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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML
><HEAD
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>pg_listen</TITLE
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><H1
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></A
>pg_listen</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN1106"
></A
><H2
>Name</H2
>pg_listen -- set or change a callback for asynchronous notification messages<A
NAME="IX-PGTCL-PGLISTEN-2"
></A
></DIV
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN1111"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>pg_listen <VAR
CLASS="PARAMETER"
>conn</VAR
> <VAR
CLASS="PARAMETER"
>notifyName</VAR
> ?<SPAN
CLASS="OPTIONAL"
><VAR
CLASS="PARAMETER"
>callbackCommand</VAR
></SPAN
>?</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN1117"
></A
><H2
>Description</H2
><P
> <CODE
CLASS="FUNCTION"
>pg_listen</CODE
> creates, changes, or cancels a
request to listen for asynchronous notification messages from the
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server. With a
<VAR
CLASS="PARAMETER"
>callbackCommand</VAR
> parameter, the request is
established, or the command string of an already existing request
is replaced. With no <VAR
CLASS="PARAMETER"
>callbackCommand</VAR
> parameter, a
prior request is canceled.
</P
><P
> After a <CODE
CLASS="FUNCTION"
>pg_listen</CODE
> request is established, the
specified command string is executed whenever a notification
message bearing the given name arrives from the server. This
occurs when any <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> client
application issues a
<TT
CLASS="COMMAND"
>NOTIFY</TT
><A
NAME="AEN1128"
></A
> command referencing that name. The
command string is executed from the Tcl idle loop. That is the
normal idle state of an application written with Tk. In non-Tk Tcl
shells, you can execute <CODE
CLASS="FUNCTION"
>update</CODE
> or
<CODE
CLASS="FUNCTION"
>vwait</CODE
> to cause the idle loop to be entered.
</P
><P
> You should not invoke the SQL statements <TT
CLASS="COMMAND"
>LISTEN</TT
>
or <TT
CLASS="COMMAND"
>UNLISTEN</TT
> directly when using
<CODE
CLASS="FUNCTION"
>pg_listen</CODE
>. <SPAN
CLASS="APPLICATION"
>pgtcl</SPAN
>
takes care of issuing those statements for you. But if you want to
send a notification message yourself, invoke the SQL
<TT
CLASS="COMMAND"
>NOTIFY</TT
> statement using
<CODE
CLASS="FUNCTION"
>pg_exec</CODE
>.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN1140"
></A
><H2
>Arguments</H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><VAR
CLASS="PARAMETER"
>conn</VAR
></DT
><DD
><P
> The handle of the connection on which to listen for notifications.
</P
></DD
><DT
><VAR
CLASS="PARAMETER"
>notifyName</VAR
></DT
><DD
><P
> The name of the notification condition to start or stop
listening to.
</P
></DD
><DT
><VAR
CLASS="PARAMETER"
>callbackCommand</VAR
></DT
><DD
><P
> If present, provides the command string to execute when a
matching notification arrives.
</P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN1158"
></A
><H2
>Return Value</H2
><P
> None
</P
></DIV
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