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Filename : itemptr.h
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/*-------------------------------------------------------------------------
 *
 * itemptr.h
 *	  POSTGRES disk item pointer definitions.
 *
 *
 * Portions Copyright (c) 1996-2012, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/storage/itemptr.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef ITEMPTR_H
#define ITEMPTR_H

#include "storage/block.h"
#include "storage/off.h"

/*
 * ItemPointer:
 *
 * This is a pointer to an item within a disk page of a known file
 * (for example, a cross-link from an index to its parent table).
 * blkid tells us which block, posid tells us which entry in the linp
 * (ItemIdData) array we want.
 *
 * Note: because there is an item pointer in each tuple header and index
 * tuple header on disk, it's very important not to waste space with
 * structure padding bytes.  The struct is designed to be six bytes long
 * (it contains three int16 fields) but a few compilers will pad it to
 * eight bytes unless coerced.  We apply appropriate persuasion where
 * possible, and to cope with unpersuadable compilers, we try to use
 * "SizeOfIptrData" rather than "sizeof(ItemPointerData)" when computing
 * on-disk sizes.
 */
typedef struct ItemPointerData
{
	BlockIdData ip_blkid;
	OffsetNumber ip_posid;
}

#ifdef __arm__
__attribute__((packed))			/* Appropriate whack upside the head for ARM */
#endif
ItemPointerData;

#define SizeOfIptrData	\
	(offsetof(ItemPointerData, ip_posid) + sizeof(OffsetNumber))

typedef ItemPointerData *ItemPointer;

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

/*
 * ItemPointerIsValid
 *		True iff the disk item pointer is not NULL.
 */
#define ItemPointerIsValid(pointer) \
	((bool) (PointerIsValid(pointer) && ((pointer)->ip_posid != 0)))

/*
 * ItemPointerGetBlockNumber
 *		Returns the block number of a disk item pointer.
 */
#define ItemPointerGetBlockNumber(pointer) \
( \
	AssertMacro(ItemPointerIsValid(pointer)), \
	BlockIdGetBlockNumber(&(pointer)->ip_blkid) \
)

/*
 * ItemPointerGetOffsetNumber
 *		Returns the offset number of a disk item pointer.
 */
#define ItemPointerGetOffsetNumber(pointer) \
( \
	AssertMacro(ItemPointerIsValid(pointer)), \
	(pointer)->ip_posid \
)

/*
 * ItemPointerSet
 *		Sets a disk item pointer to the specified block and offset.
 */
#define ItemPointerSet(pointer, blockNumber, offNum) \
( \
	AssertMacro(PointerIsValid(pointer)), \
	BlockIdSet(&((pointer)->ip_blkid), blockNumber), \
	(pointer)->ip_posid = offNum \
)

/*
 * ItemPointerSetBlockNumber
 *		Sets a disk item pointer to the specified block.
 */
#define ItemPointerSetBlockNumber(pointer, blockNumber) \
( \
	AssertMacro(PointerIsValid(pointer)), \
	BlockIdSet(&((pointer)->ip_blkid), blockNumber) \
)

/*
 * ItemPointerSetOffsetNumber
 *		Sets a disk item pointer to the specified offset.
 */
#define ItemPointerSetOffsetNumber(pointer, offsetNumber) \
( \
	AssertMacro(PointerIsValid(pointer)), \
	(pointer)->ip_posid = (offsetNumber) \
)

/*
 * ItemPointerCopy
 *		Copies the contents of one disk item pointer to another.
 */
#define ItemPointerCopy(fromPointer, toPointer) \
( \
	AssertMacro(PointerIsValid(toPointer)), \
	AssertMacro(PointerIsValid(fromPointer)), \
	*(toPointer) = *(fromPointer) \
)

/*
 * ItemPointerSetInvalid
 *		Sets a disk item pointer to be invalid.
 */
#define ItemPointerSetInvalid(pointer) \
( \
	AssertMacro(PointerIsValid(pointer)), \
	BlockIdSet(&((pointer)->ip_blkid), InvalidBlockNumber), \
	(pointer)->ip_posid = InvalidOffsetNumber \
)

/* ----------------
 *		externs
 * ----------------
 */

extern bool ItemPointerEquals(ItemPointer pointer1, ItemPointer pointer2);
extern int32 ItemPointerCompare(ItemPointer arg1, ItemPointer arg2);

#endif   /* ITEMPTR_H */