From 7b416c8c1a682ea1703d0e1da470a23c95f8124b Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Sat, 23 Feb 2008 19:11:55 +0000 Subject: [PATCH] Change the declaration of struct varlena so that the length word is represented as "char ...[4]" not "int32". Since the length word is never supposed to be accessed via this struct member anyway, this won't break any existing code that is following the rules. The advantage is that C compilers will no longer assume that a pointer to struct varlena is word-aligned, which prevents incorrect optimizations in TOAST-pointer access and perhaps other places. gcc doesn't seem to do this (at least not at -O2), but the problem is demonstrable on some other compilers. I changed struct inet as well, but didn't bother to touch a lot of other struct definitions in which it wouldn't make any difference because there were other fields forcing int alignment anyway. Hopefully none of those struct definitions are used for accessing unaligned Datums. --- doc/src/sgml/xtypes.sgml | 30 ++++++++++++++++++++-------- src/backend/access/heap/tuptoaster.c | 5 +++-- src/include/c.h | 4 ++-- src/include/utils/inet.h | 4 ++-- 4 files changed, 29 insertions(+), 14 deletions(-) diff --git a/doc/src/sgml/xtypes.sgml b/doc/src/sgml/xtypes.sgml index bc84ccd1c4b..f81f72fe60c 100644 --- a/doc/src/sgml/xtypes.sgml +++ b/doc/src/sgml/xtypes.sgml @@ -1,4 +1,4 @@ - + User-Defined Types @@ -219,7 +219,7 @@ CREATE TYPE complex ( PostgreSQL automatically provides support for arrays of that type.arrayof user-defined - type For historical reasons, the array type + type The array type typically has the same name as the base type with the underscore character (_) prepended. @@ -240,15 +240,16 @@ CREATE TYPE complex ( If the values of your data type vary in size (in internal form), you should make the data type TOAST-able (see ). You should do this even if the data are always - too small to be compressed or stored externally because - Postgres can save space on small data using - TOAST as well. + too small to be compressed or stored externally, because + TOAST can save space on small data too, by reducing header + overhead. To do this, the internal representation must follow the standard layout for - variable-length data: the first four bytes must be an int32 - which is never accessed directly (customarily named vl_len_). You + variable-length data: the first four bytes must be a char[4] + field which is never accessed directly (customarily named + vl_len_). You must use SET_VARSIZE() to store the size of the datum in this field and VARSIZE() to retrieve it. The C functions operating on the data type must always be careful to unpack any @@ -265,12 +266,25 @@ CREATE TYPE complex ( to avoid some of the overhead of PG_DETOAST_DATUM. You can use PG_DETOAST_DATUM_PACKED instead (customarily hidden by defining a GETARG_DATATYPE_PP macro) and using the macros - VARSIZE_ANY_EXHDR and VARDATA_ANY macros. + VARSIZE_ANY_EXHDR and VARDATA_ANY to access + a potentially-packed datum. Again, the data returned by these macros is not aligned even if the data type definition specifies an alignment. If the alignment is important you must go through the regular PG_DETOAST_DATUM interface. + + + Older code frequently declares vl_len_ as an + int32 field instead of char[4]. This is OK as long as + the struct definition has other fields that have at least int32 + alignment. But it is dangerous to use such a struct definition when + working with a potentially unaligned datum; the compiler may take it as + license to assume the datum actually is aligned, leading to core dumps on + architectures that are strict about alignment. + + + For further details see the description of the command. diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c index ca49ec9d937..e729e77a54e 100644 --- a/src/backend/access/heap/tuptoaster.c +++ b/src/backend/access/heap/tuptoaster.c @@ -8,7 +8,7 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/access/heap/tuptoaster.c,v 1.81 2008/01/01 19:45:46 momjian Exp $ + * $PostgreSQL: pgsql/src/backend/access/heap/tuptoaster.c,v 1.81.2.1 2008/02/23 19:11:55 tgl Exp $ * * * INTERFACE ROUTINES @@ -65,7 +65,8 @@ #define VARATT_EXTERNAL_GET_POINTER(toast_pointer, attr) \ do { \ varattrib_1b_e *attre = (varattrib_1b_e *) (attr); \ - Assert(VARSIZE_ANY_EXHDR(attre) == sizeof(toast_pointer)); \ + Assert(VARATT_IS_EXTERNAL(attre)); \ + Assert(VARSIZE_EXTERNAL(attre) == sizeof(toast_pointer) + VARHDRSZ_EXTERNAL); \ memcpy(&(toast_pointer), VARDATA_EXTERNAL(attre), sizeof(toast_pointer)); \ } while (0) diff --git a/src/include/c.h b/src/include/c.h index b66006200f8..074411a3ba9 100644 --- a/src/include/c.h +++ b/src/include/c.h @@ -12,7 +12,7 @@ * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $PostgreSQL: pgsql/src/include/c.h,v 1.222 2008/01/01 19:45:56 momjian Exp $ + * $PostgreSQL: pgsql/src/include/c.h,v 1.222.2.1 2008/02/23 19:11:55 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -410,7 +410,7 @@ typedef struct */ struct varlena { - int32 vl_len_; /* Do not touch this field directly! */ + char vl_len_[4]; /* Do not touch this field directly! */ char vl_dat[1]; }; diff --git a/src/include/utils/inet.h b/src/include/utils/inet.h index d102573f05f..5b7b619dc7e 100644 --- a/src/include/utils/inet.h +++ b/src/include/utils/inet.h @@ -7,7 +7,7 @@ * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group * Portions Copyright (c) 1994, Regents of the University of California * - * $PostgreSQL: pgsql/src/include/utils/inet.h,v 1.28 2008/01/01 19:45:59 momjian Exp $ + * $PostgreSQL: pgsql/src/include/utils/inet.h,v 1.28.2.1 2008/02/23 19:11:55 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -49,7 +49,7 @@ typedef struct */ typedef struct { - int32 vl_len_; /* Do not touch this field directly! */ + char vl_len_[4]; /* Do not touch this field directly! */ inet_struct inet_data; } inet;