Note,
however, that despite the similarity of the naming convention,
actual data stored in these files has not the same format as
in the databases created by other @code{dbm} or @code{ndbm}
libraries.  In other words, you cannot access a standard UNIX
@code{dbm} file with GNU @code{dbm}!

GNU @code{dbm} files are not @code{sparse}.  You can copy them with
the usual @code{cp} command and they will not expand in the copying
process.

@menu
* ndbm::  NDBM interface functions.
* dbm::   DBM interface functions.
@end menu

@node ndbm
@section NDBM interface functions.
@cindex NDBM functions

The functions below implement the @acronym{POSIX} @samp{ndbm} interface:

@deftypefn {ndbm} {DBM *} dbm_open (char *@var{file}, int @var{flags}, int @var{mode})
Opens a database.  The @var{file} argument is the full name of the
database file to be opened.  The function opens two files:
@file{@var{file}.pag} and @file{@var{file}.dir}.  The @var{flags} and
@var{mode} arguments have the same meaning as the second and third
arguments of 
@ifhtml
@uref{http://www.manpagez.com/man/2/open,,open(2)},
@end ifhtml
@ifnothtml
@code{open} (@pxref{open,,open a file,open(2), open(2) man page}),
@end ifnothtml
except that a database opened for write-only access opens the files
for read and write access and the behavior of the @code{O_APPEND} flag is
unspecified.

The function returns a pointer to the @code{DBM} structure describing
the database.  This pointer is used to refer to this database in all
operations described below.

Any error detected will cause a return value of @samp{NULL} and an
appropriate value will be stored in @code{gdbm_errno}
(@pxref{Variables}).
@end deftypefn

@deftypefn {ndbm} void dbm_close (DBM *@var{dbf})
Closes the database.  The @var{dbf} argument must be a pointer
returned by an earlier call to @code{dbm_open}.
@end deftypefn

@deftypefn {ndbm} datum dbm_fetch (DBM *@var{dbf}, datum @var{key})
Reads a record from the database with the matching key.  The @var{key}
argument supplies the key that is being looked for.

If no matching record is found, the @code{dptr} member of the returned
datum is @samp{NULL}.  Otherwise, the @code{dptr} member of the
returned datum points to the memory managed by the compatibility
library.  The application should never free it.
@end deftypefn

@deftypefn {ndbm} int dbm_store (DBM *@var{dbf}, datum @var{key}, @
                                 datum @var{content}, int @var{mode})
Writes a key/value pair to the database.  The argument @var{dbf} is a
pointer to the @code{DBM} structure returned from a call to
@code{dbm_open}.  The @var{key} and @var{content} provide the values
for the record key and content.  The @var{mode} argument controls
the behavior of @code{dbm_store} in case a matching record already
exists in the database.  It can have one of the following two values:

@table @code
@kwindex DBM_REPLACE
@item DBM_REPLACE
Replace existing record with the new one.

@kwindex DBM_INSERT
@item DBM_INSERT
The existing record is left unchanged, and the function returns
@samp{1}.
@end table

If no matching record exists in the database, new record will be
inserted no matter what the value of the @var{mode} is.
@end deftypefn
                                 
@deftypefn {ndbm} int dbm_delete (DBM *@var{dbf}, datum @var{key})
Deletes the record with the matching key from the database.  If the
function succeeds, @samp{0} is returned.  Otherwise, if no matching
record is found or if an error occurs, @samp{-1} is returned.
@end deftypefn

@deftypefn {ndbm} datum dbm_firstkey (DBM *@var{dbf})
Initializes iteration over the keys from the database and returns
the first key.  Note, that the word @samp{first} does not imply any
specific ordering of the keys.

If there are no records in the database, the @code{dptr} member of the
returned datum is @samp{NULL}.  Otherwise, the @code{dptr} member of
the returned datum points to the memory managed by the compatibility
library.  The application should never free it.
@end deftypefn

@deftypefn {ndbm} datum dbm_nextkey (DBM *@var{dbf})
Continues the iteration started by @code{dbm_firstkey}.  Returns the
next key in the database.  If the iteration covered all keys in the
database, the @code{dptr} member of the returned datum is @samp{NULL}.
Otherwise, the @code{dptr} member of the returned datum points to the
memory managed by the compatibility library.  The application should
never free it.

@cindex sequential access, using @samp{NDBM}
@cindex iteration loop, using @samp{NDBM}
The usual way of iterating over all the records in the database is:

@example
for (key = dbm_firstkey (dbf);
     key.ptr;
     key = dbm_nextkey (dbf))
  @{
    /* do something with the key */
  @}
@end example

The loop above should not try to delete any records from the database,
otherwise the iteration is not guaranteed to cover all the keys.
@xref{Sequential}, for a detailed discussion of this.
@end deftypefn

@deftypefn {ndbm} int dbm_error (DBM *@var{dbf})
Returns the error condition of the database: @samp{0} if no errors
occurred so far while manipulating the database, and a non-zero value
otherwise.
@end deftypefn

@deftypefn {ndbm} void dbm_clearerr (DBM *@var{dbf})
Clears the error condition of the database.
@end deftypefn

@deftypefn {ndbm} int dbm_dirfno (DBM *@var{dbf})
Returns the file descriptor of the @samp{dir} file of the database.
It is guaranteed to be different from the descriptor returned by
the @code{dbm_pagfno} function (see below).

The application can lock this descriptor to serialize accesses to the
database.
@end deftypefn

@deftypefn {ndbm} int dbm_pagfno (DBM *@var{dbf})
Returns the file descriptor of the @samp{pag} file of the database.
See also @code{dbm_dirfno}.
@end deftypefn

@deftypefn {ndbm} int dbm_rdonly (DBM *@var{dbf})
Returns @samp{1} if the database @var{dbf} is open in a read-only mode
and @samp{0} otherwise.
@end deftypefn

@node dbm
@section DBM interface functions.
@cindex DBM functions

The functions below are provided for compatibility with the old
UNIX @samp{DBM} interface.  Only one database at a time can be
manipulated using them.

@deftypefn {dbm} int dbminit (char *@var{file})
Opens a database.  The @var{file} argument is the full name of the
database file to be opened.  The function opens two files:
@file{@var{file}.pag} and @file{@var{file}.dir}.  If any of
them does not exist, the function fails.  It never attempts to create
the files.

The database is opened in the read-write mode, if its disk permissions
permit.

The application must ensure that the functions described below in
this section are called only after a successful call to @code{dbminit}.
@end deftypefn

@deftypefn {dbm} int dbmclose (void)
Closes the database opened by an earlier call to @code{dbminit}.
@end deftypefn

@deftypefn {dbm} datum fetch (datum @var{key})
Reads a record from the database with the matching key.  The @var{key}
argument supplies the key that is being looked for.

If no matching record is found, the @code{dptr} member of the returned
datum is @samp{NULL}.  Otherwise, the @code{dptr} member of the
returned datum points to the memory managed by the compatibility
library.  The application should never free it.
@end deftypefn

@deftypefn {dbm} int store (datum @var{key}, datum @var{content})
Stores the key/value pair in the database.  If a record with the
matching key already exists, its content will be replaced with the new
one.

Returns @samp{0} on success and @samp{-1} on error.
@end deftypefn

@deftypefn {dbm} int delete (datum @var{key})
Deletes a record with the matching key. 

If the function succeeds, @samp{0} is returned.  Otherwise, if no
matching record is found or if an error occurs, @samp{-1} is
returned.
@end deftypefn

@deftypefn {dbm} datum firstkey (void)
Initializes iteration over the keys from the database and returns
the first key.  Note, that the word @samp{first} does not imply any
specific ordering of the keys.

If there are no records in the database, the @code{dptr} member of the
returned datum is @samp{NULL}.  Otherwise, the @code{dptr} member of
the returned datum points to the memory managed by the compatibility
library.  The application should never free it.
@end deftypefn

@deftypefn {dbm} datum nextkey (datum @var{key})
Continues the iteration started by a call to @code{firstkey}.  Returns
the next key in the database.  If the iteration covered all keys in the
database, the @code{dptr} member of the returned datum is @samp{NULL}.
Otherwise, the @code{dptr} member of the returned datum points to the
memory managed by the compatibility library.  The application should
never free it.
@end deftypefn

@node Bugs
@chapter Problems and bugs.

If you have problems with GNU @code{dbm} or think you've found a bug,
please report it.  Before reporting a bug, make sure you've actually
found a real bug.  Carefully reread the documentation and see if it
really says you can do what you're trying to do.  If it's not clear
whether you should be able to do something or not, report that too; it's
a bug in the documentation!

Before reporting a bug or trying to fix it yourself, try to isolate it
to the smallest possible input file that reproduces the problem.  Then
send us the input file and the exact results @code{gdbm} gave you.  Also
say what you expected to occur; this will help us decide whether the
problem was really in the documentation.

Once you've got a precise problem, send e-mail to
@email{bug-gdbm@@gnu.org}.

Please include the version number of GNU @code{dbm} you are using.  You can get
this information by printing the variable @code{gdbm_version}
(@pxref{Variables}).

Non-bug suggestions are always welcome as well.  If you have questions
about things that are unclear in the documentation or are just obscure
features, please report them too.

You may contact the authors and maintainers by e-mail:
@example
@email{phil@@cs.wwu.edu}, @email{downsj@@downsj.com}, @email{gray@@gnu.org.ua}
@end example

@node Resources
@chapter Additional resources

For the latest updates and pointers to additional resources, visit
@uref{http://www.gnu.org/software/gdbm}.

In particular, a copy of @code{gdbm} documentation in various formats
is available online at @uref{http://www.gnu.org/software/gdbm/manual}.

Latest versions of @code{gdbm} can be downloaded from anonymous FTP:
@uref{ftp://ftp.gnu.org/gnu/gdbm}, or via HTTP from
@uref{http://ftp.gnu.org/gnu/gdbm}, or from any
@ifhtml
@uref{http://www.gnu.org/order/ftp.html,,GNU mirror} worldwide.
@end ifhtml
@ifnothtml
GNU mirror worldwide.  See @uref{http://www.gnu.org/order/ftp.html},
for a list of mirrors.
@end ifnothtml

To track @code{gdbm} development, visit
@uref{http://puszcza.gnu.org.ua/projects/gdbm}.

@node GNU Free Documentation License
@appendix GNU Free Documentation License

@include fdl.texi

@node Index
@unnumbered Index

@printindex cp

@bye