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Commitfbc1c12

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Add a crude facility for dealing with relative pointers.
C doesn't have any sort of built-in understanding of a pointerrelative to some arbitrary base address, but dynamic shared memorysegments can be mapped at different addresses in different processes,so any sort of shared data structure stored within a dynamic sharedmemory segment can't use absolute pointers. We could use somethinglike Size to represent a relative pointer, but then the compilerprovides no type-checking. Use stupid macro tricks to get sometype-checking.Patch originally by me. Concept suggested by Andres Freund. Recentlyresubmitted as part of Thomas Munro's work on dynamic shared memoryallocation.Discussion: 20131205144434.GG12398@alap2.anarazel.deDiscussion: CAEepm=1z5WLuNoJ80PaCvz6EtG9dN0j-KuHcHtU6QEfcPP5-qA@mail.gmail.com
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‎src/include/utils/relptr.h

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/*-------------------------------------------------------------------------
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*
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* relptr.h
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* This file contains basic declarations for relative pointers.
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*
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* Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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* src/include/utils/relptr.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndefRELPTR_H
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#defineRELPTR_H
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/*
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* Relative pointers are intended to be used when storing an address that may
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* be relative either to the base of the processes address space or some
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* dynamic shared memory segment mapped therein.
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*
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* The idea here is that you declare a relative pointer as relptr(type)
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* and then use relptr_access to dereference it and relptr_store to change
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* it. The use of a union here is a hack, because what's stored in the
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* relptr is always a Size, never an actual pointer. But including a pointer
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* in the union allows us to use stupid macro tricks to provide some measure
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* of type-safety.
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*/
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#definerelptr(type) union { type *relptr_type; Size relptr_off; }
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/*
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* pgindent gets confused by declarations of the type relptr(type), so it's
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* useful to give them a name that doesn't include parentheses.
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*/
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#definerelptr_declare(type,name) \
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typedef union { type *relptr_type; Size relptr_off; } name;
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#ifdefHAVE__BUILTIN_TYPES_COMPATIBLE_P
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#definerelptr_access(base,rp) \
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(AssertVariableIsOfTypeMacro(base, char *), \
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(__typeof__((rp).relptr_type)) ((rp).relptr_off == 0 ? NULL : \
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(base + (rp).relptr_off)))
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#else
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/*
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* If we don't have __builtin_types_compatible_p, assume we might not have
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* __typeof__ either.
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*/
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#definerelptr_access(base,rp) \
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(AssertVariableIsOfTypeMacro(base, char *), \
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(void *) ((rp).relptr_off == 0 ? NULL : (base + (rp).relptr_off)))
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#endif
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#definerelptr_is_null(rp) \
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((rp).relptr_off == 0)
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#ifdefHAVE__BUILTIN_TYPES_COMPATIBLE_P
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#definerelptr_store(base,rp,val) \
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(AssertVariableIsOfTypeMacro(base, char *), \
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AssertVariableIsOfTypeMacro(val, __typeof__((rp).relptr_type)), \
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(rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
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#else
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/*
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* If we don't have __builtin_types_compatible_p, assume we might not have
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* __typeof__ either.
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*/
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#definerelptr_store(base,rp,val) \
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(AssertVariableIsOfTypeMacro(base, char *), \
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(rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
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#endif
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#definerelptr_copy(rp1,rp2) \
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((rp1).relptr_off = (rp2).relptr_off)
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#endif/* RELPTR_H */

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