gdbm(3)
GDBM(3) GDBM User Reference GDBM(3)
NAME
GDBM - The GNU database manager. Includes dbm and ndbm com-
patibility.
SYNOPSIS
#include <gdbm.h>
extern gdbm_error gdbm_errno;
extern char *gdbm_version;
GDBM_FILE gdbm_open (const char *name, int block_size,
int flags, int mode,
void (*fatal_func)(const char *));
void gdbm_close (GDBM_FILE dbf);
int gdbm_store (GDBM_FILE dbf, datum key, datum content
datum gdbm_fetch (GDBM_FILE dbf, datum key);
int gdbm_delete (GDBM_FILE dbf, datum key);
datum gdbm_firstkey (GDBM_FILE dbf);
datum gdbm_nextkey (GDBM_FILE dbf, datum key);
int gdbm_reorganize (GDBM_FILE dbf);
void gdbm_sync (GDBM_FILE dbf);
int gdbm_exists (GDBM_FILE dbf, datum key);
const char *gdbm_strerror (gdbm_error errno);
int gdbm_setopt (GDBM_FILE dbf, int option, int value
int gdbm_fdesc (GDBM_FILE dbf);
DBM Compatibility routines:
#include <dbm.h>
int dbminit (const char *name);
int store (datum key, datum content);
datum fetch (datum key);
int delete (datum key);
datum firstkey (void);
datum nextkey (datum key);
int dbmclose (void);
NDBM Compatibility routines:
#include <ndbm.h>
DBM *dbm_open (const char *name, int flags, int mode
void dbm_close (DBM *file); datumdbm_fetch(DBM* file , datum
int dbm_store (DBM *file, datum key, datum content
int dbm_delete (DBM *file, datum key);
datum dbm_firstkey (DBM *file);
datum dbm_nextkey (DBM *file, datum key);
int dbm_error (DBM *file);
int dbm_clearerr (DBM *file);
int dbm_pagfno (DBM *file);
int dbm_dirfno (DBM *file);
int dbm_rdonly (DBM *file);
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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.
This manpage is a short description of the GDBM library.
For a detailed discussion, including examples of the confi-
guration and usage recommendations, refer to the GDBM Manual
available in Texinfo format. To access it, run:
info gdbm
Should any discrepancies occur between this manpage and the
GDBM Manual, the later shall be considered the authoritative
source.
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 open-
ing a gdbm file is:
GDBM_FILE gdbm_open (const char *name, int block_size,
int flags, int mode,
void (*fatal_func)(const char *));
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 parame-
ter 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
The GDBM_NOMMAP added to read_write by bitwise or instructs
gdbm_open to disable the use of mmap(2).
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For the last three (writers of the database) the following
may be added added to read_write by bitwise or:
GDBM_SYNC
Causes all database operations to be synchronized to
the disk,
GDBM_NOLOCK
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 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:
void gdbm_close (GDBM_FILE dbf);
The database is used by 3 primary routines. The first
stores data in the database.
int gdbm_store (GDBM_FILE dbf, datum key, datum content
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
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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 in dbm or
ndbm. Your data can be as large as you want.
To search for some data, use:
datum gdbm_fetch (GDBM_FILE dbf, datum key);
Dbf is the pointer returned by gdbm_open. Key is the key
data.
If the dptr element of the return value is NULL, the
gdbm_errno variable should be examined. The value of
GDBM_ITEM_NOT_FOUND means no data was found for that key.
Other value means an error occurred.
Otherwise the return value is a pointer to the found data.
The storage space for the dptr element is allocated using
malloc(3). 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:
int gdbm_exists (GDBM_FILE dbf, datum 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
will be true. If nothing appropriate is found, false is
returned. This routine is useful for checking for the
existence of a record, without performing the memory alloca-
tion done by gdbm_fetch.
To remove some data from the database:
int gdbm_delete (GDBM_FILE dbf, datum 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.
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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.)
datum gdbm_firstkey (GDBM_FILE dbf);
datum gdbm_nextkey (GDBM_FILE dbf, datum 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 ele-
ment of the return value is NULL, inspect the gdbm_errno.
If it is GDBM_ITEM_NOT_FOUND, there is no first key or next
key. Otherwise, an error occurred.
Again, notice that dptr points to data allocated by mal-
loc(3) 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 ori-
ginal 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.
int gdbm_reorganize (GDBM_FILE dbf);
If you have had a lot of deletions and would like to shrink
the space used by the gdbm file, this routine will reorgan-
ize 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
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before continuing. The following routine can be used to
guarantee that the database is physically written to the
disk file.
void gdbm_sync (GDBM_FILE 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:
const char *gdbm_strerror (gdbm_error errno);
Gdbm now supports the ability to set certain options on an
already open database.
int gdbm_setopt (GDBM_FILE dbf, int option, int value
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 data-
base. 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
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handled free blocks. If set, this option causes adja-
cent 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 data-
base file in order to prevent multiple writers operating on
the same file simultaneously.
In order to support this, the gdbm_fdesc routine is pro-
vided.
int gdbm_fdesc (GDBM_FILE 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 informa-
tion.
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 compatibil-
ity 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
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open it as a reader. All returned pointers in datum struc-
tures 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 exam-
ple:
gcc -o prog proc.c -lgdbm -lgdbm_compat
BUG REPORTS
Send bug reports to <bug-gdbm@gnu.org>.
SEE ALSO
gdbm_dump(1), gdbm_load(1), gdbmtool(1).
AUTHORS
by Philip A. Nelson, Jason Downs and Sergey Poznyakoff.
COPYRIGHT
Copyright 8c9 1990 - 2016 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 pub-
lished 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. If not, see
<http://gnu.org/licenses/gpl.html>
CONTACTS
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 maintainers by:
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e-mail: downsj@downsj.com
and
e-mail: gray@gnu.org
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