off.h

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/*-------------------------------------------------------------------------
 *
 * off.h
 *    POSTGRES disk "offset" definitions.
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/storage/off.h
 *
 *-------------------------------------------------------------------------
 */
#ifndef OFF_H
#define OFF_H

#include "storage/itemid.h"
/*
 * OffsetNumber:
 *
 * this is a 1-based index into the linp (ItemIdData) array in the
 * header of each disk page.
 */
typedef uint16 OffsetNumber;

#define InvalidOffsetNumber     ((OffsetNumber) 0)
#define FirstOffsetNumber       ((OffsetNumber) 1)
#define MaxOffsetNumber         ((OffsetNumber) (BLCKSZ / sizeof(ItemIdData)))
#define OffsetNumberMask        (0xffff)        /* valid uint16 bits */

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

/*
 * OffsetNumberIsValid
 *      True iff the offset number is valid.
 */
#define OffsetNumberIsValid(offsetNumber) \
    ((bool) ((offsetNumber != InvalidOffsetNumber) && \
             (offsetNumber <= MaxOffsetNumber)))

/*
 * OffsetNumberNext
 * OffsetNumberPrev
 *      Increments/decrements the argument.  These macros look pointless
 *      but they help us disambiguate the different manipulations on
 *      OffsetNumbers (e.g., sometimes we subtract one from an
 *      OffsetNumber to move back, and sometimes we do so to form a
 *      real C array index).
 */
#define OffsetNumberNext(offsetNumber) \
    ((OffsetNumber) (1 + (offsetNumber)))
#define OffsetNumberPrev(offsetNumber) \
    ((OffsetNumber) (-1 + (offsetNumber)))

#endif   /* OFF_H */