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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"><html><head><title>Obtain Aggregate Function Context</title><style type="text/css">body { margin: auto; font-family: "Verdana" "sans-serif"; padding: 8px 1%;}a { color: #45735f }a:visited { color: #734559 }.logo { position:absolute; margin:3px; }.tagline { float:right; text-align:right; font-style:italic; width:240px; margin:12px; margin-top:58px;}.toolbar { font-variant: small-caps; text-align: center; line-height: 1.6em; margin: 0; padding:1px 8px;}.toolbar a { color: white; text-decoration: none; padding: 6px 12px; }.toolbar a:visited { color: white; }.toolbar a:hover { color: #80a796; background: white; }.content { margin: 5%; }.content dt { font-weight:bold; }.content dd { margin-bottom: 25px; margin-left:20%; }.content ul { padding:0px; padding-left: 15px; margin:0px; }/* rounded corners */.se { background: url(../images/se.png) 100% 100% no-repeat #80a796}.sw { background: url(../images/sw.png) 0% 100% no-repeat }.ne { background: url(../images/ne.png) 100% 0% no-repeat }.nw { background: url(../images/nw.png) 0% 0% no-repeat }</style><meta http-equiv="content-type" content="text/html; charset=UTF-8"> </head><body><div><!-- container div to satisfy validator --><a href="../index.html"><img class="logo" src="../images/SQLite.gif" alt="SQLite Logo" border="0"></a><div><!-- IE hack to prevent disappearing logo--></div><div class="tagline">Small. Fast. Reliable.<br>Choose any three.</div><table width=100% style="clear:both"><tr><td> <div class="se"><div class="sw"><div class="ne"><div class="nw"> <div class="toolbar"> <a href="../about.html">About</a> <a href="../sitemap.html">Sitemap</a> <a href="../docs.html">Documentation</a> <a href="../download.html">Download</a> <a href="../copyright.html">License</a> <a href="../news.html">News</a> <a href="http://www.sqlite.org/cvstrac/index">Developers</a> <a href="../support.html">Support</a> </div></div></div></div></div></td></tr></table> <a href="intro.html"><h2>SQLite C Interface</h2></a><h2>Obtain Aggregate Function Context</h2><blockquote><pre>void *sqlite3_aggregate_context(sqlite3_context*, int nBytes);</pre></blockquote><p>The implementation of aggregate SQL functions use this routine to allocatea structure for storing their state.The first time the sqlite3_aggregate_context() routine isis called for a particular aggregate, SQLite allocates nBytes of memoryzeros that memory, and returns a pointer to it.On second and subsequent calls to sqlite3_aggregate_context()for the same aggregate function index, the same buffer is returned.The implementationof the aggregate can use the returned buffer to accumulate data.</p><p>SQLite automatically frees the allocated buffer when the aggregatequery concludes.</p><p>The first parameter should be a copy of the<a href="../c3ref/context.html">SQL function context</a> that is the firstparameter to the callback routine that implements the aggregatefunction.</p><p>This routine must be called from the same thread in whichthe aggregate SQL function is running.</p><p><h3>Invariants:</h3><table border="0" cellpadding="5" cellspacing="0"><tr><td valign="top">F16211</td> <td valign="top">The first invocation of <a href="../c3ref/aggregate_context.html">sqlite3_aggregate_context(C,N)</a> fora particular instance of an aggregate function (for a particularcontext C) causes SQLite to allocation N bytes of memory,zero that memory, and return a pointer to the allocationedmemory.</td></tr><tr><td valign="top">F16213</td> <td valign="top">If a memory allocation error occurs during<a href="../c3ref/aggregate_context.html">sqlite3_aggregate_context(C,N)</a> then the function returns 0.</td></tr><tr><td valign="top">F16215</td> <td valign="top">Second and subsequent invocations of<a href="../c3ref/aggregate_context.html">sqlite3_aggregate_context(C,N)</a> for the same context pointer Cignore the N parameter and return a pointer to the sameblock of memory returned by the first invocation.</td></tr><tr><td valign="top">F16217</td> <td valign="top">The memory allocated by <a href="../c3ref/aggregate_context.html">sqlite3_aggregate_context(C,N)</a> isautomatically freed on the next call to <a href="../c3ref/reset.html">sqlite3_reset()</a>or <a href="../c3ref/finalize.html">sqlite3_finalize()</a> for the <a href="../c3ref/stmt.html">prepared statement</a> containingthe aggregate function associated with context C.</td></tr></table></p><p>See also lists of <a href="objlist.html">Objects</a>, <a href="constlist.html">Constants</a>, and <a href="funclist.html">Functions</a>.</p><hr><small><i>This page last modified 2008/05/12 13:08:44 UTC</i></small></div></body></html>⌨️ 快捷键说明
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