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You are here: manpages

GDBM

Section: GDBM User Reference (3)
Updated: July 8, 2016
Index Return to Main Contents

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 *));


void 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);


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 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); datumdbm_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 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 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 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, int 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 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 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 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.
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 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.

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, 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 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.

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@gnu.org>.

AUTHORS

by Philip A. Nelson, Jason Downs and Sergey Poznyakoff.

COPYRIGHT

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. 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:

e-mail: downsj@downsj.com
and
e-mail: gray@gnu.org