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/*-------------------------------------------------------------------------
*
* relptr.h
* This file contains basic declarations for relative pointers.
*
* Portions Copyright (c) 1996-2018, 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__BUILTIN_TYPES_COMPATIBLE_P
#define relptr_access(base, rp) \
(AssertVariableIsOfTypeMacro(base, char *), \
(__typeof__((rp).relptr_type)) ((rp).relptr_off == 0 ? NULL : \
(base + (rp).relptr_off)))
#else
/*
* If we don't have __builtin_types_compatible_p, assume we might not have
* __typeof__ either.
*/
#define relptr_access(base, rp) \
(AssertVariableIsOfTypeMacro(base, char *), \
(void *) ((rp).relptr_off == 0 ? NULL : (base + (rp).relptr_off)))
#endif
#define relptr_is_null(rp) \
((rp).relptr_off == 0)
/* 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;
}
}
#ifdef HAVE__BUILTIN_TYPES_COMPATIBLE_P
#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
/*
* If we don't have __builtin_types_compatible_p, assume we might not have
* __typeof__ either.
*/
#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 */
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