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Update comments for PG_DETOAST_PACKED and VARDATA_ANY on a structures

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that require alignment.

Add a paragraph to the "User-Defined Types" chapter on using these
macros since it seems like they're a hit.

Gregory Stark
This commit is contained in:
Bruce Momjian
2007-05-15 17:39:54 +00:00
parent 39712d1184
commit 178214d2ae
3 changed files with 47 additions and 15 deletions
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9
src/include/fmgr.h
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@ -11,7 +11,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/fmgr.h,v 1.50 2007/04/06 04:21:44 tgl Exp $
* $PostgreSQL: pgsql/src/include/fmgr.h,v 1.51 2007/05/15 17:39:54 momjian Exp $
*
*-------------------------------------------------------------------------
*/
@ -158,6 +158,12 @@ extern void fmgr_info_copy(FmgrInfo *dstinfo, FmgrInfo *srcinfo,
* The resulting datum can be accessed using VARSIZE_ANY() and VARDATA_ANY()
* (beware of multiple evaluations in those macros!)
*
* WARNING: It is only safe to use PG_DETOAST_DATUM_UNPACKED() and
* VARDATA_ANY() if you really don't care about the alignment. Either because
* you're working with something like text where the alignment doesn't matter
* or because you're not going to access its constituent parts and just use
* things like memcpy on it anyways.
*
* Note: it'd be nice if these could be macros, but I see no way to do that
* without evaluating the arguments multiple times, which is NOT acceptable.
*/
@ -174,6 +180,7 @@ extern struct varlena *pg_detoast_datum_packed(struct varlena * datum);
#define PG_DETOAST_DATUM_SLICE(datum,f,c) \
pg_detoast_datum_slice((struct varlena *) DatumGetPointer(datum), \
(int32) f, (int32) c)
/* WARNING -- unaligned pointer */
#define PG_DETOAST_DATUM_PACKED(datum) \
pg_detoast_datum_packed((struct varlena *) DatumGetPointer(datum))

9
src/include/postgres.h
View File

@ -10,7 +10,7 @@
* Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
* Portions Copyright (c) 1995, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/postgres.h,v 1.80 2007/05/04 02:01:02 tgl Exp $
* $PostgreSQL: pgsql/src/include/postgres.h,v 1.81 2007/05/15 17:39:54 momjian Exp $
*
*-------------------------------------------------------------------------
*/
@ -235,6 +235,12 @@ typedef struct
* use VARSIZE_ANY/VARSIZE_ANY_EXHDR/VARDATA_ANY. The other macros here
* should usually be used only by tuple assembly/disassembly code and
* code that specifically wants to work with still-toasted Datums.
*
* WARNING: It is only safe to use VARDATA_ANY() -- typically with
* PG_DETOAST_DATUM_UNPACKED() -- if you really don't care about the alignment.
* Either because you're working with something like text where the alignment
* doesn't matter or because you're not going to access its constituent parts
* and just use things like memcpy on it anyways.
*/
#define VARDATA(PTR) VARDATA_4B(PTR)
#define VARSIZE(PTR) VARSIZE_4B(PTR)
@ -265,6 +271,7 @@ typedef struct
VARSIZE_4B(PTR)-4))
/* caution: this will not work on an external or compressed-in-line Datum */
/* caution: this will return a possibly unaligned pointer */
#define VARDATA_ANY(PTR) \
(VARATT_IS_1B(PTR) ? VARDATA_1B(PTR) : VARDATA_4B(PTR))