src/include/utils/relptr.h

   1 /*-------------------------------------------------------------------------
   2  *
   3  * relptr.h
   4  *        This file contains basic declarations for relative pointers.
   5  *
   6  * Portions Copyright (c) 1996-2023, 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 - 1))
  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 - 1))
  54 #endif
  55
  56 #define relptr_is_null(rp) \
  57         ((rp).relptr_off == 0)
  58
  59 #define relptr_offset(rp) \
  60         ((rp).relptr_off - 1)
  61
  62 /* We use this inline to avoid double eval of "val" in relptr_store */
  63 static inline Size
  64 relptr_store_eval(char *base, char *val)
  65 {
  66         if (val == NULL)
  67                 return 0;
  68         else
  69         {
  70                 Assert(val >= base);
  71                 return val - base + 1;
  72         }
  73 }
  74
  75 #ifdef HAVE__BUILTIN_TYPES_COMPATIBLE_P
  76 #define relptr_store(base, rp, val) \
  77         (AssertVariableIsOfTypeMacro(base, char *), \
  78          AssertVariableIsOfTypeMacro(val, __typeof__((rp).relptr_type)), \
  79          (rp).relptr_off = relptr_store_eval((base), (char *) (val)))
  80 #else
  81 /*
  82  * If we don't have __builtin_types_compatible_p, assume we might not have
  83  * __typeof__ either.
  84  */
  85 #define relptr_store(base, rp, val) \
  86         (AssertVariableIsOfTypeMacro(base, char *), \
  87          (rp).relptr_off = relptr_store_eval((base), (char *) (val)))
  88 #endif
  89
  90 #define relptr_copy(rp1, rp2) \
  91         ((rp1).relptr_off = (rp2).relptr_off)
  92
  93 #endif                                                  /* RELPTR_H */