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><H1
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><A
NAME="LIBPQ-NOTIFY"
>31.8. Asynchronous Notification</A
></H1
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> offers asynchronous notification
via the <TT
CLASS="COMMAND"
>LISTEN</TT
> and <TT
CLASS="COMMAND"
>NOTIFY</TT
>
commands. A client session registers its interest in a particular
notification channel with the <TT
CLASS="COMMAND"
>LISTEN</TT
> command (and
can stop listening with the <TT
CLASS="COMMAND"
>UNLISTEN</TT
> command). All
sessions listening on a particular channel will be notified
asynchronously when a <TT
CLASS="COMMAND"
>NOTIFY</TT
> command with that
channel name is executed by any session. A <SPAN
CLASS="QUOTE"
>"payload"</SPAN
> string can
be passed to communicate additional data to the listeners.
</P
><P
> <SPAN
CLASS="APPLICATION"
>libpq</SPAN
> applications submit
<TT
CLASS="COMMAND"
>LISTEN</TT
>, <TT
CLASS="COMMAND"
>UNLISTEN</TT
>,
and <TT
CLASS="COMMAND"
>NOTIFY</TT
> commands as
ordinary SQL commands. The arrival of <TT
CLASS="COMMAND"
>NOTIFY</TT
>
messages can subsequently be detected by calling
<CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>.
</P
><P
> The function <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> returns the next notification
from a list of unhandled notification messages received from the server.
It returns a null pointer if there are no pending notifications. Once a
notification is returned from <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>, it is considered
handled and will be removed from the list of notifications.
</P><PRE
CLASS="SYNOPSIS"
>PGnotify *PQnotifies(PGconn *conn);
typedef struct pgNotify
{
char *relname; /* notification channel name */
int be_pid; /* process ID of notifying server process */
char *extra; /* notification payload string */
} PGnotify;</PRE
><P>
After processing a <TT
CLASS="STRUCTNAME"
>PGnotify</TT
> object returned
by <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>, be sure to free it with
<CODE
CLASS="FUNCTION"
>PQfreemem</CODE
>. It is sufficient to free the
<TT
CLASS="STRUCTNAME"
>PGnotify</TT
> pointer; the
<TT
CLASS="STRUCTFIELD"
>relname</TT
> and <TT
CLASS="STRUCTFIELD"
>extra</TT
>
fields do not represent separate allocations. (The names of these fields
are historical; in particular, channel names need not have anything to
do with relation names.)
</P
><P
> <A
HREF="libpq-example.html#LIBPQ-EXAMPLE-2"
>Example 31-2</A
> gives a sample program that illustrates
the use of asynchronous notification.
</P
><P
> <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> does not actually read data from the
server; it just returns messages previously absorbed by another
<SPAN
CLASS="APPLICATION"
>libpq</SPAN
> function. In prior releases of
<SPAN
CLASS="APPLICATION"
>libpq</SPAN
>, the only way to ensure timely receipt
of <TT
CLASS="COMMAND"
>NOTIFY</TT
> messages was to constantly submit commands, even
empty ones, and then check <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> after each
<CODE
CLASS="FUNCTION"
>PQexec</CODE
>. While this still works, it is deprecated
as a waste of processing power.
</P
><P
> A better way to check for <TT
CLASS="COMMAND"
>NOTIFY</TT
> messages when you have no
useful commands to execute is to call
<CODE
CLASS="FUNCTION"
>PQconsumeInput</CODE
>, then check
<CODE
CLASS="FUNCTION"
>PQnotifies</CODE
>. You can use
<CODE
CLASS="FUNCTION"
>select()</CODE
> to wait for data to arrive from the
server, thereby using no <ACRONYM
CLASS="ACRONYM"
>CPU</ACRONYM
> power unless there is
something to do. (See <CODE
CLASS="FUNCTION"
>PQsocket</CODE
> to obtain the file
descriptor number to use with <CODE
CLASS="FUNCTION"
>select()</CODE
>.) Note that
this will work OK whether you submit commands with
<CODE
CLASS="FUNCTION"
>PQsendQuery</CODE
>/<CODE
CLASS="FUNCTION"
>PQgetResult</CODE
> or
simply use <CODE
CLASS="FUNCTION"
>PQexec</CODE
>. You should, however, remember
to check <CODE
CLASS="FUNCTION"
>PQnotifies</CODE
> after each
<CODE
CLASS="FUNCTION"
>PQgetResult</CODE
> or <CODE
CLASS="FUNCTION"
>PQexec</CODE
>, to
see if any notifications came in during the processing of the command.
</P
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