database/postgres
database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-05-15 19:15:29 +03:00
Code

Add a crude facility for dealing with relative pointers.

Browse Source 
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
This commit is contained in:
Robert Haas 2016-12-02 11:29:01 -05:00
parent e63d414988
commit fbc1c12a94
1 changed files with 74 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

74
src/include/utils/relptr.h Normal file
View File

@ -0,0 +1,74 @@
/*-------------------------------------------------------------------------
*
* 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 */