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authorTom Lane2008-02-23 19:11:45 +0000
committerTom Lane2008-02-23 19:11:45 +0000
commit9713c063191099a29b6e1762b2b8e24ad4a95a83 (patch)
treee71087e67f97dd668b3c6136e09a78af4756e4b2
parent870993e8717f928437dfb466901a135c63686905 (diff)
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.
-rw-r--r--doc/src/sgml/xtypes.sgml30
-rw-r--r--src/backend/access/heap/tuptoaster.c5
-rw-r--r--src/include/c.h4
-rw-r--r--src/include/utils/inet.h4
4 files changed, 29 insertions, 14 deletions
diff --git a/doc/src/sgml/xtypes.sgml b/doc/src/sgml/xtypes.sgml
index bc84ccd1c4b..c80126a1195 100644
--- a/doc/src/sgml/xtypes.sgml
+++ b/doc/src/sgml/xtypes.sgml
@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/xtypes.sgml,v 1.29 2007/05/15 17:39:54 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/xtypes.sgml,v 1.30 2008/02/23 19:11:45 tgl Exp $ -->
<sect1 id="xtypes">
<title>User-Defined Types</title>
@@ -219,7 +219,7 @@ CREATE TYPE complex (
<productname>PostgreSQL</productname> automatically provides support
for arrays of that
type.<indexterm><primary>array</primary><secondary>of user-defined
- type</secondary></indexterm> For historical reasons, the array type
+ type</secondary></indexterm> The array type typically
has the same name as the base type with the underscore character
(<literal>_</>) prepended.
</para>
@@ -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 <acronym>TOAST</>-able (see <xref
linkend="storage-toast">). You should do this even if the data are always
- too small to be compressed or stored externally because
- <productname>Postgres</> can save space on small data using
- <acronym>TOAST</> as well.
+ too small to be compressed or stored externally, because
+ <acronym>TOAST</> can save space on small data too, by reducing header
+ overhead.
</para>
<para>
To do this, the internal representation must follow the standard layout for
- variable-length data: the first four bytes must be an <type>int32</type>
- which is never accessed directly (customarily named <literal>vl_len_</>). You
+ variable-length data: the first four bytes must be a <type>char[4]</type>
+ field which is never accessed directly (customarily named
+ <structfield>vl_len_</>). You
must use <function>SET_VARSIZE()</function> to store the size of the datum
in this field and <function>VARSIZE()</function> 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 <function>PG_DETOAST_DATUM</>. You can use
<function>PG_DETOAST_DATUM_PACKED</> instead (customarily hidden by
defining a <function>GETARG_DATATYPE_PP</> macro) and using the macros
- <function>VARSIZE_ANY_EXHDR</> and <function>VARDATA_ANY</> macros.
+ <function>VARSIZE_ANY_EXHDR</> and <function>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 <function>PG_DETOAST_DATUM</> interface.
</para>
+ <note>
+ <para>
+ Older code frequently declares <structfield>vl_len_</> as an
+ <type>int32</> field instead of <type>char[4]</>. This is OK as long as
+ the struct definition has other fields that have at least <type>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.
+ </para>
+ </note>
+
<para>
For further details see the description of the
<xref linkend="sql-createtype" endterm="sql-createtype-title"> command.
diff --git a/src/backend/access/heap/tuptoaster.c b/src/backend/access/heap/tuptoaster.c
index ca49ec9d937..a7fd6d0984d 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 1.82 2008/02/23 19:11:45 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..07a29d9cdf4 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 1.223 2008/02/23 19:11:45 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..793e1207857 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 1.29 2008/02/23 19:11:45 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;