Add a crude facility for dealing with relative pointers.

C doesn't have any sort of built-in understanding of a pointer
relative to some arbitrary base address, but dynamic shared memory
segments can be mapped at different addresses in different processes,
so any sort of shared data structure stored within a dynamic shared
memory segment can't use absolute pointers.  We could use something
like Size to represent a relative pointer, but then the compiler
provides no type-checking.  Use stupid macro tricks to get some
type-checking.

Patch originally by me.  Concept suggested by Andres Freund.  Recently
resubmitted as part of Thomas Munro's work on dynamic shared memory
allocation.

Discussion: 20131205144434.GG12398@alap2.anarazel.de
Discussion: CAEepm=1z5WLuNoJ80PaCvz6EtG9dN0j-KuHcHtU6QEfcPP5-qA@mail.gmail.com

src/include/utils/relptr.h

+/*-------------------------------------------------------------------------
+ *
+ * relptr.h
+ *	  This file contains basic declarations for relative pointers.
+ *
+ * Portions Copyright (c) 1996-2016, 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 processes 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 of the type relptr(type), so it's
+ * useful to give them a name that doesn't include parentheses.
+ */
+#define relptr_declare(type, name) \
+	typedef union { type *relptr_type; Size relptr_off; } name;
+
+#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)
+
+#ifdef HAVE__BUILTIN_TYPES_COMPATIBLE_P
+#define relptr_store(base, rp, val) \
+	(AssertVariableIsOfTypeMacro(base, char *), \
+	 AssertVariableIsOfTypeMacro(val, __typeof__((rp).relptr_type)), \
+	 (rp).relptr_off = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
+#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 = ((val) == NULL ? 0 : ((char *) (val)) - (base)))
+#endif
+
+#define relptr_copy(rp1, rp2) \
+	((rp1).relptr_off = (rp2).relptr_off)
+
+#endif   /* RELPTR_H */