/* gdbmdefs.h - The include file for dbm. Defines structure and constants. */
/* This file is part of GDBM, the GNU data base manager, by Philip A. Nelson.
Copyright (C) 1990, 1991 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 author by:
e-mail: phil@cs.wwu.edu
us-mail: Philip A. Nelson
Computer Science Department
Western Washington University
Bellingham, WA 98226
phone: (206) 676-3035
*************************************************************************/
/* Include the standard include files needed for gdbm. */
#include <stdio.h>
#include <sys/types.h>
#include <sys/file.h>
#include <sys/stat.h>
#include "gdbmconst.h"
#include "systems.h"
/* The type definitions are next. */
/* The data and key structure. This structure is defined for compatibility. */
typedef struct {
char *dptr;
int dsize;
} datum;
/* The available file space is stored in an "avail" table. The one with
most activity is contained in the file header. (See below.) When that
one filles up, it is split in half and half is pushed on an "avail
stack." When the active avail table is empty and the "avail stack" is
not empty, the top of the stack is popped into the active avail table. */
/* The following structure is the element of the avaliable table. */
typedef struct {
int av_size; /* The size of the available block. */
long av_adr; /* The file address of the available block. */
} avail_elem;
/* This is the actual table. The in-memory images of the avail blocks are
allocated by malloc using a calculated size. */
typedef struct {
int size; /* The number of avail elements in the table.*/
int count; /* The number of entries in the table. */
long next_block; /* The file address of the next avail block. */
avail_elem av_table[1]; /* The table. Make it look like an array. */
} avail_block;
/* The dbm file header keeps track of the current location of the hash
directory and the free space in the file. */
typedef struct {
long header_magic; /* 0x13579ace to make sure the header is good. */
int block_size; /* The optimal i/o blocksize from stat. */
long dir; /* File address of hash directory table. */
int dir_size; /* Size in bytes of the table. */
int dir_bits; /* The number of address bits used in the table.*/
int bucket_size; /* Size in bytes of a hash bucket struct. */
int bucket_elems; /* Number of elements in a hash bucket. */
long next_block; /* The next unallocated block address. */
avail_block avail; /* This must be last because of the psuedo
array in avail. This avail grows to fill
the entire block. */
} gdbm_file_header;
/* The dbm hash bucket element contains the full 31 bit hash value, the
"pointer" to the key and data (stored together) with their sizes. It also
has a small part of the actual key value. It is used to verify the first
part of the key has the correct value without having to read the actual
key. */
typedef struct {
long hash_value; /* The complete 31 bit value. */
char key_start[SMALL]; /* Up to the first SMALL bytes of the key. */
long data_pointer; /* The file address of the key record. The
data record directly follows the key. */
int key_size; /* Size of key data in the file. */
int data_size; /* Size of associated data in the file. */
} bucket_element;
/* A bucket is a small hash table. This one consists of a number of
bucket elements plus some bookkeeping fields. The number of elements
depends on the optimum blocksize for the storage device and on a
parameter given at file creation time. This bucket takes one block.
When one of these tables gets full, it is split into two hash buckets.
The contents are split between them by the use of the first few bits
of the 31 bit hash function. The location in a bucket is the hash
value modulo the size of the bucket. The in-memory images of the
buckets are allocated by malloc using a calculated size depending of
the file system buffer size. To speed up write, each bucket will have
BUCKET_AVAIL avail elements with the bucket. */
typedef struct {
int av_count; /* The number of bucket_avail entries. */
avail_elem bucket_avail[BUCKET_AVAIL]; /* Distributed avail. */
int bucket_bits; /* The number of bits used to get here. */
int count; /* The number of element buckets full. */
bucket_element h_table[1]; /* The table. Make it look like an array.*/
} hash_bucket;
/* We want to keep from reading buckets as much as possible. The following is
to implement a bucket cache. When full, buckets will be dropped in a
least recently read from disk order. */
/* To speed up fetching and "sequential" access, we need to implement a
data cache for key/data pairs read from the file. To find a key, we
must exactly match the key from the file. To reduce overhead, the
data will be read at the same time. Both key and data will be stored
in a data cache. Each bucket cached will have a one element data
cache. */
typedef struct {
long hash_val;
int data_size;
int key_size;
char *dptr;
int elem_loc;
} data_cache_elem;
typedef struct {
hash_bucket * ca_bucket;
long ca_adr;
char ca_changed; /* Data in the bucket changed. */
data_cache_elem ca_data;
} cache_elem;
/* This final structure contains all main memory based information for
a gdbm file. This allows multiple gdbm files to be opened at the same
time by one program. */
typedef struct {
/* Global variables and pointers to dynamic variables used by gdbm. */
/* The file name. */
char *name;
/* The reader/writer status. */
int read_write;
/* The fatal error handling routine. */
void (*fatal_err) ();
/* The gdbm file descriptor which is set in gdbm_open. */
int desc;
/* The file header holds information about the database. */
gdbm_file_header *header;
/* The hash table directory from extendible hashing. See Fagin et al,
ACM Trans on Database Systems, Vol 4, No 3. Sept 1979, 315-344 */
long *dir;
/* The bucket cache. */
cache_elem bucket_cache [CACHE_SIZE];
int last_read;
/* Points to the current hash bucket in the cache. */
hash_bucket *bucket;
/* The directory entry used to get the current hash bucket. */
long bucket_dir;
/* Pointer to the current bucket's cache entry. */
cache_elem *cache_entry;
/* Bookkeeping of things that need to be written back at the
end of an update. */
char header_changed;
char directory_changed;
char bucket_changed;
char second_changed;
} gdbm_file_info;