32.9. Asynchronous Notification
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## 32.9. Asynchronous Notification #

PostgreSQL™ offers asynchronous notification via the LISTEN and NOTIFY commands. A client session registers its interest in a particular notification channel with the LISTEN command (and can stop listening with the UNLISTEN command). All sessions listening on a particular channel will be notified asynchronously when a NOTIFY command with that channel name is executed by any session. A “payload” string can be passed to communicate additional data to the listeners.

libpq applications submit LISTEN, UNLISTEN, and NOTIFY commands as ordinary SQL commands. The arrival of NOTIFY messages can subsequently be detected by calling `PQnotifies`.

The function `PQnotifies` 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 `PQnotifies`, it is considered handled and will be removed from the list of notifications.

    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;

After processing a PGnotify object returned by `PQnotifies`, be sure to free it with `PQfreemem`. It is sufficient to free the PGnotify pointer; the `relname` and `extra` 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.)

Example 32.2 gives a sample program that illustrates the use of asynchronous notification.

`PQnotifies` does not actually read data from the server; it just returns messages previously absorbed by another libpq function. In ancient releases of libpq, the only way to ensure timely receipt of NOTIFY messages was to constantly submit commands, even empty ones, and then check `PQnotifies` after each `PQexec`. While this still works, it is deprecated as a waste of processing power.

A better way to check for NOTIFY messages when you have no useful commands to execute is to call `PQconsumeInput` , then check `PQnotifies`. You can use `select()` to wait for data to arrive from the server, thereby using no CPU power unless there is something to do. (See `PQsocket` to obtain the file descriptor number to use with `select()`.) Note that this will work OK whether you submit commands with `PQsendQuery`/`PQgetResult` or simply use `PQexec`. You should, however, remember to check `PQnotifies` after each `PQgetResult` or `PQexec`, to see if any notifications came in during the processing of the command.

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