168 lines
6.8 KiB
HTML
168 lines
6.8 KiB
HTML
<!DOCTYPE html>
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<html><head>
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<meta http-equiv="content-type" content="text/html; charset=UTF-8">
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<link href="../sqlite.css" rel="stylesheet">
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<title>Impose A Limit On Heap Size</title>
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</head>
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<body>
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Small. Fast. Reliable.<br>Choose any three.
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<!-- keywords: sqlite3_hard_heap_limit64 sqlite3_soft_heap_limit64 -->
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<div class=nosearch>
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<a href="intro.html"><h2>SQLite C Interface</h2></a>
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<h2>Impose A Limit On Heap Size</h2>
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</div>
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<blockquote><pre>
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sqlite3_int64 sqlite3_soft_heap_limit64(sqlite3_int64 N);
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sqlite3_int64 sqlite3_hard_heap_limit64(sqlite3_int64 N);
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</pre></blockquote>
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<p>
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These interfaces impose limits on the amount of heap memory that will be
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by all database connections within a single process.</p>
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<p>The sqlite3_soft_heap_limit64() interface sets and/or queries the
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soft limit on the amount of heap memory that may be allocated by SQLite.
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SQLite strives to keep heap memory utilization below the soft heap
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limit by reducing the number of pages held in the page cache
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as heap memory usages approaches the limit.
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The soft heap limit is "soft" because even though SQLite strives to stay
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below the limit, it will exceed the limit rather than generate
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an <a href="../rescode.html#nomem">SQLITE_NOMEM</a> error. In other words, the soft heap limit
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is advisory only.</p>
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<p>The sqlite3_hard_heap_limit64(N) interface sets a hard upper bound of
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N bytes on the amount of memory that will be allocated. The
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sqlite3_hard_heap_limit64(N) interface is similar to
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sqlite3_soft_heap_limit64(N) except that memory allocations will fail
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when the hard heap limit is reached.</p>
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<p>The return value from both sqlite3_soft_heap_limit64() and
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sqlite3_hard_heap_limit64() is the size of
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the heap limit prior to the call, or negative in the case of an
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error. If the argument N is negative
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then no change is made to the heap limit. Hence, the current
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size of heap limits can be determined by invoking
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sqlite3_soft_heap_limit64(-1) or sqlite3_hard_heap_limit(-1).</p>
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<p>Setting the heap limits to zero disables the heap limiter mechanism.</p>
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<p>The soft heap limit may not be greater than the hard heap limit.
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If the hard heap limit is enabled and if sqlite3_soft_heap_limit(N)
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is invoked with a value of N that is greater than the hard heap limit,
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the the soft heap limit is set to the value of the hard heap limit.
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The soft heap limit is automatically enabled whenever the hard heap
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limit is enabled. When sqlite3_hard_heap_limit64(N) is invoked and
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the soft heap limit is outside the range of 1..N, then the soft heap
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limit is set to N. Invoking sqlite3_soft_heap_limit64(0) when the
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hard heap limit is enabled makes the soft heap limit equal to the
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hard heap limit.</p>
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<p>The memory allocation limits can also be adjusted using
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<a href="../pragma.html#pragma_soft_heap_limit">PRAGMA soft_heap_limit</a> and <a href="../pragma.html#pragma_hard_heap_limit">PRAGMA hard_heap_limit</a>.</p>
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<p>The heap limits are not enforced in the current implementation
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if one or more of following conditions are true:</p>
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<p><ul>
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<li> The limit value is set to zero.
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<li> Memory accounting is disabled using a combination of the
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<a href="../c3ref/config.html">sqlite3_config</a>(<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigmemstatus">SQLITE_CONFIG_MEMSTATUS</a>,...) start-time option and
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the <a href="../compile.html#default_memstatus">SQLITE_DEFAULT_MEMSTATUS</a> compile-time option.
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<li> An alternative page cache implementation is specified using
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<a href="../c3ref/config.html">sqlite3_config</a>(<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigpcache2">SQLITE_CONFIG_PCACHE2</a>,...).
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<li> The page cache allocates from its own memory pool supplied
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by <a href="../c3ref/config.html">sqlite3_config</a>(<a href="../c3ref/c_config_covering_index_scan.html#sqliteconfigpagecache">SQLITE_CONFIG_PAGECACHE</a>,...) rather than
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from the heap.
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</ul></p>
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<p>The circumstances under which SQLite will enforce the heap limits may
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changes in future releases of SQLite.
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</p><p>See also lists of
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<a href="objlist.html">Objects</a>,
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<a href="constlist.html">Constants</a>, and
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<a href="funclist.html">Functions</a>.</p>
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