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GDBM(3) 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;
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
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
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 charac-
ters to this name). Block_size is the size of a single transfer from disk to mem-
ory. 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.
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 ele-
ment is allocated using malloc(3C). Gdbm does not automatically free this 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 check-
ing 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 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 exe-
cuted:
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.
ret = gdbm_setopt ( dbf, option, value, size )
Where dbf is the return value from a previous call to gdbm_open, and option speci-
fies 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 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 exter-
nal 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
BUGS
SEE ALSO
dbm, ndbm
AUTHOR
by Philip A. Nelson and Jason Downs. Copyright (C) 1990 - 1999 Free Software Foun-
dation, 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 AT cs.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 AT downsj.com
5/19/99 GDBM(3)
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Under GNU General Public License
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