GDBM(3) UNIX System V (5/19/99) GDBM(3)
NAME
GDBM - The GNU database manager. Includes dbm and ndbm
compatability. (Version 1.8.)
SYNOPSIS
#include <gdbm.h>
extern gdbm_error
gdbm_errno
extern char
*gdbm_version
GDBM_FILE
gdbm_open (name, block_size, read_write, mode, fatal_func)
char * name;
int block_size, read_write, mode;
void (*fatal_func) ();
void
gdbm_close (dbf)
GDBM_FILE dbf;
int
gdbm_store (dbf, key, content, flag)
GDBM_FILE dbf;
datum key, content;
int flag;
datum
gdbm_fetch (dbf, key)
GDBM_FILE dbf;
datum key;
int
gdbm_delete (dbf, key)
GDBM_FILE dbf;
datum key;
datum
gdbm_firstkey (dbf)
GDBM_FILE dbf;
datum
gdbm_nextkey (dbf, key)
GDBM_FILE dbf;
datum key;
int
gdbm_reorganize (dbf)
GDBM_FILE dbf;
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void
gdbm_sync (dbf)
GDBM_FILE dbf;
int
gdbm_exists (dbf, key)
GDBM_FILE dbf;
datum key;
char *
gdbm_strerror (errno)
gdbm_error errno;
int
gdbm_setopt (dbf, option, value, size)
GDBM_FILE dbf;
int option;
int *value;
int size;
int
gdbm_fdesc (dbf)
GDBM_FILE dbf;
DBM Compatability routines:
#include <dbm.h>
int
dbminit (name)
char *name;
int
store (key, content)
datum key, content;
datum
fetch (key)
datum key;
int
delete (key)
datum key;
datum
firstkey ()
datum
nextkey (key)
datum key;
int
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dbmclose ()
NDBM Compatability routines:
#include <ndbm.h>
DBM
*dbm_open (name, flags, mode)
char *name;
int flags, mode;
void
dbm_close (file)
DBM *file;
datum
dbm_fetch (file, key)
DBM *file;
datum key;
int
dbm_store (file, key, content, flags)
DBM *file;
datum key, content;
int flags;
int
dbm_delete (file, key)
DBM *file;
datum key;
datum
dbm_firstkey (file)
DBM *file;
datum
dbm_nextkey (file)
DBM *file;
int
dbm_error (file)
DBM *file;
int
dbm_clearerr (file)
DBM *file;
int
dbm_pagfno (file)
DBM *file;
int
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dbm_dirfno (file)
DBM *file;
int
dbm_rdonly (file)
DBM *file;
DESCRIPTION
GNU dbm is a library of routines that manages data files
that contain key/data pairs. The access provided is that of
storing, retrieval, and deletion by key and a non-sorted
traversal of all keys. A process is allowed to use multiple
data files at the same time.
A process that opens a gdbm file is designated as a "reader"
or a "writer". Only one writer may open a gdbm file and
many readers may open the file. Readers and writers can not
open the gdbm file at the same time. The procedure for
opening a gdbm file is:
GDBM_FILE dbf;
dbf = gdbm_open ( name, block_size, read_write, mode,
fatal_func )
Name is the name of the file (the complete name, gdbm does
not append any characters to this name). Block_size is the
size of a single transfer from disk to memory. This
parameter is ignored unless the file is a new file. The
minimum size is 512. If it is less than 512, dbm will use
the stat block size for the file system. Read_write can
have one of the following values:
GDBM_READER reader
GDBM_WRITER writer
GDBM_WRCREAT writer - if database does not exist create new
one
GDBM_NEWDB writer - create new database regardless if one
exists
For the last three (writers of the database) the following
may be added added to read_write by bitwise or: 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 gdbm defaults to no-sync
mode.
Mode is the file mode (see chmod(2) and open(2)) if the file
is created. (*Fatal_func) () is a function for dbm to call
if it detects a fatal error. The only parameter of this
function is a string. If the value of 0 is provided, gdbm
will use a default function.
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The return value dbf is the pointer needed by all other
routines to access that gdbm file. If the return is the
NULL pointer, gdbm_open was not successful. The errors can
be found in gdbm_errno for gdbm errors and in errno for
system errors. (For error codes, see gdbmerrno.h.)
In all of the following calls, the parameter dbf refers to
the pointer returned from gdbm_open.
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:
gdbm_close (dbf);
The database is used by 3 primary routines. The first
stores data in the database.
ret = gdbm_store ( dbf, key, content, flag )
Dbf is the pointer returned by gdbm_open. Key is the key
data. Content is the data to be associated with the key.
Flag can have one of the following values:
GDBM_INSERT insert only, generate an error if key exists
GDBM_REPLACE replace contents if key exists.
If a reader calls gdbm_store, the return value will be -1.
If called with GDBM_INSERT and key is in the database, the
return value will be 1. Otherwise, the return value is 0.
NOTICE: If you store data for a key that is already in the
data base, gdbm replaces the old data with the new data if
called with GDBM_REPLACE. You do not get two data items for
the same key and you do not get an error from gdbm_store.
NOTICE: The size in gdbm is not restricted like dbm or ndbm.
Your data can be as large as you want.
To search for some data:
content = gdbm_fetch ( dbf, key )
Dbf is the pointer returned by gdbm_open. Key is the key
data.
If the dptr element of the return value is NULL, no data was
found. Otherwise the return value is a pointer to the found
data. The storage space for the dptr element is allocated
using malloc(3C). Gdbm does not automatically free this
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data. It is the programmer's responsibility to free this
storage when it is no longer needed.
To search for some data, without retrieving it:
ret = gdbm_exists ( dbf, key )
Dbf is the pointer returned by gdbm_open. Key is the key
data to search for.
If the key is found within the database, the return value
ret will be true. If nothing appropiate is found, ret will
be false. This routine is useful for checking for the
existance of a record, without performing the memory
allocation done by gdbm_fetch.
To remove some data from the database:
ret = gdbm_delete ( dbf, key )
Dbf is the pointer returned by gdbm_open. Key is the key
data.
The return value is -1 if the item is not present or the
requester is a reader. The return value is 0 if there was a
successful delete.
The next two routines allow for accessing all items in the
database. 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.)
key = gdbm_firstkey ( dbf )
nextkey = gdbm_nextkey ( dbf, key )
Dbf is the pointer returned by gdbm_open. Key is the key
data.
The return values are both of type datum. If the dptr
element of the return value is NULL, there is no first key
or next key. Again notice that dptr points to data
allocated by malloc(3C) and gdbm will not free it for you.
These functions were intended to visit the database in
read-only algorithms, for instance, to validate the database
or similar operations.
File `visiting' is based on a `hash table'. gdbm_delete
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re-arranges the hash table to make sure that any collisions
in the table do not leave some item `un-findable'. The
original key order is NOT guaranteed to remain unchanged in
ALL instances. It is possible that some key will not be
visited if a loop like the following is executed:
key = gdbm_firstkey ( dbf );
while ( key.dptr ) {
nextkey = gdbm_nextkey ( dbf, key );
if ( some condition ) {
gdbm_delete ( dbf, key );
free ( key.dptr );
}
key = nextkey;
}
The following routine should be used very infrequently.
ret = gdbm_reorganize ( dbf )
If you have had a lot of deletions and would like to shrink
the space used by the gdbm file, this routine will
reorganize the database. Gdbm will not shorten the length
of a gdbm file except by using this reorganization.
(Deleted file space will be reused.)
Unless your database was opened with the GDBM_SYNC flag,
gdbm does not wait for writes to be flushed to the disk
before continuing. The following routine can be used to
guarantee that the database is physically written to the
disk file.
gdbm_sync ( dbf )
It will not return until the disk file state is syncronized
with the in-memory state of the database.
To convert a gdbm error code into English text, use this
routine:
ret = gdbm_strerror ( errno )
Where errno is of type gdbm_error, usually the global
variable gdbm_errno. The appropiate phrase is returned.
Gdbm now supports the ability to set certain options on an
already open database.
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ret = gdbm_setopt ( dbf, option, value, size )
Where dbf is the return value from a previous call to
gdbm_open, and option specifies which option to set. The
valid options are currently:
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.
GDBM_FASTMODE - Set fast mode to either on or off. This
allows fast mode to be toggled on an already open and
active database. value (see below) should be set to either
TRUE or FALSE. This option is now obsolete.
GDBM_SYNCMODE - Turn on or off file system synchronization
operations.
This setting defaults to off; value (see below) should be
set to either
TRUE or FALSE.
GDBM_CENTFREE - Set central free block pool to either on
or off.
The default is off, which is how previous versions of Gdbm
handled free blocks. If set, this option causes all
subsequent free
blocks to be placed in the global pool, allowing (in
thoery)
more file space to be reused more quickly. value (see
below) should
be set to either TRUE or FALSE.
NOTICE: This feature is still under study.
GDBM_COALESCEBLKS - Set free block merging to either on or
off.
The default is off, which is how previous versions of Gdbm
handled free blocks. If set, this option causes adjacent
free blocks
to be merged. This can become a CPU expensive process with
time, though,
especially if used in conjunction with GDBM_CENTFREE.
value
(see below) should be set to either TRUE or FALSE.
NOTICE: This feature is still under study.
value is the value to set option to, specified as an integer
pointer. size is the size of the data pointed to by value.
The return value will be -1 upon failure, or 0 upon success.
The global variable gdbm_errno will be set upon failure.
For instance, to set a database to use a cache of 10, after
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opening it with gdbm_open, but prior to accessing it in any
way, the following code could be used:
int value = 10;
ret = gdbm_setopt( dbf, GDBM_CACHESIZE, &value,
sizeof(int));
If the database was opened with the GDBM_NOLOCK flag, the
user may wish 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 gdbm_fdesc routine is
provided.
ret = gdbm_fdesc ( dbf )
Where dbf is the return value from a previous call to
gdbm_open. The return value will be the file descriptor of
the database.
The following two external variables may be useful:
gdbm_errno is the variable that contains more information
about gdbm errors. (gdbm.h has the definitions of the error
values and defines gdbm_errno as an external variable.)
gdbm_version is the string containing the version
information.
There are a few more things of interest. First, gdbm files
are not "sparse". You can copy them with the UNIX cp(1)
command and they will not expand in the copying process.
Also, there is a compatibility mode for use with programs
that already use UNIX dbm. In this compatibility mode, no
gdbm file pointer is required by the programmer, and only
one file may be opened at a time. All users in
compatibility mode are assumed to be writers. If the gdbm
file is a read only, it will fail as a writer, but will also
try to open it as a reader. All returned pointers in datum
structures point to data that gdbm WILL free. They should
be treated as static pointers (as standard UNIX dbm does).
LINKING
This library is accessed by specifying -lgdbm as the last
parameter to the compile line, e.g.:
gcc -o prog prog.c -lgdbm
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BUGS
SEE ALSO
dbm, ndbm
AUTHOR
by Philip A. Nelson and Jason Downs. Copyright (C) 1990 -
1999 Free Software Foundation, Inc.
GDBM is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 1,
or (at your option) any later version.
GDBM is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See
the GNU General Public License for more details.
You should have received a copy of the GNU General Public
License along with GDBM; see the file COPYING. If not,
write to the Free Software Foundation, 675 Mass Ave,
Cambridge, MA 02139, USA.
You may contact the original author by:
e-mail: phil@cs.wwu.edu
us-mail: Philip A. Nelson
Computer Science Department
Western Washington University
Bellingham, WA 98226
You may contact the current maintainer by:
e-mail: downsj@downsj.com
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