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<sect1 id="xfunc">

<title>User-Defined Functions</title>

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>