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><H1
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><A
NAME="LIBPQ-NOTIFY"
>29.7. Asynchronous Notification</A
></H1
><A
NAME="AEN28284"
></A
><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 condition 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 condition will be notified asynchronously when a <TT
CLASS="COMMAND"
>NOTIFY</TT
> command with that
condition name is executed by any session.  No additional information is
passed from the notifier to the listener.  Thus, typically, any actual data
that needs to be communicated is transferred through a database table.
Commonly, the condition name is the same as the associated table, but it is
not necessary for there to be any associated table.</P
><P
><SPAN
CLASS="APPLICATION"
>libpq</SPAN
> applications submit
<TT
CLASS="COMMAND"
>LISTEN</TT
> and <TT
CLASS="COMMAND"
>UNLISTEN</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
>.<A
NAME="AEN28300"
></A
></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 condition name */
    int  be_pid;                /* process ID of notifying server process */
    char *extra;                /* notification parameter */
} 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.
(At present, the <TT
CLASS="STRUCTFIELD"
>extra</TT
> field is unused and will
always point to an empty string.)</P
><P
><A
HREF="libpq-example.html#LIBPQ-EXAMPLE-2"
>Example 29-2</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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