dbm_fetch (3)

#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;

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).

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

dbm, ndbm

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:  [email protected]

  us-mail:  Philip A. Nelson

Computer Science Department

Western Washington University

Bellingham, WA 98226

You may contact the current maintainer by:

   e-mail:  [email protected]