src/include/storage/block.h

/*-------------------------------------------------------------------------
 *
 * block.h
 *	  POSTGRES disk block definitions.
 *
 *
 * Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * $PostgreSQL: pgsql/src/include/storage/block.h,v 1.25 2009/01/01 17:24:01 momjian Exp $
 *
 *-------------------------------------------------------------------------
 */
#ifndef BLOCK_H
#define BLOCK_H

/*
 * BlockNumber:
 *
 * each data file (heap or index) is divided into postgres disk blocks
 * (which may be thought of as the unit of i/o -- a postgres buffer
 * contains exactly one disk block).  the blocks are numbered
 * sequentially, 0 to 0xFFFFFFFE.
 *
 * InvalidBlockNumber is the same thing as P_NEW in buf.h.
 *
 * the access methods, the buffer manager and the storage manager are
 * more or less the only pieces of code that should be accessing disk
 * blocks directly.
 */
typedef uint32 BlockNumber;

#define InvalidBlockNumber		((BlockNumber) 0xFFFFFFFF)

#define MaxBlockNumber			((BlockNumber) 0xFFFFFFFE)

/*
 * BlockId:
 *
 * this is a storage type for BlockNumber.	in other words, this type
 * is used for on-disk structures (e.g., in HeapTupleData) whereas
 * BlockNumber is the type on which calculations are performed (e.g.,
 * in access method code).
 *
 * there doesn't appear to be any reason to have separate types except
 * for the fact that BlockIds can be SHORTALIGN'd (and therefore any
 * structures that contains them, such as ItemPointerData, can also be
 * SHORTALIGN'd).  this is an important consideration for reducing the
 * space requirements of the line pointer (ItemIdData) array on each
 * page and the header of each heap or index tuple, so it doesn't seem
 * wise to change this without good reason.
 */
typedef struct BlockIdData
{
	uint16		bi_hi;
	uint16		bi_lo;
} BlockIdData;

typedef BlockIdData *BlockId;	/* block identifier */

/* ----------------
 *		support macros
 * ----------------
 */

/*
 * BlockNumberIsValid
 *		True iff blockNumber is valid.
 */
#define BlockNumberIsValid(blockNumber) \
	((bool) ((BlockNumber) (blockNumber) != InvalidBlockNumber))

/*
 * BlockIdIsValid
 *		True iff the block identifier is valid.
 */
#define BlockIdIsValid(blockId) \
	((bool) PointerIsValid(blockId))

/*
 * BlockIdSet
 *		Sets a block identifier to the specified value.
 */
#define BlockIdSet(blockId, blockNumber) \
( \
	AssertMacro(PointerIsValid(blockId)), \
	(blockId)->bi_hi = (blockNumber) >> 16, \
	(blockId)->bi_lo = (blockNumber) & 0xffff \
)

/*
 * BlockIdCopy
 *		Copy a block identifier.
 */
#define BlockIdCopy(toBlockId, fromBlockId) \
( \
	AssertMacro(PointerIsValid(toBlockId)), \
	AssertMacro(PointerIsValid(fromBlockId)), \
	(toBlockId)->bi_hi = (fromBlockId)->bi_hi, \
	(toBlockId)->bi_lo = (fromBlockId)->bi_lo \
)

/*
 * BlockIdEquals
 *		Check for block number equality.
 */
#define BlockIdEquals(blockId1, blockId2) \
	((blockId1)->bi_hi == (blockId2)->bi_hi && \
	 (blockId1)->bi_lo == (blockId2)->bi_lo)

/*
 * BlockIdGetBlockNumber
 *		Retrieve the block number from a block identifier.
 */
#define BlockIdGetBlockNumber(blockId) \
( \
	AssertMacro(BlockIdIsValid(blockId)), \
	(BlockNumber) (((blockId)->bi_hi << 16) | ((uint16) (blockId)->bi_lo)) \
)

#endif   /* BLOCK_H */