1/*-------------------------------------------------------------------------
2 *
3 * relptr.h
4 * This file contains basic declarations for relative pointers.
5 *
6 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
7 * Portions Copyright (c) 1994, Regents of the University of California
8 *
9 * src/include/utils/relptr.h
10 *
11 *-------------------------------------------------------------------------
12 */
13
14#ifndef RELPTR_H
15#define RELPTR_H
16
17/*
18 * Relative pointers are intended to be used when storing an address that may
19 * be relative either to the base of the process's address space or some
20 * dynamic shared memory segment mapped therein.
21 *
22 * The idea here is that you declare a relative pointer as relptr(type)
23 * and then use relptr_access to dereference it and relptr_store to change
24 * it. The use of a union here is a hack, because what's stored in the
25 * relptr is always a Size, never an actual pointer. But including a pointer
26 * in the union allows us to use stupid macro tricks to provide some measure
27 * of type-safety.
28 */
29#define relptr(type) union { type *relptr_type; Size relptr_off; }
30
31/*
32 * pgindent gets confused by declarations that use "relptr(type)" directly,
33 * so preferred style is to write
34 * typedef struct ... SomeStruct;
35 * relptr_declare(SomeStruct, RelptrSomeStruct);
36 * and then declare pointer variables as "RelptrSomeStruct someptr".
37 */
38#define relptr_declare(type, relptrtype) \
39 typedef relptr(type) relptrtype
40
41#ifdef HAVE__BUILTIN_TYPES_COMPATIBLE_P
42#define relptr_access(base, rp) \
43 (AssertVariableIsOfTypeMacro(base, char *), \
44 (__typeof__((rp).relptr_type)) ((rp).relptr_off == 0 ? NULL : \
45 (base + (rp).relptr_off)))
46#else
47/*
48 * If we don't have __builtin_types_compatible_p, assume we might not have
49 * __typeof__ either.
50 */
51#define relptr_access(base, rp) \
52 (AssertVariableIsOfTypeMacro(base, char *), \
53 (void *) ((rp).relptr_off == 0 ? NULL : (base + (rp).relptr_off)))
54#endif
55
56#define relptr_is_null(rp) \
57 ((rp).relptr_off == 0)
58
59#ifdef HAVE__BUILTIN_TYPES_COMPATIBLE_P
60#define relptr_store(base, rp, val) \
61 (AssertVariableIsOfTypeMacro(base, char *), \
62 AssertVariableIsOfTypeMacro(val, __typeof__((rp).relptr_type)), \
63 (rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
64#else
65/*
66 * If we don't have __builtin_types_compatible_p, assume we might not have
67 * __typeof__ either.
68 */
69#define relptr_store(base, rp, val) \
70 (AssertVariableIsOfTypeMacro(base, char *), \
71 (rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
72#endif
73
74#define relptr_copy(rp1, rp2) \
75 ((rp1).relptr_off = (rp2).relptr_off)
76
77#endif /* RELPTR_H */
78