gdbm(3)
NAME
GDBM - The GNU database manager. Includes dbm and ndbm compatability. (Version 1.8.3.)
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 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.
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 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 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.
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 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
If you wish to use the dbm or ndbm compatibility routines, you must
link in the gdbm_compat library as well. For example:
gcc -o prog proc.c -lgdbm -lgdbm_compat
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
10/15/2002 GDBM(3)
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