Baoding-FenShuaJiang
/
Green_GoodERP18_FSJ_Standard
İzle 1
Yıldızla 0
Wiki 捐赠
gooderp18绿色标准版
You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
Ağaç: 0f75e31513
Dallar Biçim İmleri
${ item.name }
Branş ${ searchTerm } oluştur
'0f75e31513'den
${ noResults }
Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/libpq-notify.html

72 satır
6.6KB
Ham Suçlama Geçmiş 

  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  2. <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>33.8. Asynchronous Notification</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><link rel="prev" href="libpq-fastpath.html" title="33.7. The Fast-Path Interface" /><link rel="next" href="libpq-copy.html" title="33.9. Functions Associated with the COPY Command" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">33.8. Asynchronous Notification</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-fastpath.html" title="33.7. The Fast-Path Interface">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 33. libpq - C Library">Up</a></td><th width="60%" align="center">Chapter 33. <span xmlns="http://www.w3.org/1999/xhtml" class="application">libpq</span> - C Library</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-copy.html" title="33.9. Functions Associated with the COPY Command">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LIBPQ-NOTIFY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">33.8. Asynchronous Notification</h2></div></div></div><a id="id-1.7.3.15.2" class="indexterm"></a><p>

  3.    <span class="productname">PostgreSQL</span> offers asynchronous notification

  4.    via the <code class="command">LISTEN</code> and <code class="command">NOTIFY</code>

  5.    commands.  A client session registers its interest in a particular

  6.    notification channel with the <code class="command">LISTEN</code> command (and

  7.    can stop listening with the <code class="command">UNLISTEN</code> command).  All

  8.    sessions listening on a particular channel will be notified

  9.    asynchronously when a <code class="command">NOTIFY</code> command with that

  10.    channel name is executed by any session. A <span class="quote">“<span class="quote">payload</span>”</span> string can

  11.    be passed to communicate additional data to the listeners.

  12.   </p><p>

  13.    <span class="application">libpq</span> applications submit

  14.    <code class="command">LISTEN</code>, <code class="command">UNLISTEN</code>,

  15.    and <code class="command">NOTIFY</code> commands as

  16.    ordinary SQL commands.  The arrival of <code class="command">NOTIFY</code>

  17.    messages can subsequently be detected by calling

  18.    <code class="function">PQnotifies</code>.<a id="id-1.7.3.15.4.7" class="indexterm"></a>

  19.   </p><p>

  20.    The function <code class="function">PQnotifies</code> returns the next notification

  21.    from a list of unhandled notification messages received from the server.

  22.    It returns a null pointer if there are no pending notifications.  Once a

  23.    notification is returned from <code class="function">PQnotifies</code>, it is considered

  24.    handled and will be removed from the list of notifications.

  25. 

  26. </p><pre class="synopsis">

  27. PGnotify *PQnotifies(PGconn *conn);

  28. 

  29. typedef struct pgNotify

  30. {

  31.     char *relname;              /* notification channel name */

  32.     int  be_pid;                /* process ID of notifying server process */

  33.     char *extra;                /* notification payload string */

  34. } PGnotify;

  35. </pre><p>

  36. 

  37.    After processing a <code class="structname">PGnotify</code> object returned

  38.    by <code class="function">PQnotifies</code>, be sure to free it with

  39.    <code class="function">PQfreemem</code>.  It is sufficient to free the

  40.    <code class="structname">PGnotify</code> pointer; the

  41.    <code class="structfield">relname</code> and <code class="structfield">extra</code>

  42.    fields do not represent separate allocations.  (The names of these fields

  43.    are historical; in particular, channel names need not have anything to

  44.    do with relation names.)

  45.   </p><p>

  46.    <a class="xref" href="libpq-example.html#LIBPQ-EXAMPLE-2" title="Example 33.2. libpq Example Program 2">Example 33.2</a> gives a sample program that illustrates

  47.    the use of asynchronous notification.

  48.   </p><p>

  49.    <code class="function">PQnotifies</code> does not actually read data from the

  50.    server; it just returns messages previously absorbed by another

  51.    <span class="application">libpq</span> function.  In ancient releases of

  52.    <span class="application">libpq</span>, the only way to ensure timely receipt

  53.    of <code class="command">NOTIFY</code> messages was to constantly submit commands, even

  54.    empty ones, and then check <code class="function">PQnotifies</code> after each

  55.    <code class="function">PQexec</code>.  While this still works, it is deprecated

  56.    as a waste of processing power.

  57.   </p><p>

  58.    A better way to check for <code class="command">NOTIFY</code> messages when you have no

  59.    useful commands to execute is to call

  60.    <code class="function">PQconsumeInput</code>, then check

  61.    <code class="function">PQnotifies</code>.  You can use

  62.    <code class="function">select()</code> to wait for data to arrive from the

  63.    server, thereby using no <acronym class="acronym">CPU</acronym> power unless there is

  64.    something to do.  (See <code class="function">PQsocket</code> to obtain the file

  65.    descriptor number to use with <code class="function">select()</code>.) Note that

  66.    this will work OK whether you submit commands with

  67.    <code class="function">PQsendQuery</code>/<code class="function">PQgetResult</code> or

  68.    simply use <code class="function">PQexec</code>.  You should, however, remember

  69.    to check <code class="function">PQnotifies</code> after each

  70.    <code class="function">PQgetResult</code> or <code class="function">PQexec</code>, to

  71.    see if any notifications came in during the processing of the command.

  72.   </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="libpq-fastpath.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="libpq.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="libpq-copy.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">33.7. The Fast-Path Interface </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 33.9. Functions Associated with the <code class="command">COPY</code> Command</td></tr></table></div></body></html>
上海开阖软件有限公司 沪ICP备12045867号-1