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><H1
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><A
NAME="LIBPQ-NOTIFY"
>30.7. Asynchronous Notification</A
></H1
><A
NAME="AEN32083"
></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="AEN32099"
></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 30-2</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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