GDBM(3) GDBM User Reference GDBM(3)
NAME
GDBM - The GNU database manager. Includes dbm and ndbm compatibility.
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 *));
int gdbm_close (GDBM_FILE dbf);
int gdbm_store (GDBM_FILE dbf, datum key, datum content, int flag);
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);
int 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 size);
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);
datum dbm_fetch (DBM *file, datum key);
int dbm_store (DBM *file, datum key, datum content, int flags);
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);
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 configuration 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 Man-
ual, 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 opening 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 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
The GDBM_NOMMAP added to read_write by bitwise or instructs gdbm_open
to disable the use of mmap(2).
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 cre-
ated. (*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:
int 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, int flag);
Dbf is the pointer returned by gdbm_open. Key is the key data. Con-
tent 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 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 vari-
able 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 allocation 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.
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 element 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 malloc(3) and gdbm
will not free it for you.
These functions were intended to visit the database in read-only algo-
rithms, 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 guaran-
teed 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 reorganize the database. Gdbm
will not shorten the length of a gdbm file except by using this reorga-
nization. (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 fol-
lowing routine can be used to guarantee that the database is physically
written to the disk file.
int 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, int 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 automati-
cally 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 pre-
vent multiple writers operating on the same file simultaneously.
In order to support this, the gdbm_fdesc routine is provided.
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 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
BUG REPORTS
Send bug reports to <bug-gdbm AT gnu.org>.
SEE ALSO
gdbm_dump(1), gdbm_load(1), gdbmtool(1).
AUTHORS
by Philip A. Nelson, Jason Downs and Sergey Poznyakoff.
COPYRIGHT
Copyright (C) 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 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 FIT-
NESS 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 AT cs.edu
us-mail: Philip A. Nelson
Computer Science Department
Western Washington University
Bellingham, WA 98226
You may contact the current maintainers by:
e-mail: downsj AT downsj.com
and
e-mail: gray AT gnu.org
GDBM July 1, 2018 GDBM(3)
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