| PostgreSQL 9.1.14 Documentation | ||||
|---|---|---|---|---|
| Prev | Up | Chapter 31. libpq - C Library | Next | |
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 31-2 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 prior 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.