relptr.h

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/*-------------------------------------------------------------------------
 *
 * relptr.h
 *    This file contains basic declarations for relative pointers.
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/include/utils/relptr.h
 *
 *-------------------------------------------------------------------------
 */
 
#ifndef RELPTR_H
#define RELPTR_H
 
/*
 * Relative pointers are intended to be used when storing an address that may
 * be relative either to the base of the process's address space or some
 * dynamic shared memory segment mapped therein.
 *
 * The idea here is that you declare a relative pointer as relptr(type)
 * and then use relptr_access to dereference it and relptr_store to change
 * it.  The use of a union here is a hack, because what's stored in the
 * relptr is always a Size, never an actual pointer.  But including a pointer
 * in the union allows us to use stupid macro tricks to provide some measure
 * of type-safety.
 */
#define relptr(type)     union { type *relptr_type; Size relptr_off; }
 
/*
 * pgindent gets confused by declarations that use "relptr(type)" directly,
 * so preferred style is to write
 *      typedef struct ... SomeStruct;
 *      relptr_declare(SomeStruct, RelptrSomeStruct);
 * and then declare pointer variables as "RelptrSomeStruct someptr".
 */
#define relptr_declare(type, relptrtype) \
    typedef relptr(type) relptrtype
 
#ifdef HAVE_TYPEOF
#define relptr_access(base, rp) \
    (AssertVariableIsOfTypeMacro(base, char *), \
     (typeof((rp).relptr_type)) ((rp).relptr_off == 0 ? NULL : \
        (base) + (rp).relptr_off - 1))
#else
#define relptr_access(base, rp) \
    (AssertVariableIsOfTypeMacro(base, char *), \
     (void *) ((rp).relptr_off == 0 ? NULL : (base) + (rp).relptr_off - 1))
#endif
 
#define relptr_is_null(rp) \
    ((rp).relptr_off == 0)
 
#define relptr_offset(rp) \
    ((rp).relptr_off - 1)
 
/* We use this inline to avoid double eval of "val" in relptr_store */
static inline Size
relptr_store_eval(char *base, char *val)
{
    if (val == NULL)
        return 0;
    else
    {
        Assert(val >= base);
        return val - base + 1;
    }
}
 
#ifdef HAVE_TYPEOF
#define relptr_store(base, rp, val) \
    (AssertVariableIsOfTypeMacro(base, char *), \
     AssertVariableIsOfTypeMacro(val, typeof((rp).relptr_type)), \
     (rp).relptr_off = relptr_store_eval((base), (char *) (val)))
#else
#define relptr_store(base, rp, val) \
    (AssertVariableIsOfTypeMacro(base, char *), \
     (rp).relptr_off = relptr_store_eval((base), (char *) (val)))
#endif
 
#define relptr_copy(rp1, rp2) \
    ((rp1).relptr_off = (rp2).relptr_off)
 
#endif                          /* RELPTR_H */