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relptr.h
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1 /*-------------------------------------------------------------------------
2  *
3  * relptr.h
4  * This file contains basic declarations for relative pointers.
5  *
6  * Portions Copyright (c) 1996-2020, 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 */