gdbm (3)





NAME

       GDBM  - The GNU database manager.  Includes dbm and ndbm compatability.
       (Version 1.8.)


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_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;

       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
       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  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  gdbmer-
       rno.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.   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 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 responsibil-

       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 perform-
       ing 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 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.

         ret = gdbm_reorganize ( dbf )

       If you have had a lot of deletions and would like to shrink  the  space

         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 opera-
       tions.
         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.

         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.

         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


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.

       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

                                    5/19/99                            gdbm(3)