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Shared Memory Hooks Documentation

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This patch, against xfunc.sgml, adds a new subsection 33.9.12, Shared
Memory and LWLocks in C-Language Functions, describing how shared memory
and lwlocks may be requested by C add-in functions.

Marc Munro
This commit is contained in:
Bruce Momjian 2006-11-23 03:52:05 +00:00
parent 3e0c96b2ec
commit 5f78aa5acf
1 changed files with 49 additions and 1 deletions
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50
doc/src/sgml/xfunc.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/xfunc.sgml,v 1.117 2006/09/16 00:30:16 momjian Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/xfunc.sgml,v 1.118 2006/11/23 03:52:05 momjian Exp $ -->
<sect1 id="xfunc">
<title>User-Defined Functions</title>
@ -2906,6 +2906,54 @@ make_array(PG_FUNCTION_ARGS)
CREATE FUNCTION make_array(anyelement) RETURNS anyarray
AS '<replaceable>DIRECTORY</replaceable>/funcs', 'make_array'
LANGUAGE C IMMUTABLE;
</programlisting>
</para>
</sect2>
<sect2>
<title>Shared Memory and LWLocks in C-Language Functions</title>
<para>
Add-ins may reserve LWLocks and an allocation of shared memory on server
startup. The add-in's shared library must be preloaded, by specifying
it in
<xref linkend="guc-shared-preload-libraries"><indexterm><primary>shared-preload-libraries</></>,
and the shared memory must be reserved by calling:
<programlisting>
void RequestAddinShmemSpace(int size)
</programlisting>
from your <literal>_PG_init</> function.
</para>
<para>
LWLocks are reserved by calling:
<programlisting>
void RequestAddinLWLocks(int n)
</programlisting>
from <literal>_PG_init</>.
</para>
<para>
To avoid possible race-conditions, each backend should use the LWLock
<literal>AddinShmemInitLock</> when connecting to and intializing
its allocation of shared memory, as shown here:
<programlisting>
static mystruct *ptr = NULL;
if (!ptr)
{
bool found;
LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
ptr = ShmemInitStruct("my struct name", size, &amp;found);
if (!ptr)
elog(ERROR, "out of shared memory");
if (!found)
{
initialize contents of shmem area;
acquire any requested LWLocks using:
ptr->mylockid = LWLockAssign();
}
LWLockRelease(AddinShmemInitLock);
}
</programlisting>
</para>
</sect2>