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db_include/utils/relptr.h

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+/*-------------------------------------------------------------------------
+ *
+ * relptr.h
+ *	  This file contains basic declarations for relative pointers.
+ *
+ * Portions Copyright (c) 1996-2021, 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 process's 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 that use "relptr(type)" directly,
+ * so preferred style is to write
+ *		typedef struct ... SomeStruct;
+ *		relptr_declare(SomeStruct, RelptrSomeStruct);
+ * and then declare pointer variables as "RelptrSomeStruct someptr".
+ */
+#define relptr_declare(type, relptrtype) \
+	typedef relptr(type) relptrtype
+
+#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 */